stap(1)
NAME
stap - systemtap script translator/driver
SYNOPSIS
stap [ OPTIONS ] FILENAME [ ARGUMENTS ] stap [ OPTIONS ] - [ ARGUMENTS ] stap [ OPTIONS ] -e SCRIPT [ ARGUMENTS ] stap [ OPTIONS ] -l PROBE [ ARGUMENTS ] stap [ OPTIONS ] -L PROBE [ ARGUMENTS ] stap [ OPTIONS ] --dump-probe-types stap [ OPTIONS ] --dump-probe-aliases stap [ OPTIONS ] --dump-functions
DESCRIPTION
The stap program is the front-end to the Systemtap tool. It accepts probing instructions written in a simple domain-specific language, translates those instructions into C code, compiles this C code, and loads the resulting module into a running Linux kernel or a DynInst user-space mutator, to perform the requested system trace/probe functions. You can supply the script in a named file (FILENAME), from standard input (use - instead of FILENAME), or from the command line (using -e SCRIPT). The program runs until it is interrupted by the user, or if the script voluntarily invokes the exit() function, or by sufficient number of soft errors. The language, which is described the SCRIPT LANGUAGE section below, is strictly typed, expressive, declaration free, procedural, prototyping- friendly, and inspired by awk and C. It allows source code points or events in the system to be associated with handlers, which are subroutines that are executed synchronously. It is somewhat similar conceptually to "breakpoint command lists" in the gdb debugger.
DOCUMENTATION OVERVIEW
systemtap comes with a variety of educational, documentation and
reference resources. They come online and/or packaged for offline use.
For online documentation, see the project web site,
https://sourceware.org/systemtap/
man pages
stap (this page) language syntax, concepts, operation, options
stapprobes probe points and their $context variables
stapref quick reference to language syntax
stappaths list of directories, including books & references
stap-prep program to install auxiliary dependencies like
kernel debuginfo
tapset::* generated list of tapsets
probe::* generated list of tapset probe aliases
function::* generated list of tapset functions
macro::* generated list of tapset macros
stapvars some of the tapset global variables
staprun, stapdyn programs for executing compiled systemtap scripts
systemtap initscript, boot-time probing
stap-server compilation server
stapex a few very basic script examples
books
Beginner's Guide tutorial book, language essentials, examples
Tutorial shorter tutorial, exercises
Language Reference detailed language manual, covers statistics/analysis
Tapset Reference the tapset man pages, reformatted into a book
references
example scripts over a hundred directly usable sysadmin tools, toys,
hacks to learn from
OPTIONS
The systemtap translator supports the following options. Any other
option prints a list of supported options. Options may be given on the
command line, as usual. If the file $SYSTEMTAP_DIR/rc exist, options
are also loaded from there and interpreted first. ($SYSTEMTAP_DIR
defaults to $HOME/.systemtap if unset.)
In some cases, the default value of an option depends on particular
system configuration and thus can't be mentioned here directly. In
some of those cases running "stap --help" might display the default.
- Use standard input instead of a given FILENAME as probe language
input, unless -e SCRIPT is given.
-h --help
Show help message.
-V --version
Show version message.
-p NUM Stop after pass NUM. The passes are numbered 1-5: parse,
elaborate, translate, compile, run. See the PROCESSING section
for details.
-v Increase verbosity for all passes. Produce a larger volume of
informative (?) output each time option repeated.
--vp ABCDE
Increase verbosity on a per-pass basis. For example, "--vp 002"
adds 2 units of verbosity to pass 3 only. The combination
"-v --vp 00004" adds 1 unit of verbosity for all passes, and 4
more for pass 5.
-k Keep the temporary directory after all processing. This may be
useful in order to examine the generated C code, or to reuse the
compiled kernel object.
-g Guru mode. Enable parsing of unsafe expert-level constructs
like embedded C.
-P Prologue-searching mode. This is equivalent to
--prologue-searching=always. Activate heuristics to work around
incorrect debugging information for function parameter $context
variables.
-u Unoptimized mode. Disable unused code elision and many other
optimizations during elaboration / translation.
-w Suppressed warnings mode. Disables all warning messages.
-W Treat all warnings as errors.
-b Use bulk mode (percpu files) for kernel-to-user data transfer.
Use the stap-merge program to multiplex them back together
later.
-t Collect timing information on the number of times probe executes
and average amount of time spent in each probe-point. Also shows
the derivation for each probe-point.
-s NUM Use NUM megabyte buffers for kernel-to-user data transfer. On a
multiprocessor in bulk mode, this is a per-processor amount.
-I DIR Add the given directory to the tapset search directory. See the
description of pass 2 for details.
-D NAME=VALUE
Add the given C preprocessor directive to the module Makefile.
These can be used to override limit parameters described below.
-B NAME=VALUE
In kernel-runtime mode, add the given make directive to the
kernel module build's make invocation. These can be used to add
or override kconfig options. For example, use
-B CONFIG_DEBUG_INFO=y
to add debugging information.
-B FLAG
In dyninst-runtime mode, add the given parameter to the compiler
CFLAGS used for building the dyninst shared library. For
example, use
-B -g
to add debugging information.
-a ARCH
Use a cross-compilation mode for the given target architecture.
This requires access to the cross-compiler and the kernel build
tree, and goes along with the
-B CROSS_COMPILE=arch-tool-prefix-
and
-r /build/tree
options.
--modinfo NAME=VALUE
Add the name/value pair as a MODULE_INFO macro call to the
generated module. This may be useful to inform or override
various module-related checks in the kernel.
-G NAME=VALUE
Sets the value of global variable NAME to VALUE when staprun is
invoked. This applies to scalar variables declared global in
the script/tapset.
-R DIR Look for the systemtap runtime sources in the given directory.
Your DIR default can be seen using "stap --help".
-r /DIR
Build for kernel in given build tree. Can also be set with the
SYSTEMTAP_RELEASE environment variable.
-r RELEASE
Build for kernel in build tree /lib/modules/RELEASE/build. Can
also be set with the SYSTEMTAP_RELEASE environment variable.
-m MODULE
Use the given name for the generated kernel object module,
instead of a unique randomized name. The generated kernel
object module is copied to the current directory.
-d MODULE
Add symbol/unwind information for the given module into the
kernel object module. This may enable symbolic tracebacks from
those modules/programs, even if they do not have an explicit
probe placed into them.
--ldd Add symbol/unwind information for all user-space shared
libraries suspected by ldd to be necessary for user-space
binaries being probed or listed with the -d option. Caution:
this can make the probe modules considerably larger. Note that
this option does not deal with kernel-space modules: see instead
--all-modules below.
--all-modules
Equivalent to specifying "-dkernel" and a "-d" for each kernel
module that is currently loaded. Caution: this can make the
probe modules considerably larger.
-o FILE
Send standard output to named file. In bulk mode, percpu files
will start with FILE_ (FILE_cpu with -F) followed by the cpu
number. This supports strftime(3) formats for FILE.
-c CMD Start the probes, run CMD, and exit when CMD finishes. This
also has the effect of setting target() to the pid of the
command ran.
-x PID Sets target() to PID. This allows scripts to be written that
filter on a specific process. Scripts run independent of the
PID's lifespan.
-e SCRIPT
Run the given SCRIPT specified on the command line.
-E SCRIPT
Run the given SCRIPT specified. This SCRIPT is run in addition
to the main script specified, through -e, or as a script file.
This option can be repeated to run multiple scripts, and can be
used in listing mode (-l/-L).
-l PROBE
Instead of running a probe script, just list all available probe
points matching the given single probe point. The pattern may
include wildcards and aliases, but not comma-separated multiple
probe points. The process result code will indicate failure if
there are no matches.
-L PROBE
Similar to "-l", but list probe points and script-level local
variables.
-F Without -o option, load module and start probes, then detach
from the module leaving the probes running. With -o option, run
staprun in background as a daemon and show its pid.
-S size[,N]
Sets the maximum size of output file and the maximum number of
output files. If the size of output file will exceed size ,
systemtap switches output file to the next file. And if the
number of output files exceed N , systemtap removes the oldest
output file. You can omit the second argument.
--skip-badvars
Ignore unresolvable or run-time-inaccessible context variables
and substitute with 0, without errors.
--prologue-searching[=WHEN]
Prologue-searching mode. Activate heuristics to work around
incorrect debugging information for function parameter
$context variables. WHEN can be either "never", "always", or
"auto" (i.e. enabled by heuristic). If WHEN is missing, then
"always" is assumed. If the option is missing, then "auto" is
assumed.
--suppress-handler-errors
Wrap all probe handlers into something like this
try { ... } catch { next }
block, which causes any runtime errors to be quietly suppressed.
Suppressed errors do not count against MAXERRORS limits. In
this mode, the MAXSKIPPED limits are also suppressed, so that
many errors and skipped probes may be accumulated during a
script's runtime. Any overall counts will still be reported at
shutdown.
--compatible VERSION
Suppress recent script language or tapset changes which are
incompatible with given older version of systemtap. This may be
useful if a much older systemtap script fails to run. See the
DEPRECATION section for more details.
--check-version
This option is used to check if the active script has any
constructs that may be systemtap version specific. See the
DEPRECATION section for more details.
--clean-cache
This option prunes stale entries from the cache directory. This
is normally done automatically after successful runs, but this
option will trigger the cleanup manually and then exit. See the
CACHING section for more details about cache limits.
--color[=WHEN], --colour[=WHEN]
This option controls coloring of error messages. WHEN can be
either "never", "always", or "auto" (i.e. enable only if at a
terminal). If WHEN is missing, then "always" is assumed. If the
option is missing, then "auto" is assumed.
Colors can be modified using the SYSTEMTAP_COLORS environment
variable. The format must be of the form
key1=val1:key2=val2:key3=val3 ...etc. Valid keys are "error",
"warning", "source", "caret", and "token". Values constitute
Select Graphic Rendition (SGR) parameter(s). Consult the
documentation of your terminal for the SGRs it supports. As an
example, the default colors would be expressed as
error=01;31:warning=00;33:source=00;34:caret=01:token=01. If
SYSTEMTAP_COLORS is absent, the default colors will be used. If
it is empty or invalid, coloring is turned off.
--disable-cache
This option disables all use of the cache directory. No files
will be either read from or written to the cache.
--poison-cache
This option treats files in the cache directory as invalid. No
files will be read from the cache, but resulting files from this
run will still be written to the cache. This is meant as a
troubleshooting aid when stap's cached behavior seems to be
misbehaving.
--privilege[=stapusr | =stapsys | =stapdev]
This option instructs stap to examine the script looking for
constructs which are not allowed for the specified privilege
level (see UNPRIVILEGED USERS). Compilation fails if any such
constructs are used. If stapusr or stapsys are specified when
using a compile server (see --use-server), the server will
examine the script and, if compilation succeeds, the server will
cryptographically sign the resulting kernel module, certifying
that is it safe for use by users at the specified privilege
level.
If --privilege has not been specified, -pN has not been
specified with N < 5, and the invoking user is not root, and is
not a member of the group stapdev, then stap will automatically
add the appropriate --privilege option to the options already
specified.
--unprivileged
This option is equivalent to --privilege=stapusr.
--use-server[=HOSTNAME[:PORT] | =IP_ADDRESS[:PORT] | =CERT_SERIAL]
Specify compile-server(s) to be used for compilation and/or in
conjunction with --list-servers and --trust-servers (see below)
for listing. If no argument is supplied, then the default in
unprivileged mode (see --privilege) is to select compatible
servers which are trusted as SSL peers and as module signers and
currently online. Otherwise the default is to select compatible
servers which are trusted as SSL peers and currently online.
--use-server may be specified more than once, in which case a
list of servers is accumulated in the order specified. Servers
may be specified by host name, ip address, or by certificate
serial number (obtained using --list-servers). The latter is
most commonly used when adding or revoking trust in a server
(see --trust-servers below). If a server is specified by host
name or ip address, then an optional port number may be
specified. This is useful for accessing servers which are not on
the local network or to specify a particular server.
IP addresses may be IPv4 or IPv6 addresses.
If a particular IPv6 address is link local and exists on more
than one interface, the intended interface may be specified by
appending the address with a percent sign (%) followed by the
intended interface name. For example,
"fe80::5eff:35ff:fe07:55ca%eth0".
In order to specify a port number with an IPv6 address, it is
necessary to enclose the IPv6 address in square brackets ([]) in
order to separate the port number from the rest of the address.
For example, "[fe80::5eff:35ff:fe07:55ca]:5000" or
"[fe80::5eff:35ff:fe07:55ca%eth0]:5000".
If --use-server has not been specified, -pN has not been
specified with N < 5, and the invoking user not root, is not a
member of the group stapdev, but is a member of the group
stapusr, then stap will automatically add --use-server to the
options already specified.
--use-server-on-error[=yes|=no]
Instructs stap to retry compilation of a script using a compile
server if compilation on the local host fails in a manner which
suggests that it might succeed using a server. If this option
is not specified, the default is no. If no argument is
provided, then the default is yes. Compilation will be retried
for certain types of errors (e.g. insufficient data or
resources) which may not occur during re-compilation by a
compile server. Compile servers will be selected automatically
for the re-compilation attempt as if --use-server was specified
with no arguments.
--list-servers[=SERVERS]
Display the status of the requested SERVERS, where SERVERS is a
comma-separated list of server attributes. The list of
attributes is combined to filter the list of servers displayed.
Supported attributes are:
all specifies all known servers (trusted SSL peers, trusted
module signers, online servers).
specified
specifies servers specified using --use-server.
online filters the output by retaining information about servers
which are currently online.
trusted
filters the output by retaining information about servers
which are trusted as SSL peers.
signer filters the output by retaining information about servers
which are trusted as module signers (see --privilege).
compatible
filters the output by retaining information about servers
which are compatible with the current kernel release and
architecture.
If no argument is provided, then the default is specified. If
no servers were specified using --use-server, then the default
servers for --use-server are listed.
Note that --list-servers uses the avahi-daemon service to detect
online servers. If this service is not available, then
--list-servers will fail to detect any online servers. In order
for --list-servers to detect servers listening on IPv6
addresses, the avahi-daemon configuration file /etc/avahi/avahi-
daemon.conf must contain an active "use-ipv6=yes" line. The
service must be restarted after adding this line in order for
IPv6 to be enabled.
--trust-servers[=TRUST_SPEC]
Grant or revoke trust in compile-servers, specified using
--use-server as specified by TRUST_SPEC, where TRUST_SPEC is a
comma-separated list specifying the trust which is to be granted
or revoked. Supported elements are:
ssl trust the specified servers as SSL peers.
signer trust the specified servers as module signers (see
--privilege). Only root can specify signer.
all-users
grant trust as an ssl peer for all users on the local
host. The default is to grant trust as an ssl peer for
the current user only. Trust as a module signer is always
granted for all users. Only root can specify all-users.
revoke revoke the specified trust. The default is to grant it.
no-prompt
do not prompt the user for confirmation before carrying
out the requested action. The default is to prompt the
user for confirmation.
If no argument is provided, then the default is ssl. If no
servers were specified using --use-server, then no trust will be
granted or revoked.
Unless no-prompt has been specified, the user will be prompted
to confirm the trust to be granted or revoked before the
operation is performed.
--dump-probe-types
Dumps a list of supported probe types and exits. If
--privilege=stapusr is also specified, the list will be limited
to probe types available to unprivileged users.
--dump-probe-aliases
Dumps a list of all probe aliases found in library files and
exits.
--dump-functions
Dumps a list of all the public functions found in library files
and exits. Also includes their parameters and types. A function
of type 'unknown' indicates a function that does not return a
value. Note that not all function/parameter types may be
resolved (these are also shown by 'unknown'). This features is
very memory-intensive and thus may not work properly with --use-
server if the target server imposes an rlimit on process memory
(i.e. through the ~stap-server/.systemtap/rc configuration file,
see stap-server(8)).
--remote URL
Set the execution target to the given host. This option may be
repeated to target multiple execution targets. Passes 1-4 are
completed locally as normal to build the script, and then pass 5
will copy the module to the target and run it. Acceptable URL
forms include:
[USER@]HOSTNAME, ssh://[USER@]HOSTNAME
This mode uses ssh, optionally using a username not
matching your own. If a custom ssh_config file is in use,
add SendEnv LANG to retain internationalization
functionality.
libvirt://DOMAIN, libvirt://DOMAIN/LIBVIRT_URI
This mode uses stapvirt to execute the script on a domain
managed by libvirt. Optionally, LIBVIRT_URI may be
specified to connect to a specific driver and/or a remote
host. For example, to connect to the local privileged
QEMU driver, use:
--remote libvirt://MyDomain/qemu:///system
See the page at <http://libvirt.org/uri.html> for
supported URIs. Also see stapvirt(1) for more information
on how to prepare the domain for stap probing.
unix:PATH
This mode connects to a UNIX socket. This can be used
with a QEMU virtio-serial port for executing scripts
inside a running virtual machine.
direct://
Special loopback mode to run on the local host.
--remote-prefix
Prefix each line of remote output with "N: ", where N is the
index of the remote execution target from which the given line
originated.
--download-debuginfo[=OPTION]
Enable, disable or set a timeout for the automatic debuginfo
downloading feature offered by abrt as specified by OPTION,
where OPTION is one of the following:
yes enable automatic downloading of debuginfo with no
timeout. This is the same as not providing an OPTION
value to --download-debuginfo
no explicitly disable automatic downloading of debuginfo.
This is the same as not using the option at all.
ask show abrt output, and ask before continuing download. No
timeout will be set.
<timeout>
specify a timeout as a positive number to stop the
download if it is taking longer than <timeout> seconds.
--rlimit-as=NUM
Specify the maximum size of the process's virtual memory
(address space), in bytes. If nothing is specified, no limits
are imposed.
--rlimit-cpu=NUM
Specify the CPU time limit, in seconds. If nothing is specified,
no limits are imposed.
--rlimit-nproc=NUM
Specify the maximum number of processes that can be created. If
nothing is specified, no limits are imposed.
--rlimit-stack=NUM
Specify the maximum size of the process stack, in bytes. If
nothing is specified, no limits are imposed.
--rlimit-fsize=NUM
Specify the maximum size of files that the process may create,
in bytes. If nothing is specified, no limits are imposed.
--sysroot=DIR
Specify sysroot directory where target files (executables,
libraries, etc.) are located. With -r RELEASE, the sysroot
will be searched for the appropriate kernel build directory.
With -r /DIR, however, the sysroot will not be used to find the
kernel build.
--sysenv=VAR=VALUE
Provide an alternate value for an environment variable where the
value on a remote system differs. Path variables (e.g. PATH,
LD_LIBRARY_PATH) are assumed to be relative to the directory
provided by --sysroot, if provided.
--suppress-time-limits
Disable -DSTP_OVERLOAD related options as well as -DMAXACTION
and -DMAXTRYLOCK. This option requires guru mode.
--runtime=MODE
Set the pass-5 runtime mode. Valid options are kernel (default)
and dyninst. See ALTERNATE RUNTIMES below for more information.
--dyninst
Shorthand for --runtime=dyninst.
--save-uprobes
On machines that require SystemTap to build its own uprobes
module (kernels prior to version 3.5), this option instructs
SystemTap to also save a copy of the module in the current
directory (creating a new "uprobes" directory first).
--target-namespaces=PID
Allow for a set of target namespaces to be set based on the
namespaces the given PID is in. This is for namespace-aware
tapset functions. If the target namespaces was not set, the
target defaults to the stap process' namespaces.
--monitor=INTERVAL
Enables an interface to display status information about the
module(uptime, module name, invoker uid, memory sizes, global
variables, list of probes with their statistics). An optional
argument INTERVAL can be supplied to set the refresh rate in
seconds of the status window. The module can also be controlled
by a list of commands using the following keys:
c Resets all global variables to their initial values or
zeroes them if they did not have an initial value.
s Rotates the attribute used to sort the list of probes.
t Brings up a prompt to allow toggling(on/off) of probes by
index. Probe points are still affected by their
conditions.
r Resumes the script by toggling on all probes.
p Pauses the script by toggling off all probes.
x Hides/shows the status window. This allows for more
output to be seen.
navigation-keys
The j/k/ArrowDown/ArrowUp keys can be used to scroll
through the probe list and the
d,D/u,U/PageDown,End/PageUp,Home keys can be used to
scroll through the module output.
ARGUMENTS
Any additional arguments on the command line are passed to the script parser for substitution. See below.
SCRIPT LANGUAGE
The systemtap script language resembles awk and C. There are two main
outermost constructs: probes and functions. Within these, statements
and expressions use C-like operator syntax and precedence.
GENERAL SYNTAX
Whitespace is ignored. Three forms of comments are supported:
# ... shell style, to the end of line, except for $# and @#
// ... C++ style, to the end of line
/* ... C style ... */
Literals are either strings enclosed in double-quotes (passing through
the usual C escape codes with backslashes, and with adjacent string
literals glued together, also as in C), or integers (in decimal,
hexadecimal, or octal, using the same notation as in C). All strings
are limited in length to some reasonable value (a few hundred bytes).
Integers are 64-bit signed quantities, although the parser also accepts
(and wraps around) values above positive 2**63.
In addition, script arguments given at the end of the command line may
be inserted. Use $1 ... $<NN> for insertion unquoted, @1 ... @<NN> for
insertion as a string literal. The number of arguments may be accessed
through $# (as an unquoted number) or through @# (as a quoted number).
These may be used at any place a token may begin, including within the
preprocessing stage. Reference to an argument number beyond what was
actually given is an error.
PREPROCESSING
A simple conditional preprocessing stage is run as a part of parsing.
The general form is similar to the cond ? exp1 : exp2 ternary operator:
%( CONDITION %? TRUE-TOKENS %)
%( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)
The CONDITION is either an expression whose format is determined by its
first keyword, or a string literals comparison or a numeric literals
comparison. It can be also composed of many alternatives and
conjunctions of CONDITIONs (meant as in previous sentence) using || and
&& respectively. However, parentheses are not supported yet, so
remembering that conjunction takes precedence over alternative is
important.
If the first part is the identifier kernel_vr or kernel_v to refer to
the kernel version number, with ("2.6.13-1.322FC3smp") or without
("2.6.13") the release code suffix, then the second part is one of the
six standard numeric comparison operators <, <=, ==, !=, >, and >=, and
the third part is a string literal that contains an RPM-style version-
release value. The condition is deemed satisfied if the version of the
target kernel (as optionally overridden by the -r option) compares to
the given version string. The comparison is performed by the glibc
function strverscmp. As a special case, if the operator is for simple
equality (==), or inequality (!=), and the third part contains any
wildcard characters (* or ? or [), then the expression is treated as a
wildcard (mis)match as evaluated by fnmatch.
If, on the other hand, the first part is the identifier arch to refer
to the processor architecture (as named by the kernel build system
ARCH/SUBARCH), then the second part is one of the two string comparison
operators == or !=, and the third part is a string literal for matching
it. This comparison is a wildcard (mis)match.
Similarly, if the first part is an identifier like CONFIG_something to
refer to a kernel configuration option, then the second part is == or
!=, and the third part is a string literal for matching the value
(commonly "y" or "m"). Nonexistent or unset kernel configuration
options are represented by the empty string. This comparison is also a
wildcard (mis)match.
If the first part is the identifier systemtap_v, the test refers to the
systemtap compatibility version, which may be overridden for old
scripts with the --compatible flag. The comparison operator is as is
for kernel_v and the right operand is a version string. See also the
DEPRECATION section below.
If the first part is the identifier systemtap_privilege, the test
refers to the privilege level that the systemtap script is compiled
with. Here the second part is == or !=, and the third part is a string
literal, either "stapusr" or "stapsys" or "stapdev".
If the first part is the identifier guru_mode, the test refers to if
the systemtap script is compiled with guru_mode. Here the second part
is == or !=, and the third part is a number, either 1 or 0.
If the first part is the identifier runtime, the test refers to the
systemtap runtime mode. See ALTERNATE RUNTIMES below for more
information on runtimes. The second part is one of the two string
comparison operators == or !=, and the third part is a string literal
for matching it. This comparison is a wildcard (mis)match.
Otherwise, the CONDITION is expected to be a comparison between two
string literals or two numeric literals. In this case, the arguments
are the only variables usable.
The TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens
(possibly including nested preprocessor conditionals), and are passed
into the input stream if the condition is true or false. For example,
the following code induces a parse error unless the target kernel
version is newer than 2.6.5:
%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence
The following code might adapt to hypothetical kernel version drift:
probe kernel.function (
%( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
%( kernel_vr == "2.6.13*smp" %? "do_page_fault" %:
UNSUPPORTED %) %)
) { /* ... */ }
%( arch == "ia64" %?
probe syscall.vliw = kernel.function("vliw_widget") {}
%)
PREPROCESSOR MACROS
The preprocessor also supports a simple macro facility, run as a
separate pass before conditional preprocessing.
Macros are defined using the following construct:
@define NAME %( BODY %)
@define NAME(PARAM_1, PARAM_2, ...) %( BODY %)
Macros, and parameters inside a macro body, are both invoked by
prefixing the macro name with an @ symbol:
@define foo %( x %)
@define add(a,b) %( ((@a)+(@b)) %)
@foo = @add(2,2)
Macro expansion is currently performed in a separate pass before
conditional compilation. Therefore, both TRUE- and FALSE-tokens in
conditional expressions will be macroexpanded regardless of how the
condition is evaluated. This can sometimes lead to errors:
// The following results in a conflict:
%( CONFIG_UTRACE == "y" %?
@define foo %( process.syscall %)
%:
@define foo %( **ERROR** %)
%)
// The following works properly as expected:
@define foo %(
%( CONFIG_UTRACE == "y" %? process.syscall %: **ERROR** %)
%)
The first example is incorrect because both @defines are evaluated in a
pass prior to the conditional being evaluated.
Normally, a macro definition is local to the file it occurs in. Thus,
defining a macro in a tapset does not make it available to the user of
the tapset. Publically available library macros can be defined by
including .stpm files on the tapset search path. These files may only
contain @define constructs, which become visible across all tapsets and
user scripts. Optionally, within the .stpm files, a public macro
definition can be surrounded by a preprocessor conditional as described
above.
VARIABLES
Identifiers for variables and functions are an alphanumeric sequence,
and may include _ and $ characters. They may not start with a plain
digit, as in C. Each variable is by default local to the probe or
function statement block within which it is mentioned, and therefore
its scope and lifetime is limited to a particular probe or function
invocation.
Scalar variables are implicitly typed as either string or integer.
Associative arrays also have a string or integer value, and a tuple of
strings and/or integers serving as a key. Here are a few basic
expressions.
var1 = 5
var2 = "bar"
array1 [pid()] = "name" # single numeric key
array2 ["foo",4,i++] += 5 # vector of string/num/num keys
if (["hello",5,4] in array2) println ("yes") # membership test
The translator performs type inference on all identifiers, including
array indexes and function parameters. Inconsistent type-related use
of identifiers signals an error.
Variables may be declared global, so that they are shared amongst all
probes and functions and live as long as the entire systemtap session.
There is one namespace for all global variables, regardless of which
script file they are found within. Concurrent access to global
variables is automatically protected with locks, see the SAFETY AND
SECURITY section for more details. A global declaration may be written
at the outermost level anywhere, not within a block of code. Global
variables which are written but never read will be displayed
automatically at session shutdown. The translator will infer for each
its value type, and if it is used as an array, its key types.
Optionally, scalar globals may be initialized with a string or number
literal. The following declaration marks variables as global.
global var1, var2, var3=4
Global variables can also be set as module options. One can do this by
either using the -G option, or the module must first be compiled using
stap -p4. Global variables can then be set on the command line when
calling staprun on the module generated by stap -p4. See staprun(8) for
more information.
The scope of a global variable may be limited to a tapset or user
script file using private keyword. The global keyword is optional when
defining a private global variable. Following declaration marks var1
and var2 private globals.
private global var1=2
private var2
Arrays are limited in size by the MAXMAPENTRIES variable -- see the
SAFETY AND SECURITY section for details. Optionally, global arrays may
be declared with a maximum size in brackets, overriding MAXMAPENTRIES
for that array only. Note that this doesn't indicate the type of keys
for the array, just the size.
global tiny_array[10], normal_array, big_array[50000]
Arrays may be configured for wrapping using the '%' suffix. This
causes older elements to be overwritten if more elements are inserted
than the array can hold. This works for both associative and statistics
typed arrays.
global wrapped_array1%[10], wrapped_array2%
Many types of probe points provide context variables, which are run-
time values, safely extracted from the kernel or userspace program
being probed. These are prefixed with the $ character. The CONTEXT
VARIABLES section in stapprobes(3stap) lists what is available for each
type of probe point. These context variables become normal string or
numeric scalars once they are stored in normal script variables. See
the TYPECASTING section below on how to to turn them back into typed
pointers for further processing as context variables.
STATEMENTS
Statements enable procedural control flow. They may occur within
functions and probe handlers. The total number of statements executed
in response to any single probe event is limited to some number defined
by the MAXACTION macro in the translated C code, and is in the
neighbourhood of 1000.
EXP Execute the string- or integer-valued expression and throw away
the value.
{ STMT1 STMT2 ... }
Execute each statement in sequence in this block. Note that
separators or terminators are generally not necessary between
statements.
; Null statement, do nothing. It is useful as an optional
separator between statements to improve syntax-error detection
and to handle certain grammar ambiguities.
if (EXP) STMT1 [ else STMT2 ]
Compare integer-valued EXP to zero. Execute the first (non-
zero) or second STMT (zero).
while (EXP) STMT
While integer-valued EXP evaluates to non-zero, execute STMT.
for (EXP1; EXP2; EXP3) STMT
Execute EXP1 as initialization. While EXP2 is non-zero, execute
STMT, then the iteration expression EXP3.
foreach (VAR in ARRAY [ limit EXP ]) STMT
Loop over each element of the named global array, assigning
current key to VAR. The array may not be modified within the
statement. By adding a single + or - operator after the VAR or
the ARRAY identifier, the iteration will proceed in a sorted
order, by ascending or descending index or value. If the array
contains statistics aggregates, adding the desired @operator
between the ARRAY identifier and the + or - will specify the
sorting aggregate function. See the STATISTICS section below
for the ones available. Default is @count. Using the optional
limit keyword limits the number of loop iterations to EXP times.
EXP is evaluated once at the beginning of the loop.
foreach ([VAR1, VAR2, ...] in ARRAY [ limit EXP ]) STMT
Same as above, used when the array is indexed with a tuple of
keys. A sorting suffix may be used on at most one VAR or ARRAY
identifier.
foreach ([VAR1, VAR2, ...] in ARRAY [INDEX1, INDEX2, ...] [ limit EXP
]) STMT
Same as above, where iterations are limited to elements in the
array where the keys match the index values specified. The
symbol * can be used to specify an index and will be treated as
a wildcard.
foreach (VAR0 = VAR in ARRAY [ limit EXP ]) STMT
This variant of foreach saves current value into VAR0 on each
iteration, so it is the same as ARRAY[VAR]. This also works
with a tuple of keys. Sorting suffixes on VAR0 have the same
effect as on ARRAY.
foreach (VAR0 = VAR in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT
Same as above, where iterations are limited to elements in the
array where the keys match the index values specified. The
symbol * can be used to specify an index and will be treated as
a wildcard.
break, continue
Exit or iterate the innermost nesting loop (while or for or
foreach) statement.
return EXP
Return EXP value from enclosing function. If the function's
value is not taken anywhere, then a return statement is not
needed, and the function will have a special "unknown" type with
no return value.
next Return now from enclosing probe handler. This is especially
useful in probe aliases that apply event filtering predicates.
When used in functions, the execution will be immediately
transferred to the next overloaded function.
try { STMT1 } catch { STMT2 }
Run the statements in the first block. Upon any run-time
errors, abort STMT1 and start executing STMT2. Any errors in
STMT2 will propagate to outer try/catch blocks, if any.
try { STMT1 } catch(VAR) { STMT2 }
Same as above, plus assign the error message to the string
scalar variable VAR.
delete ARRAY[INDEX1, INDEX2, ...]
Remove from ARRAY the element specified by the index tuple. If
the index tuple contains a * in place of an index, the * is
treated as a wildcard and all elements with keys that match the
index tuple will be removed from ARRAY. The value will no
longer be available, and subsequent iterations will not report
the element. It is not an error to delete an element that does
not exist.
delete ARRAY
Remove all elements from ARRAY.
delete SCALAR
Removes the value of SCALAR. Integers and strings are cleared
to 0 and "" respectively, while statistics are reset to the
initial empty state.
EXPRESSIONS
Systemtap supports a number of operators that have the same general
syntax, semantics, and precedence as in C and awk. Arithmetic is
performed as per typical C rules for signed integers. Division by zero
or overflow is detected and results in an error.
binary numeric operators
* / % + - >> << & ^ | && ||
binary string operators
. (string concatenation)
numeric assignment operators
= *= /= %= += -= >>= <<= &= ^= |=
string assignment operators
= .=
unary numeric operators
+ - ! ~ ++ --
binary numeric, string comparison or regex matching operators
< > <= >= == != =~ !~
ternary operator
cond ? exp1 : exp2
grouping operator
( exp )
function call
fn ([ arg1, arg2, ... ])
array membership check
exp in array
[exp1, exp2, ...] in array
[*, *, ... ]in array
REGULAR EXPRESSION MATCHING
The scripting language supports regular expression matching. The basic
syntax is as follows:
exp =~ regex
exp !~ regex
(The first operand must be an expression evaluating to a string; the
second operand must be a string literal containing a syntactically
valid regular expression.)
The regular expression syntax supports most of the features of POSIX
Extended Regular Expressions, except for subexpression reuse ("\1")
functionality. The ability to capture and extract the contents of the
matched string and subexpressions has not yet been implemented.
PROBES
The main construct in the scripting language identifies probes. Probes
associate abstract events with a statement block ("probe handler") that
is to be executed when any of those events occur. The general syntax
is as follows:
probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
probe PROBEPOINT [, PROBEPOINT] if (CONDITION) { [STMT ...] }
Events are specified in a special syntax called "probe points". There
are several varieties of probe points defined by the translator, and
tapset scripts may define further ones using aliases. Probe points may
be wildcarded, grouped, or listed in preference sequences, or declared
optional. More details on probe point syntax and semantics are listed
on the stapprobes(3stap) manual page.
The probe handler is interpreted relative to the context of each event.
For events associated with kernel code, this context may include
variables defined in the source code at that spot. These "context
variables" are presented to the script as variables whose names are
prefixed with "$". They may be accessed only if the kernel's compiler
preserved them despite optimization. This is the same constraint that
a debugger user faces when working with optimized code. In addition,
the objects must exist in paged-in memory at the moment of the
systemtap probe handler's execution, because systemtap must not cause
(suppresses) any additional paging. Some probe types have very little
context. See the stapprobes(3stap) man pages to see the kinds of
context variables available at each kind of probe point.
Probes may be decorated with an arming condition, consisting of a
simple boolean expression on read-only global script variables. While
disarmed (inactive, condition evaluates to false), some probe types
reduce or eliminate their run-time overheads. When an arming condition
evaluates to true, probes will be soon re-armed, and their probe
handlers will start getting called as the events fire. (Some events
may be lost during the arming interval. If this is unacceptable, do
not use arming conditions for those probes.) Example of the syntax:
probe timer.us(TIMER) if (enabled) {
}
New probe points may be defined using "aliases". Probe point aliases
look similar to probe definitions, but instead of activating a probe at
the given point, it just defines a new probe point name as an alias to
an existing one. There are two types of alias, i.e. the prologue style
and the epilogue style which are identified by "=" and "+="
respectively.
For prologue style alias, the statement block that follows an alias
definition is implicitly added as a prologue to any probe that refers
to the alias. While for the epilogue style alias, the statement block
that follows an alias definition is implicitly added as an epilogue to
any probe that refers to the alias. For example:
probe syscall.read = kernel.function("sys_read") {
fildes = $fd
if (execname() == "init") next # skip rest of probe
}
defines a new probe point syscall.read, which expands to
kernel.function("sys_read"), with the given statement as a prologue,
which is useful to predefine some variables for the alias user and/or
to skip probe processing entirely based on some conditions. And
probe syscall.read += kernel.function("sys_read") {
if (tracethis) println ($fd)
}
defines a new probe point with the given statement as an epilogue,
which is useful to take actions based upon variables set or left over
by the the alias user. Please note that in each case, the statements
in the alias handler block are treated ordinarily, so that variables
assigned there constitute mere initialization, not a macro
substitution.
An alias is used just like a built-in probe type.
probe syscall.read {
printf("reading fd=%d\n", fildes)
if (fildes > 10) tracethis = 1
}
FUNCTIONS
Systemtap scripts may define subroutines to factor out common work.
Functions take any number of scalar (integer or string) arguments, and
must return a single scalar (integer or string). An example function
declaration looks like this:
function thisfn (arg1, arg2) {
return arg1 + arg2
}
Note the general absence of type declarations, which are instead
inferred by the translator. However, if desired, a function definition
may include explicit type declarations for its return value and/or its
arguments. This is especially helpful for embedded-C functions. In
the following example, the type inference engine need only infer type
type of arg2 (a string).
function thatfn:string (arg1:long, arg2) {
return sprint(arg1) . arg2
}
Functions may call others or themselves recursively, up to a fixed
nesting limit. This limit is defined by the MAXNESTING macro in the
translated C code and is in the neighbourhood of 10.
Functions may be marked private using the private keyword to limit
their scope to the tapset or user script file they are defined in. An
example definition of a private function follows:
private function three:long () { return 3 }
Functions terminating without reaching an explicit return statement
will return an implicit 0 or "", determined by type inference.
Functions may be overloaded during both runtime and compile time.
Runtime overloading allows the executed function to be selected while
the module is running based on runtime conditions and is achieved using
the "next" statement in script functions and STAP_NEXT macro for
embedded-C functions. For example,
function f() { if (condition) next; print("first function") }
function f() %{ STAP_NEXT; print("second function") %}
function f() { print("third function") }
During a functioncall f(), the execution will transfer to the third
function if condition evaluates to true and print "third function".
Note that the second function is unconditionally nexted.
Parameter overloading allows the function to be executed to be selected
at compile time based on the number of arguments provided to the
functioncall. For example,
function g() { print("first function") }
function g(x) { print("second function") }
g() -> "first function"
g(1) -> "second function"
Note that runtime overloading does not occur in the above example, as
exactly one function will be resolved for the functioncall. The use of
a next statement inside a function while no more overloads remain will
trigger a runtime exception Runtime overloading will only occur if the
functions have the same arity, functions with the same name but
different number of parameters are completely unrelated.
Execution order is determined by a priority value which may be
specified. If no explicit priority is specified, user script functions
are given a higher priority than library functions. User script
functions and library functions are assigned a default priority value
of 0 and 1 respectively. Functions with the same priority are executed
in declaration order. For example,
function f():3 { if (condition) next; print("first function") }
function f():1 { if (condition) next; print("second function") }
function f():2 { print("third function") }
Since the second function has highest priority, it is executed first.
The first function is never executed as there no "next" statements in
the third function to transfer execution.
PRINTING
There are a set of function names that are specially treated by the
translator. They format values for printing to the standard systemtap
output stream in a more convenient way (note that data generated in the
kernel module need to get transferred to user-space in order to get
printed).
The sprint* variants return the formatted string instead of printing
it.
print, sprint
Print one or more values of any type, concatenated directly
together.
println, sprintln
Print values like print and sprint, but also append a newline.
printd, sprintd
Take a string delimiter and two or more values of any type, and
print the values with the delimiter interposed. The delimiter
must be a literal string constant.
printdln, sprintdln
Print values with a delimiter like printd and sprintd, but also
append a newline.
printf, sprintf
Take a formatting string and a number of values of corresponding
types, and print them all. The format must be a literal string
constant.
The printf formatting directives similar to those of C, except that
they are fully type-checked by the translator:
%b Writes a binary blob of the value given, instead of ASCII
text. The width specifier determines the number of bytes
to write; valid specifiers are %b %1b %2b %4b %8b.
Default (%b) is 8 bytes.
%c Character.
%d,%i Signed decimal.
%m Safely reads kernel memory at the given address, outputs
its content. The optional precision specifier (not field
width) determines the number of bytes to read - default
is 1 byte. %10.4m prints 4 bytes of the memory in a
10-character-wide field.
%M Same as %m, but outputs in hexadecimal. The minimal size
of output is double the optional precision specifier -
default is 1 byte (2 hex chars). %10.4M prints 4 bytes
of the memory as 8 hexadecimal characters in a
10-character-wide field. %.*M hex-dumps a given number
of bytes from a given buffer.
%o Unsigned octal.
%p Unsigned pointer address.
%s String.
%u Unsigned decimal.
%x Unsigned hex value, in all lower-case.
%X Unsigned hex value, in all upper-case.
%% Writes a %.
The # flag selects the alternate forms. For octal, this prefixes a 0.
For hex, this prefixes 0x or 0X, depending on case. For characters,
this escapes non-printing values with either C-like escapes or raw
octal.
Examples:
a = "alice", b = "bob", p = 0x1234abcd, i = 123, j = -1, id[a] = 1234, id[b] = 4567
print("hello")
Prints: hello
println(b)
Prints: bob\n
println(a . " is " . sprint(16))
Prints: alice is 16
foreach (name in id) printdln("|", strlen(name), name, id[name])
Prints: 5|alice|1234\n3|bob|4567
printf("%c is %s; %x or %X or %p; %d or %u\n",97,a,p,p,p,j,j)
Prints: a is alice; 1234abcd or 1234ABCD or 0x1234abcd; -1 or 18446744073709551615\n
printf("2 bytes of kernel buffer at address %p: %2m", p, p)
Prints: 2 byte of kernel buffer at address 0x1234abcd: <binary data>
printf("%4b", p)
Prints (these values as binary data): 0x1234abcd
printf("%#o %#x %#X\n", 1, 2, 3)
Prints: 01 0x2 0X3
printf("%#c %#c %#c\n", 0, 9, 42)
Prints: \000 \t *
STATISTICS
It is often desirable to collect statistics in a way that avoids the
penalties of repeatedly exclusive locking the global variables those
numbers are being put into. Systemtap provides a solution using a
special operator to accumulate values, and several pseudo-functions to
extract the statistical aggregates.
The aggregation operator is <<<, and resembles an assignment, or a C++
output-streaming operation. The left operand specifies a scalar or
array-index lvalue, which must be declared global. The right operand
is a numeric expression. The meaning is intuitive: add the given
number to the pile of numbers to compute statistics of. (The specific
list of statistics to gather is given separately, by the extraction
functions.)
foo <<< 1
stats[pid()] <<< memsize
The extraction functions are also special. For each appearance of a
distinct extraction function operating on a given identifier, the
translator arranges to compute a set of statistics that satisfy it.
The statistics system is thereby "on-demand". Each execution of an
extraction function causes the aggregation to be computed for that
moment across all processors.
Here is the set of extractor functions. The first argument of each is
the same style of lvalue used on the left hand side of the accumulate
operation. The @count(v), @sum(v), @min(v), @max(v), @avg(v) extractor
functions compute the number/total/minimum/maximum/average of all
accumulated values. The resulting values are all simple integers.
Arrays containing aggregates may be sorted and iterated. See the
foreach construct above.
Histograms are also available, but are more complicated because they
have a vector rather than scalar value.
@hist_linear(v,start,stop,interval) represents a linear histogram from
"start" to "stop" by increments of "interval". The interval must be
positive. Similarly, @hist_log(v) represents a base-2 logarithmic
histogram. Printing a histogram with the print family of functions
renders a histogram object as a tabular "ASCII art" bar chart.
probe timer.profile {
x[1] <<< pid()
x[2] <<< uid()
y <<< tid()
}
global x // an array containing aggregates
global y // a scalar
probe end {
foreach ([i] in x @count+) {
printf ("x[%d]: avg %d = sum %d / count %d\n",
i, @avg(x[i]), @sum(x[i]), @count(x[i]))
println (@hist_log(x[i]))
}
println ("y:")
println (@hist_log(y))
}
TYPECASTING
Once a pointer (see the CONTEXT VARIABLES section of stapprobes(3stap))
has been saved into a script integer variable, the translator loses the
type information necessary to access members from that pointer. Using
the @cast() operator tells the translator how to interpret the number
as a typed pointer.
@cast(p, "type_name"[, "module"])->member
This will interpret p as a pointer to a struct/union named type_name
and dereference the member value. Further ->subfield expressions may
be appended to dereference more levels. NOTE: the same dereferencing
operator -> is used to refer to both direct containment or pointer
indirection. Systemtap automatically determines which. The optional
module tells the translator where to look for information about that
type. Multiple modules may be specified as a list with : separators.
If the module is not specified, it will default either to the probe
module for dwarf probes, or to "kernel" for functions and all other
probes types.
The translator can create its own module with type information from a
header surrounded by angle brackets, in case normal debuginfo is not
available. For kernel headers, prefix it with "kernel" to use the
appropriate build system. All other headers are built with default GCC
parameters into a user module. Multiple headers may be specified in
sequence to resolve a codependency.
@cast(tv, "timeval", "<sys/time.h>")->tv_sec
@cast(task, "task_struct", "kernel<linux/sched.h>")->tgid
@cast(task, "task_struct",
"kernel<linux/sched.h><linux/fs_struct.h>")->fs->umask
Values acquired by @cast may be pretty-printed by the $ and $$ suffix
operators, the same way as described in the CONTEXT VARIABLES section
of the stapprobes(3stap) manual page.
When in guru mode, the translator will also allow scripts to assign new
values to members of typecasted pointers.
Typecasting is also useful in the case of void* members whose type may
be determinable at runtime.
probe foo {
if ($var->type == 1) {
value = @cast($var->data, "type1")->bar
} else {
value = @cast($var->data, "type2")->baz
}
print(value)
}
EMBEDDED C
When in guru mode, the translator accepts embedded C code in the top
level of the script. Such code is enclosed between %{ and %} markers,
and is transcribed verbatim, without analysis, in some sequence, into
the top level of the generated C code. At the outermost level, this
may be useful to add #include instructions, and any auxiliary
definitions for use by other embedded code.
Another place where embedded code is permitted is as a function body.
In this case, the script language body is replaced entirely by a piece
of C code enclosed again between %{ and %} markers. This C code may do
anything reasonable and safe. There are a number of undocumented but
complex safety constraints on atomicity, concurrency, resource
consumption, and run time limits, so this is an advanced technique.
The memory locations set aside for input and output values are made
available to it using macros STAP_ARG_* and STAP_RETVALUE. Errors may
be signalled with STAP_ERROR. Output may be written with STAP_PRINTF.
The function may return early with STAP_RETURN. Here are some
examples:
function integer_ops (val) %{
STAP_PRINTF("%d\n", STAP_ARG_val);
STAP_RETVALUE = STAP_ARG_val + 1;
if (STAP_RETVALUE == 4)
STAP_ERROR("wrong guess: %d", (int) STAP_RETVALUE);
if (STAP_RETVALUE == 3)
STAP_RETURN(0);
STAP_RETVALUE ++;
%}
function string_ops (val) %{
strlcpy (STAP_RETVALUE, STAP_ARG_val, MAXSTRINGLEN);
strlcat (STAP_RETVALUE, "one", MAXSTRINGLEN);
if (strcmp (STAP_RETVALUE, "three-two-one"))
STAP_RETURN("parameter should be three-two-");
%}
function no_ops () %{
STAP_RETURN(); /* function inferred with no return value */
%}
The function argument and return value types have to be inferred by the
translator from the call sites in order for this to work. The user
should examine C code generated for ordinary script-language functions
in order to write compatible embedded-C ones.
The last place where embedded code is permitted is as an expression
rvalue. In this case, the C code enclosed between %{ and %} markers is
interpreted as an ordinary expression value. It is assumed to be a
normal 64-bit signed number, unless the marker /* string */ is
included, in which case it's treated as a string.
function add_one (val) {
return val + %{ 1 %}
}
function add_string_two (val) {
return val . %{ /* string */ "two" %}
}
The embedded-C code may contain markers to assert optimization and
safety properties.
/* pure */
means that the C code has no side effects and may be elided
entirely if its value is not used by script code.
/* stable */
means that the C code always has the same value (in any given
probe handler invocation), so repeated calls may be
automatically replaced by memoized values. Such functions must
take no parameters, and also be pure.
/* unprivileged */
means that the C code is so safe that even unprivileged users
are permitted to use it.
/* myproc-unprivileged */
means that the C code is so safe that even unprivileged users
are permitted to use it, provided that the target of the current
probe is within the user's own process.
/* guru */
means that the C code is so unsafe that a systemtap user must
specify -g (guru mode) to use this.
/* unmangled */
in an embedded-C function, means that the legacy (pre-1.8)
argument access syntax should be made available inside the
function. Hence, in addition to STAP_ARG_foo and STAP_RETVALUE
one can use THIS->foo and THIS->__retvalue respectively inside
the function. This is useful for quickly migrating code written
for SystemTap version 1.7 and earlier.
/* unmodified-fnargs */
in an embedded-C function, means that the function arguments are
not modified inside the function body.
/* string */
in embedded-C expressions only, means that the expression has
const char * type and should be treated as a string value,
instead of the default long numeric.
Script level global variables may be accessed in embedded-C functions
and blocks. To read or write the global variable var , the /*
pragma:read:var */ or /* pragma:write:var */ marker must be first
placed in the embedded-C function or block. This provides the macros
STAP_GLOBAL_GET_* and STAP_GLOBAL_SET_* macros to allow reading and
writing, respectively. For example:
global var
global var2[100]
function increment() %{
/* pragma:read:var */ /* pragma:write:var */
/* pragma:read:var2 */ /* pragma:write:var2 */
STAP_GLOBAL_SET_var(STAP_GLOBAL_GET_var()+1); //var++
STAP_GLOBAL_SET_var2(1, 1, STAP_GLOBAL_GET_var2(1, 1)+1); //var2[1,1]++
%}
Variables may be read and set in both embedded-C functions and
expressions. Strings returned from embedded-C code are decayed to
pointers. Variables must also be assigned at script level to allow for
type inference. Map assignment does not return the value written, so
chaining does not work.
BUILT-INS
A set of builtin probe point aliases are provided by the scripts
installed in the directory specified in the stappaths(7) manual page.
The functions are described in the stapprobes(3stap) manual page.
PROCESSING
The translator begins pass 1 by parsing the given input script, and all
scripts (files named *.stp) found in a tapset directory. The
directories listed with -I are processed in sequence, each processed in
"guru mode". For each directory, a number of subdirectories are also
searched. These subdirectories are derived from the selected kernel
version (the -R option), in order to allow more kernel-version-specific
scripts to override less specific ones. For example, for a kernel
version 2.6.12-23.FC3 the following patterns would be searched, in
sequence: 2.6.12-23.FC3/*.stp, 2.6.12/*.stp, 2.6/*.stp, and finally
*.stp. Stopping the translator after pass 1 causes it to print the
parse trees.
In pass 2, the translator analyzes the input script to resolve symbols
and types. References to variables, functions, and probe aliases that
are unresolved internally are satisfied by searching through the parsed
tapset script files. If any tapset script file is selected because it
defines an unresolved symbol, then the entirety of that file is added
to the translator's resolution queue. This process iterates until all
symbols are resolved and a subset of tapset script files is selected.
Next, all probe point descriptions are validated against the wide
variety supported by the translator. Probe points that refer to code
locations ("synchronous probe points") require the appropriate kernel
debugging information to be installed. In the associated probe
handlers, target-side variables (whose names begin with "$") are found
and have their run-time locations decoded.
Next, all probes and functions are analyzed for optimization
opportunities, in order to remove variables, expressions, and functions
that have no useful value and no side-effect. Embedded-C functions are
assumed to have side-effects unless they include the magic string
/* pure */. Since this optimization can hide latent code errors such
as type mismatches or invalid $context variables, it sometimes may be
useful to disable the optimizations with the -u option.
Finally, all variable, function, parameter, array, and index types are
inferred from context (literals and operators). Stopping the
translator after pass 2 causes it to list all the probes, functions,
and variables, along with all inferred types. Any inconsistent or
unresolved types cause an error.
In pass 3, the translator writes C code that represents the actions of
all selected script files, and creates a Makefile to build that into a
kernel object. These files are placed into a temporary directory.
Stopping the translator at this point causes it to print the contents
of the C file.
In pass 4, the translator invokes the Linux kernel build system to
create the actual kernel object file. This involves running make in
the temporary directory, and requires a kernel module build system
(headers, config and Makefiles) to be installed in the usual spot
/lib/modules/VERSION/build. Stopping the translator after pass 4 is
the last chance before running the kernel object. This may be useful
if you want to archive the file.
In pass 5, the translator invokes the systemtap auxiliary program
staprun program for the given kernel object. This program arranges to
load the module then communicates with it, copying trace data from the
kernel into temporary files, until the user sends an interrupt signal.
Any run-time error encountered by the probe handlers, such as running
out of memory, division by zero, exceeding nesting or runtime limits,
results in a soft error indication. Soft errors in excess of MAXERRORS
block of all subsequent probes (except error-handling probes), and
terminate the session. Finally, staprun unloads the module, and cleans
up.
ABNORMAL TERMINATION
One should avoid killing the stap process forcibly, for example with
SIGKILL, because the stapio process (a child process of the stap
process) and the loaded module may be left running on the system. If
this happens, send SIGTERM or SIGINT to any remaining stapio processes,
then use rmmod to unload the systemtap module.
EXAMPLES
See the stapex(3stap) manual page for a brief collection of samples, or a large set of installed samples under the systemtap documentation/testsuite directories. See stappaths(7stap) for the likely location of these on the system.
CACHING
The systemtap translator caches the pass 3 output (the generated C code) and the pass 4 output (the compiled kernel module) if pass 4 completes successfully. This cached output is reused if the same script is translated again assuming the same conditions exist (same kernel version, same systemtap version, etc.). Cached files are stored in the $SYSTEMTAP_DIR/cache directory. The cache can be limited by having the file cache_mb_limit placed in the cache directory (shown above) containing only an ASCII integer representing how many MiB the cache should not exceed. In the absence of this file, a default will be created with the limit set to 256MiB. This is a 'soft' limit in that the cache will be cleaned after a new entry is added if the cache clean interval is exceeded, so the total cache size may temporarily exceed this limit. This interval can be specified by having the file cache_clean_interval_s placed in the cache directory (shown above) containing only an ASCII integer representing the interval in seconds. In the absence of this file, a default will be created with the interval set to 300 s.
SAFETY AND SECURITY
Systemtap may be used as a powerful administrative tool. It can expose
kernel internal data structures and potentially private user
information. (In dyninst runtime mode, this is not the case, see the
ALTERNATE RUNTIMES section below.)
The translator asserts many safety constraints during compilation and
more during run-time. It aims to ensure that no handler routine can
run for very long, allocate boundless memory, perform unsafe
operations, or in unintentionally interfere with the system. Uses of
script global variables are automatically read/write locked as
appropriate, to protect against manipulation by concurrent probe
handlers. (Deadlocks are detected with timeouts. Use the -t flag to
receive reports of excessive lock contention.) Experimenting with
scripts is therefore generally safe. The guru-mode -g option allows
administrators to bypass most safety measures, which permits invasive
or state-changing operations, embedded-C code, and increases the risk
of upset. By default, overload prevention is turned on for all
modules. If you would like to disable overload processing, use the
--suppress-time-limits option.
Errors that are caught at run time normally result in a clean script
shutdown and a pass-5 error message. The --suppress-handler-errors
option lets scripts tolerate soft errors without shutting down.
PERMISSIONS
For the normal linux-kernel-module runtime, to run the kernel objects
systemtap builds, a user must be one of the following:
* the root user;
* a member of the stapdev and stapusr groups;
* a member of the stapsys and stapusr groups; or
* a member of the stapusr group.
The root user or a user who is a member of both the stapdev and stapusr
groups can build and run any systemtap script.
A user who is a member of both the stapsys and stapusr groups can only
use pre-built modules under the following conditions:
* The module has been signed by a trusted signer. Trusted signers are
normally systemtap compile-servers which sign modules when the
--privilege option is specified by the client. See the
stap-server(8) manual page for more information.
* The module was built using the --privilege=stapsys or the
--privilege=stapusr options.
Members of only the stapusr group can only use pre-built modules under
the following conditions:
* The module is located in the /lib/modules/VERSION/systemtap
directory. This directory must be owned by root and not be world
writable.
or
* The module has been signed by a trusted signer. Trusted signers are
normally systemtap compile-servers which sign modules when the
--privilege option is specified by the client. See the
stap-server(8) manual page for more information.
* The module was built using the --privilege=stapusr option.
The kernel modules generated by stap program are run by the staprun
program. The latter is a part of the Systemtap package, dedicated to
module loading and unloading (but only in the white zone), and kernel-
to-user data transfer. Since staprun does not perform any additional
security checks on the kernel objects it is given, it would be unwise
for a system administrator to add untrusted users to the stapdev or
stapusr groups.
SECUREBOOT
If the current system has SecureBoot turned on in the UEFI firmware,
all kernel modules must be signed. (Some kernels may allow disabling
SecureBoot long after booting with a key sequence such as SysRq-X,
making it unnecessary to sign modules.) The systemtap compile server
can sign modules with a MOK (Machine Owner Key) that it has in common
with a client system. See the following wiki page for more details:
https://sourceware.org/systemtap/wiki/SecureBoot
RESOURCE LIMITS
Many resource use limits are set by macros in the generated C code.
These may be overridden with -D flags. A selection of these is as
follows:
MAXNESTING
Maximum number of nested function calls. Default determined by
script analysis, with a bonus 10 slots added for recursive
scripts.
MAXSTRINGLEN
Maximum length of strings, default 128.
MAXTRYLOCK
Maximum number of iterations to wait for locks on global
variables before declaring possible deadlock and skipping the
probe, default 1000.
MAXACTION
Maximum number of statements to execute during any single probe
hit (with interrupts disabled), default 1000. Note that for
straight-through probe handlers lacking loops or recursion, due
to optimization, this parameter may be interpreted too
conservatively.
MAXACTION_INTERRUPTIBLE
Maximum number of statements to execute during any single probe
hit which is executed with interrupts enabled (such as begin/end
probes), default (MAXACTION * 10).
MAXBACKTRACE
Maximum number of stack frames that will be be processed by the
stap runtime unwinder as produced by the backtrace functions in
the [u]context-unwind.stp tapsets, default 20.
MAXMAPENTRIES
Maximum number of rows in any single global array, default 2048.
Individual arrays may be declared with a larger or smaller limit
instead:
global big[10000],little[5]
or denoted with % to make them wrap-around automatically.
MAPHASHBIAS
The number of powers-of-two to add or subtract from the natural
size of the hash table backing each global associative array.
Default is 0. Try small positive numbers to get extra
performance at the cost of more memory consumption, because that
should reduce hash table collisions. Try small negative numbers
for the opposite tradeoff.
MAXERRORS
Maximum number of soft errors before an exit is triggered,
default 0, which means that the first error will exit the
script. Note that with the --suppress-handler-errors option,
this limit is not enforced.
MAXSKIPPED
Maximum number of skipped probes before an exit is triggered,
default 100. Running systemtap with -t (timing) mode gives more
details about skipped probes. With the default
-DINTERRUPTIBLE=1 setting, probes skipped due to reentrancy are
not accumulated against this limit. Note that with the
--suppress-handler-errors option, this limit is not enforced.
MINSTACKSPACE
Minimum number of free kernel stack bytes required in order to
run a probe handler, default 1024. This number should be large
enough for the probe handler's own needs, plus a safety margin.
MAXUPROBES
Maximum number of concurrently armed user-space probes
(uprobes), default somewhat larger than the number of user-space
probe points named in the script. This pool needs to be
potentially large because individual uprobe objects (about 64
bytes each) are allocated for each process for each matching
script-level probe.
STP_MAXMEMORY
Maximum amount of memory (in kilobytes) that the systemtap
module should use, default unlimited. The memory size includes
the size of the module itself, plus any additional allocations.
This only tracks direct allocations by the systemtap runtime.
This does not track indirect allocations (as done by
kprobes/uprobes/etc. internals).
STP_OVERLOAD_THRESHOLD, STP_OVERLOAD_INTERVAL
Maximum number of machine cycles spent in probes on any cpu per
given interval, before an overload condition is declared and the
script shut down. The defaults are 500 million and 1 billion,
so as to limit stap script cpu consumption at around 50%.
STP_PROCFS_BUFSIZE
Size of procfs probe read buffers (in bytes). Defaults to
MAXSTRINGLEN. This value can be overridden on a per-procfs file
basis using the procfs read probe .maxsize(MAXSIZE) parameter.
With scripts that contain probes on any interrupt path, it is possible
that those interrupts may occur in the middle of another probe handler.
The probe in the interrupt handler would be skipped in this case to
avoid reentrance. To work around this issue, execute stap with the
option -DINTERRUPTIBLE=0 to mask interrupts throughout the probe
handler. This does add some extra overhead to the probes, but it may
prevent reentrance for common problem cases. However, probes in NMI
handlers and in the callpath of the stap runtime may still be skipped
due to reentrance.
In case something goes wrong with stap or staprun after a probe has
already started running, one may safely kill both user processes, and
remove the active probe kernel module with rmmod. Any pending trace
messages may be lost.
UNPRIVILEGED USERS
Systemtap exposes kernel internal data structures and potentially
private user information. Because of this, use of systemtap's full
capabilities are restricted to root and to users who are members of the
groups stapdev and stapusr.
However, a restricted set of systemtap's features can be made available
to trusted, unprivileged users. These users are members of the group
stapusr only, or members of the groups stapusr and stapsys. These
users can load systemtap modules which have been compiled and certified
by a trusted systemtap compile-server. See the descriptions of the
options --privilege and --use-server. See README.unprivileged in the
systemtap source code for information about setting up a trusted
compile server.
The restrictions enforced when --privilege=stapsys is specified are
designed to prevent unprivileged users from:
* harming the system maliciously.
The restrictions enforced when --privilege=stapusr is specified are
designed to prevent unprivileged users from:
* harming the system maliciously.
* gaining access to information which would not normally be
available to an unprivileged user.
* disrupting the performance of processes owned by other users
of the system. Some overhead to the system in general is
unavoidable since the unprivileged user's probes will be
triggered at the appropriate times. What we would like to
avoid is targeted interruption of another user's processes
which would not normally be possible by an unprivileged
user.
PROBE RESTRICTIONS
A member of the groups stapusr and stapsys may use all probe points.
A member of only the group stapusr may use only the following probes:
* begin, begin(n)
* end, end(n)
* error(n)
* never
* process.*, where the target process is owned by the user.
* timer.{jiffies,s,sec,ms,msec,us,usec,ns,nsec}(n)*
* timer.hz(n)
SCRIPT LANGUAGE RESTRICTIONS
The following scripting language features are unavailable to all
unprivileged users:
* any feature enabled by the Guru Mode (-g) option.
* embedded C code.
RUNTIME RESTRICTIONS
The following runtime restrictions are placed upon all unprivileged
users:
* Only the default runtime code (see -R) may be used.
Additional restrictions are placed on members of only the group
stapusr:
* Probing of processes owned by other users is not permitted.
* Access of kernel memory (read and write) is not permitted.
COMMAND LINE OPTION RESTRICTIONS
Some command line options provide access to features which must not be
available to all unprivileged users:
* -g may not be specified.
* The following options may not be used by the compile-server
client:
-a, -B, -D, -I, -r, -R
ENVIRONMENT RESTRICTIONS
The following environment variables must not be set for all
unprivileged users:
SYSTEMTAP_RUNTIME
SYSTEMTAP_TAPSET
SYSTEMTAP_DEBUGINFO_PATH
TAPSET RESTRICTIONS
In general, tapset functions are only available for members of the
group stapusr when they do not gather information that an ordinary
program running with that user's privileges would be denied access to.
There are two categories of unprivileged tapset functions. The first
category consists of utility functions that are unconditionally
available to all users; these include such things as:
cpu:long ()
exit ()
str_replace:string (prnt_str:string, srch_str:string, rplc_str:string)
The second category consists of so-called myproc-unprivileged functions
that can only gather information within their own processes. Scripts
that wish to use these functions must test the result of the tapset
function is_myproc and only call these functions if the result is 1.
The script will exit immediately if any of these functions are called
by an unprivileged user within a probe within a process which is not
owned by that user. Examples of myproc-unprivileged functions include:
print_usyms (stk:string)
user_int:long (addr:long)
usymname:string (addr:long)
A compile error is triggered when any function not in either of the
above categories is used by members of only the group stapusr.
No other built-in tapset functions may be used by members of only the
group stapusr.
ALTERNATE RUNTIMES
As described above, systemtap's default runtime mode involves building
and loading kernel modules, with various security tradeoffs presented.
Systemtap now includes a new prototype backend, selected with
--runtime=dyninst, which uses Dyninst to instrument a user's own
processes at runtime. This backend does not use kernel modules, and
does not require root privileges, but is restricted with respect to the
kinds of probes and other constructs that a script may use.
The dyninst runtime operates in target-attach mode, so it does require
a -c COMMAND or -x PID process. For example:
stap --runtime=dyninst -c 'stap -V' \
-e 'probe process.function("main")
{ println("hi from dyninst!") }'
It may be necessary to disable a conflicting selinux check with
# setsebool allow_execstack 1
EXIT STATUS
The systemtap translator generally returns with a success code of 0 if the requested script was processed and executed successfully through the requested pass. Otherwise, errors may be printed to stderr and a failure code is returned. Use -v or -vp N to increase (global or per- pass) verbosity to identify the source of the trouble. In listings mode (-l and -L), error messages are normally suppressed. A success code of 0 is returned if at least one matching probe was found. A script executing in pass 5 that is interrupted with ^C / SIGINT is considered to be successful.
DEPRECATION
Over time, some features of the script language and the tapset library may undergo incompatible changes, so that a script written against an old version of systemtap may no longer run. In these cases, it may help to run systemtap with the --compatible VERSION flag, specifying the last known working version. Running systemtap with the --check-version flag will output a warning if any possible incompatible elements have been parsed. Deprecation historical details may be found in the NEWS file. The purpose of deprecation facility is to improve the experience of scripts written for newer versions of systemtap (by adding better alternatives and removing conflicting or messy older alternatives), while at the same time permitting scripts written for older versions of systemtap to continue running. Deprecation is thus intended a service to users (and an inconvenience to systemtap's developers), rather than the other way around. Please note that underscore-prefixed identifiers in the tapset sometimes undergo such changes that are difficult to preserve compatibility for, even with the deprecation mechanisms. Avoid relying on these in your scripts; instead propose them for promotion to non- underscored status.
FILES
Important files and their corresponding paths can be located in the
stappaths (7) manual page.
SEE ALSO
stapprobes(3stap), function::*(3stap), probe::*(3stap), tapset::*(3stap), stappaths(7), staprun(8), stapdyn(8), systemtap(8), stapvars(3stap), stapex(3stap), stap-server(8), stap-prep(1), stapref(1), awk(1), gdb(1)
BUGS
Use the Bugzilla link of the project web page or our mailing list.
http://sourceware.org/systemtap/, <systemtap@sourceware.org>.
error::reporting(7stap),
https://sourceware.org/systemtap/wiki/HowToReportBugs
STAP(1)
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