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NAME | DESCRIPTION | SYNTAX | DWARF DEBUGINFO | ON-THE-FLY ARMING | PROBE POINT FAMILIES | EXAMPLES | SEE ALSO | COLOPHON |
STAPPROBES(3stap) STAPPROBES(3stap)
stapprobes - systemtap probe points
The following sections enumerate the variety of probe points
supported by the systemtap translator, and some of the additional
aliases defined by standard tapset scripts. Many are individually
documented in the 3stap manual section, with the probe:: prefix.
probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
A probe declaration may list multiple comma-separated probe points in
order to attach a handler to all of the named events. Normally, the
handler statements are run whenever any of events occur. Depending
on the type of probe point, the handler statements may refer to con‐
text variables (denoted with a dollar-sign prefix like $foo) to read
or write state. This may include function parameters for function
probes, or local variables for statement probes.
The syntax of a single probe point is a general dotted-symbol se‐
quence. This allows a breakdown of the event namespace into parts,
somewhat like the Domain Name System does on the Internet. Each com‐
ponent identifier may be parametrized by a string or number literal,
with a syntax like a function call. A component may include a "*"
character, to expand to a set of matching probe points. It may also
include "**" to match multiple sequential components at once. Probe
aliases likewise expand to other probe points.
Probe aliases can be given on their own, or with a suffix. The suffix
attaches to the underlying probe point that the alias is expanded to.
For example,
syscall.read.return.maxactive(10)
expands to
kernel.function("sys_read").return.maxactive(10)
with the component maxactive(10) being recognized as a suffix.
Normally, each and every probe point resulting from wildcard- and
alias-expansion must be resolved to some low-level system instrumen‐
tation facility (e.g., a kprobe address, marker, or a timer configu‐
ration), otherwise the elaboration phase will fail.
However, a probe point may be followed by a "?" character, to indi‐
cate that it is optional, and that no error should result if it fails
to resolve. Optionalness passes down through all levels of
alias/wildcard expansion. Alternately, a probe point may be followed
by a "!" character, to indicate that it is both optional and suffi‐
cient. (Think vaguely of the Prolog cut operator.) If it does re‐
solve, then no further probe points in the same comma-separated list
will be resolved. Therefore, the "!" sufficiency mark only makes
sense in a list of probe point alternatives.
Additionally, a probe point may be followed by a "if (expr)" state‐
ment, in order to enable/disable the probe point on-the-fly. With the
"if" statement, if the "expr" is false when the probe point is hit,
the whole probe body including alias's body is skipped. The condition
is stacked up through all levels of alias/wildcard expansion. So the
final condition becomes the logical-and of conditions of all expanded
alias/wildcard. The expressions are necessarily restricted to global
variables.
These are all syntactically valid probe points. (They are generally
semantically invalid, depending on the contents of the tapsets, and
the versions of kernel/user software installed.)
kernel.function("foo").return
process("/bin/vi").statement(0x2222)
end
syscall.*
syscall.*.return.maxactive(10)
syscall.{open,close}
sys**open
kernel.function("no_such_function") ?
module("awol").function("no_such_function") !
signal.*? if (switch)
kprobe.function("foo")
Probes may be broadly classified into "synchronous" and "asynchro‐
nous". A "synchronous" event is deemed to occur when any processor
executes an instruction matched by the specification. This gives
these probes a reference point (instruction address) from which more
contextual data may be available. Other families of probe points re‐
fer to "asynchronous" events such as timers/counters rolling over,
where there is no fixed reference point that is related. Each probe
point specification may match multiple locations (for example, using
wildcards or aliases), and all them are then probed. A probe decla‐
ration may also contain several comma-separated specifications, all
of which are probed.
Brace expansion is a mechanism which allows a list of probe points to
be generated. It is very similar to shell expansion. A component may
be surrounded by a pair of curly braces to indicate that the comma-
separated sequence of one or more subcomponents will each constitute
a new probe point. The braces may be arbitrarily nested. The ordering
of expanded results is based on product order.
The question mark (?), exclamation mark (!) indicators and probe
point conditions may not be placed in any expansions that are before
the last component.
The following is an example of brace expansion.
syscall.{write,read}
# Expands to
syscall.write, syscall.read
{kernel,module("nfs")}.function("nfs*")!
# Expands to
kernel.function("nfs*")!, module("nfs").function("nfs*")!
Resolving some probe points requires DWARF debuginfo or "debug
symbols" for the specific program being instrumented. For some
others, DWARF is automatically synthesized on the fly from source
code header files. For others, it is not needed at all. Since a
systemtap script may use any mixture of probe points together, the
union of their DWARF requirements has to be met on the computer where
script compilation occurs. (See the --use-server option and the
stap-server(8) man page for information about the remote compilation
facility, which allows these requirements to be met on a different
machine.)
The following point lists many of the available probe point families,
to classify them with respect to their need for DWARF debuginfo for
the specific program for that probe point.
DWARF NON-DWARF SYMBOL-TABLE
kernel.function, .statement kernel.mark kernel.function*
module.function, .statement process.mark, process.plt module.function*
process.function, .statement begin, end, error, never process.function*
process.mark* timer
.function.callee perf
python2, python3 procfs
kernel.statement.absolute
AUTO-GENERATED-DWARF kernel.data
kprobe.function
kernel.trace process.statement.absolute
process.begin, .end
netfilter
java
The probe types marked with * asterisks mark fallbacks, where
systemtap can sometimes infer subset or substitute information. In
general, the more symbolic / debugging information available, the
higher quality probing will be available.
The following types of probe points may be armed/disarmed on-the-fly
to save overheads during uninteresting times. Arming conditions may
also be added to other types of probes, but will be treated as a
wrapping conditional and won't benefit from overhead savings.
DISARMABLE exceptions
kernel.function, kernel.statement
module.function, module.statement
process.*.function, process.*.statement
process.*.plt, process.*.mark
timer. timer.profile
java
BEGIN/END/ERROR
The probe points begin and end are defined by the translator to refer
to the time of session startup and shutdown. All "begin" probe
handlers are run, in some sequence, during the startup of the
session. All global variables will have been initialized prior to
this point. All "end" probes are run, in some sequence, during the
normal shutdown of a session, such as in the aftermath of an exit ()
function call, or an interruption from the user. In the case of an
error-triggered shutdown, "end" probes are not run. There are no
target variables available in either context.
If the order of execution among "begin" or "end" probes is
significant, then an optional sequence number may be provided:
begin(N)
end(N)
The number N may be positive or negative. The probe handlers are run
in increasing order, and the order between handlers with the same se‐
quence number is unspecified. When "begin" or "end" are given with‐
out a sequence, they are effectively sequence zero.
The error probe point is similar to the end probe, except that each
such probe handler run when the session ends after errors have oc‐
curred. In such cases, "end" probes are skipped, but each "error"
probe is still attempted. This kind of probe can be used to clean up
or emit a "final gasp". It may also be numerically parametrized to
set a sequence.
NEVER
The probe point never is specially defined by the translator to mean
"never". Its probe handler is never run, though its statements are
analyzed for symbol / type correctness as usual. This probe point
may be useful in conjunction with optional probes.
SYSCALL and ND_SYSCALL
The syscall.* and nd_syscall.* aliases define several hundred
probes, too many to detail here. They are of the general form:
syscall.NAME
nd_syscall.NAME
syscall.NAME.return
nd_syscall.NAME.return
Generally, a pair of probes are defined for each normal system call
as listed in the syscalls(2) manual page, one for entry and one for
return. Those system calls that never return do not have a corre‐
sponding .return probe. The nd_* family of probes are about the
same, except it uses non-DWARF based searching mechanisms, which may
result in a lower quality of symbolic context data (parameters), and
may miss some system calls. You may want to try them first, in case
kernel debugging information is not immediately available.
Each probe alias provides a variety of variables. Looking at the
tapset source code is the most reliable way. Generally, each vari‐
able listed in the standard manual page is made available as a
script-level variable, so syscall.open exposes filename, flags, and
mode. In addition, a standard suite of variables is available at
most aliases:
argstr A pretty-printed form of the entire argument list, without
parentheses.
name The name of the system call.
retstr For return probes, a pretty-printed form of the system-call
result.
As usual for probe aliases, these variables are all initialized once
from the underlying $context variables, so that later changes to
$context variables are not automatically reflected. Not all probe
aliases obey all of these general guidelines. Please report any
bothersome ones you encounter as a bug. Note that on some ker‐
nel/userspace architecture combinations (e.g., 32-bit userspace on
64-bit kernel), the underlying $context variables may need explicit
sign extension / masking. When this is an issue, consider using the
tapset-provided variables instead of raw $context variables.
If debuginfo availability is a problem, you may try using the non-
DWARF syscall probe aliases instead. Use the nd_syscall. prefix in‐
stead of syscall. The same context variables are available, as far
as possible.
TIMERS
There are two main types of timer probes: "jiffies" timer probes and
time interval timer probes.
Intervals defined by the standard kernel "jiffies" timer may be used
to trigger probe handlers asynchronously. Two probe point variants
are supported by the translator:
timer.jiffies(N)
timer.jiffies(N).randomize(M)
The probe handler is run every N jiffies (a kernel-defined unit of
time, typically between 1 and 60 ms). If the "randomize" component
is given, a linearly distributed random value in the range [-M..+M]
is added to N every time the handler is run. N is restricted to a
reasonable range (1 to around a million), and M is restricted to be
smaller than N. There are no target variables provided in either
context. It is possible for such probes to be run concurrently on a
multi-processor computer.
Alternatively, intervals may be specified in units of time. There
are two probe point variants similar to the jiffies timer:
timer.ms(N)
timer.ms(N).randomize(M)
Here, N and M are specified in milliseconds, but the full options for
units are seconds (s/sec), milliseconds (ms/msec), microseconds
(us/usec), nanoseconds (ns/nsec), and hertz (hz). Randomization is
not supported for hertz timers.
The actual resolution of the timers depends on the target kernel.
For kernels prior to 2.6.17, timers are limited to jiffies resolu‐
tion, so intervals are rounded up to the nearest jiffies interval.
After 2.6.17, the implementation uses hrtimers for tighter precision,
though the actual resolution will be arch-dependent. In either case,
if the "randomize" component is given, then the random value will be
added to the interval before any rounding occurs.
Profiling timers are also available to provide probes that execute on
all CPUs at the rate of the system tick (CONFIG_HZ) or at a given
frequency (hz). On some kernels, this is a one-concurrent-user-only
or disabled facility, resulting in error -16 (EBUSY) during probe
registration.
timer.profile.tick
timer.profile.freq.hz(N)
Full context information of the interrupted process is available,
making this probe suitable for a time-based sampling profiler.
It is recommended to use the tapset probe timer.profile rather than
timer.profile.tick. This probe point behaves identically to
timer.profile.tick when the underlying functionality is available,
and falls back to using perf.sw.cpu_clock on some recent kernels
which lack the corresponding profile timer facility.
Profiling timers with specified frequencies are only accurate up to
around 100 hz. You may need to provide a larger value to achieve the
desired rate.
Note that if a timer probe is set to fire at a very high rate and if
the probe body is complex, succeeding timer probes can get skipped,
since the time for them to run has already passed. Normally systemtap
reports missed probes, but it will not report these skipped probes.
DWARF
This family of probe points uses symbolic debugging information for
the target kernel/module/program, as may be found in unstripped exe‐
cutables, or the separate debuginfo packages. They allow placement
of probes logically into the execution path of the target program, by
specifying a set of points in the source or object code. When a
matching statement executes on any processor, the probe handler is
run in that context.
Probe points in the DWARF family can be identified by the target ker‐
nel module (or user process), source file, line number, function
name, or some combination of these.
Here is a list of DWARF probe points currently supported:
kernel.function(PATTERN)
kernel.function(PATTERN).call
kernel.function(PATTERN).callee(PATTERN)
kernel.function(PATTERN).callee(PATTERN).return
kernel.function(PATTERN).callee(PATTERN).call
kernel.function(PATTERN).callees(DEPTH)
kernel.function(PATTERN).return
kernel.function(PATTERN).inline
kernel.function(PATTERN).label(LPATTERN)
module(MPATTERN).function(PATTERN)
module(MPATTERN).function(PATTERN).call
module(MPATTERN).function(PATTERN).callee(PATTERN)
module(MPATTERN).function(PATTERN).callee(PATTERN).return
module(MPATTERN).function(PATTERN).callee(PATTERN).call
module(MPATTERN).function(PATTERN).callees(DEPTH)
module(MPATTERN).function(PATTERN).return
module(MPATTERN).function(PATTERN).inline
module(MPATTERN).function(PATTERN).label(LPATTERN)
kernel.statement(PATTERN)
kernel.statement(PATTERN).nearest
kernel.statement(ADDRESS).absolute
module(MPATTERN).statement(PATTERN)
process("PATH").function("NAME")
process("PATH").statement("*@FILE.c:123")
process("PATH").library("PATH").function("NAME")
process("PATH").library("PATH").statement("*@FILE.c:123")
process("PATH").library("PATH").statement("*@FILE.c:123").nearest
process("PATH").function("*").return
process("PATH").function("myfun").label("foo")
process("PATH").function("foo").callee("bar")
process("PATH").function("foo").callee("bar").return
process("PATH").function("foo").callee("bar").call
process("PATH").function("foo").callees(DEPTH)
process(PID).function("NAME")
process(PID).function("myfun").label("foo")
process(PID).plt("NAME")
process(PID).plt("NAME").return
process(PID).statement("*@FILE.c:123")
process(PID).statement("*@FILE.c:123").nearest
process(PID).statement(ADDRESS).absolute
(See the USER-SPACE section below for more information on the process
probes.)
The list above includes multiple variants and modifiers which provide
additional functionality or filters. They are:
.function
Places a probe near the beginning of the named func‐
tion, so that parameters are available as context vari‐
ables.
.return
Places a probe at the moment after the return from the
named function, so the return value is available as the
"$return" context variable.
.inline
Filters the results to include only instances of in‐
lined functions. Note that inlined functions do not
have an identifiable return point, so .return is not
supported on .inline probes.
.call Filters the results to include only non-inlined func‐
tions (the opposite set of .inline)
.exported
Filters the results to include only exported functions.
.statement
Places a probe at the exact spot, exposing those local
variables that are visible there.
.statement.nearest
Places a probe at the nearest available line number for
each line number given in the statement.
.callee
Places a probe on the callee function given in the
.callee modifier, where the callee must be a function
called by the target function given in .function. The
advantage of doing this over directly probing the
callee function is that this probe point is run only
when the callee is called from the target function (add
the -DSTAP_CALLEE_MATCHALL directive to override this
when calling stap(1)).
Note that only callees that can be statically deter‐
mined are available. For example, calls through func‐
tion pointers are not available. Additionally, calls
to functions located in other objects (e.g. libraries)
are not available (instead use another probe point).
This feature will only work for code compiled with GCC
4.7+.
.callees
Shortcut for .callee("*"), which places a probe on all
callees of the function.
.callees(DEPTH)
Recursively places probes on callees. For example,
.callees(2) will probe both callees of the target func‐
tion, as well as callees of those callees. And
.callees(3) goes one level deeper, etc... A callee
probe at depth N is only triggered when the N callers
in the callstack match those that were statically de‐
termined during analysis (this also may be overridden
using -DSTAP_CALLEE_MATCHALL).
In the above list of probe points, MPATTERN stands for a string lit‐
eral that aims to identify the loaded kernel module of interest. For
in-tree kernel modules, the name suffices (e.g. "btrfs"). The name
may also include the "*", "[]", and "?" wildcards to match multiple
in-tree modules. Out-of-tree modules are also supported by specifying
the full path to the ko file. Wildcards are not supported. The file
must follow the convention of being named <module_name>.ko (charac‐
ters ',' and '-' are replaced by '_').
LPATTERN stands for a source program label. It may also contain "*",
"[]", and "?" wildcards. PATTERN stands for a string literal that
aims to identify a point in the program. It is made up of three
parts:
· The first part is the name of a function, as would appear in the
nm program's output. This part may use the "*" and "?" wildcard‐
ing operators to match multiple names.
· The second part is optional and begins with the "@" character.
It is followed by the path to the source file containing the
function, which may include a wildcard pattern, such as mm/slab*.
If it does not match as is, an implicit "*/" is optionally added
before the pattern, so that a script need only name the last few
components of a possibly long source directory path.
· Finally, the third part is optional if the file name part was
given, and identifies the line number in the source file preceded
by a ":" or a "+". The line number is assumed to be an absolute
line number if preceded by a ":", or relative to the declaration
line of the function if preceded by a "+". All the lines in the
function can be matched with ":*". A range of lines x through y
can be matched with ":x-y". Ranges and specific lines can be
mixed using commas, e.g. ":x,y-z".
As an alternative, PATTERN may be a numeric constant, indicating an
address. Such an address may be found from symbol tables of the ap‐
propriate kernel / module object file. It is verified against known
statement code boundaries, and will be relocated for use at run time.
In guru mode only, absolute kernel-space addresses may be specified
with the ".absolute" suffix. Such an address is considered already
relocated, as if it came from /proc/kallsyms, so it cannot be checked
against statement/instruction boundaries.
CONTEXT VARIABLES
Many of the source-level context variables, such as function parame‐
ters, locals, globals visible in the compilation unit, may be visible
to probe handlers. They may refer to these variables by prefixing
their name with "$" within the scripts. In addition, a special syn‐
tax allows limited traversal of structures, pointers, and arrays.
More syntax allows pretty-printing of individual variables or their
groups. See also @cast. Note that variables may be inaccessible due
to them being paged out, or for a few other reasons. See also man
error::fault(7stap).
$var refers to an in-scope variable "var". If it's an integer-like
type, it will be cast to a 64-bit int for systemtap script
use. String-like pointers (char *) may be copied to systemtap
string values using the kernel_string or user_string func‐
tions.
@var("varname")
an alternative syntax for $varname
@var("varname@src/file.c")
refers to the global (either file local or external) variable
varname defined when the file src/file.c was compiled. The CU
in which the variable is resolved is the first CU in the mod‐
ule of the probe point which matches the given file name at
the end and has the shortest file name path (e.g. given
@var("foo@bar/baz.c") and CUs with file name paths
src/sub/module/bar/baz.c and src/bar/baz.c the second CU will
be chosen to resolve the (file) global variable foo
$var->field traversal via a structure's or a pointer's field. This
generalized indirection operator may be repeated to follow
more levels. Note that the . operator is not used for plain
structure members, only -> for both purposes. (This is be‐
cause "." is reserved for string concatenation.) Also note
that for direct dereferencing of $var pointer {kernel,us‐
er}_{char,int,...}($var) should be used. (Refer to stap‐
funcs(5) for more details.)
$return
is available in return probes only for functions that are de‐
clared with a return value, which can be determined using @de‐
fined($return).
$var[N]
indexes into an array. The index given with a literal number
or even an arbitrary numeric expression.
A number of operators exist for such basic context variable expres‐
sions:
$$vars expands to a character string that is equivalent to
sprintf("parm1=%x ... parmN=%x var1=%x ... varN=%x",
parm1, ..., parmN, var1, ..., varN)
for each variable in scope at the probe point. Some values
may be printed as =? if their run-time location cannot be
found.
$$locals
expands to a subset of $$vars for only local variables.
$$parms
expands to a subset of $$vars for only function parameters.
$$return
is available in return probes only. It expands to a string
that is equivalent to sprintf("return=%x", $return) if the
probed function has a return value, or else an empty string.
& $EXPR
expands to the address of the given context variable expres‐
sion, if it is addressable.
@defined($EXPR)
expands to 1 or 0 iff the given context variable expression is
resolvable, for use in conditionals such as
@defined($foo->bar) ? $foo->bar : 0
$EXPR$ expands to a string with all of $EXPR's members, equivalent to
sprintf("{.a=%i, .b=%u, .c={...}, .d=[...]}",
$EXPR->a, $EXPR->b)
$EXPR$$
expands to a string with all of $var's members and submembers,
equivalent to
sprintf("{.a=%i, .b=%u, .c={.x=%p, .y=%c}, .d=[%i, ...]}",
$EXPR->a, $EXPR->b, $EXPR->c->x, $EXPR->c->y, $EXPR->d[0])
MORE ON RETURN PROBES
For the kernel ".return" probes, only a certain fixed number of re‐
turns may be outstanding. The default is a relatively small number,
on the order of a few times the number of physical CPUs. If many
different threads concurrently call the same blocking function, such
as futex(2) or read(2), this limit could be exceeded, and skipped
"kretprobes" would be reported by "stap -t". To work around this,
specify a
probe FOO.return.maxactive(NNN)
suffix, with a large enough NNN to cover all expected concurrently
blocked threads. Alternately, use the
stap -DKRETACTIVE=NNNN
stap command line macro setting to override the default for all ".re‐
turn" probes.
For ".return" probes, context variables other than the "$return" may
be accessible, as a convenience for a script programmer wishing to
access function parameters. These values are snapshots taken at the
time of function entry. (Local variables within the function are not
generally accessible, since those variables did not exist in allocat‐
ed/initialized form at the snapshot moment.) These entry-snapshot
variables should be accessed via @entry($var).
In addition, arbitrary entry-time expressions can also be saved for
".return" probes using the @entry(expr) operator. For example, one
can compute the elapsed time of a function:
probe kernel.function("do_filp_open").return {
println( get_timeofday_us() - @entry(get_timeofday_us()) )
}
The following table summarizes how values related to a function pa‐
rameter context variable, a pointer named addr, may be accessed from
a .return probe.
at-entry value past-exit value
$addr not available
$addr->x->y @cast(@entry($addr),"struct zz")->x->y
$addr[0] {kernel,user}_{char,int,...}(& $addr[0])
DWARFLESS
In absence of debugging information, entry & exit points of kernel &
module functions can be probed using the "kprobe" family of probes.
However, these do not permit looking up the arguments / local vari‐
ables of the function. Following constructs are supported :
kprobe.function(FUNCTION)
kprobe.function(FUNCTION).call
kprobe.function(FUNCTION).return
kprobe.module(NAME).function(FUNCTION)
kprobe.module(NAME).function(FUNCTION).call
kprobe.module(NAME).function(FUNCTION).return
kprobe.statement(ADDRESS).absolute
Probes of type function are recommended for kernel functions, whereas
probes of type module are recommended for probing functions of the
specified module. In case the absolute address of a kernel or module
function is known, statement probes can be utilized.
Note that FUNCTION and MODULE names must not contain wildcards, or
the probe will not be registered. Also, statement probes must be run
under guru-mode only.
USER-SPACE
Support for user-space probing is available for kernels that are con‐
figured with the utrace extensions, or have the uprobes facility in
linux 3.5. (Various kernel build configuration options need to be
enabled; systemtap will advise if these are missing.)
There are several forms. First, a non-symbolic probe point:
process(PID).statement(ADDRESS).absolute
is analogous to kernel.statement(ADDRESS).absolute in that both use
raw (unverified) virtual addresses and provide no $variables. The
target PID parameter must identify a running process, and ADDRESS
should identify a valid instruction address. All threads of that
process will be probed.
Second, non-symbolic user-kernel interface events handled by utrace
may be probed:
process(PID).begin
process("FULLPATH").begin
process.begin
process(PID).thread.begin
process("FULLPATH").thread.begin
process.thread.begin
process(PID).end
process("FULLPATH").end
process.end
process(PID).thread.end
process("FULLPATH").thread.end
process.thread.end
process(PID).syscall
process("FULLPATH").syscall
process.syscall
process(PID).syscall.return
process("FULLPATH").syscall.return
process.syscall.return
process(PID).insn
process("FULLPATH").insn
process(PID).insn.block
process("FULLPATH").insn.block
A process.begin probe gets called when new process described by PID
or FULLPATH gets created. In addition, it is called once from the
context of each preexisting process, at systemtap script startup.
This is useful to track live processes. A process.thread.begin probe
gets called when a new thread described by PID or FULLPATH gets cre‐
ated. A process.end probe gets called when process described by PID
or FULLPATH dies. A process.thread.end probe gets called when a
thread described by PID or FULLPATH dies. A process.syscall probe
gets called when a thread described by PID or FULLPATH makes a system
call. The system call number is available in the $syscall context
variable, and the first 6 arguments of the system call are available
in the $argN (ex. $arg1, $arg2, ...) context variable. A
process.syscall.return probe gets called when a thread described by
PID or FULLPATH returns from a system call. The system call number
is available in the $syscall context variable, and the return value
of the system call is available in the $return context variable. A
process.insn probe gets called for every single-stepped instruction
of the process described by PID or FULLPATH. A process.insn.block
probe gets called for every block-stepped instruction of the process
described by PID or FULLPATH.
If a process probe is specified without a PID or FULLPATH, all user
threads will be probed. However, if systemtap was invoked with the
-c or -x options, then process probes are restricted to the process
hierarchy associated with the target process. If a process probe is
unspecified (i.e. without a PID or FULLPATH), but with the -c option,
the PATH of the -c cmd will be heuristically filled into the process
PATH. In that case, only command parameters are allowed in the -c
command (i.e. no command substitution allowed and no occurrences of
any of these characters: '|&;<>(){}').
Third, symbolic static instrumentation compiled into programs and
shared libraries may be probed:
process("PATH").mark("LABEL")
process("PATH").provider("PROVIDER").mark("LABEL")
process(PID).mark("LABEL")
process(PID).provider("PROVIDER").mark("LABEL")
A .mark probe gets called via a static probe which is defined in the
application by STAP_PROBE1(PROVIDER,LABEL,arg1), which are macros de‐
fined in sys/sdt.h. The PROVIDER is an arbitrary application identi‐
fier, LABEL is the marker site identifier, and arg1 is the integer-
typed argument. STAP_PROBE1 is used for probes with 1 argument,
STAP_PROBE2 is used for probes with 2 arguments, and so on. The ar‐
guments of the probe are available in the context variables $arg1,
$arg2, ... An alternative to using the STAP_PROBE macros is to use
the dtrace script to create custom macros. Additionally, the vari‐
ables $$name and $$provider are available as parts of the probe point
name. The sys/sdt.h macro names DTRACE_PROBE* are available as
aliases for STAP_PROBE*.
Finally, full symbolic source-level probes in user-space programs and
shared libraries are supported. These are exactly analogous to the
symbolic DWARF-based kernel/module probes described above. They ex‐
pose the same sorts of context $variables for function parameters,
local variables, and so on.
process("PATH").function("NAME")
process("PATH").statement("*@FILE.c:123")
process("PATH").plt("NAME")
process("PATH").library("PATH").plt("NAME")
process("PATH").library("PATH").function("NAME")
process("PATH").library("PATH").statement("*@FILE.c:123")
process("PATH").function("*").return
process("PATH").function("myfun").label("foo")
process("PATH").function("foo").callee("bar")
process("PATH").plt("NAME").return
process(PID).function("NAME")
process(PID).statement("*@FILE.c:123")
process(PID).plt("NAME")
Note that for all process probes, PATH names refer to executables
that are searched the same way shells do: relative to the working di‐
rectory if they contain a "/" character, otherwise in $PATH. If PATH
names refer to scripts, the actual interpreters (specified in the
script in the first line after the #! characters) are probed.
Tapset process probes placed in the special directory $pre‐
fix/share/systemtap/tapset/PATH/ with relative paths will have their
process parameter prefixed with the location of the tapset. For exam‐
ple,
process("foo").function("NAME")
expands to
process("/usr/bin/foo").function("NAME")
when placed in $prefix/share/systemtap/tapset/PATH/usr/bin/
If PATH is a process component parameter referring to shared li‐
braries then all processes that map it at runtime would be selected
for probing. If PATH is a library component parameter referring to
shared libraries then the process specified by the process component
would be selected. Note that the PATH pattern in a library component
will always apply to libraries statically determined to be in use by
the process. However, you may also specify the full path to any li‐
brary file even if not statically needed by the process.
A .plt probe will probe functions in the program linkage table corre‐
sponding to the rest of the probe point. .plt can be specified as a
shorthand for .plt("*"). The symbol name is available as a $$name
context variable; function arguments are not available, since PLTs
are processed without debuginfo. A .plt.return probe places a probe
at the moment after the return from the named function.
If the PATH string contains wildcards as in the MPATTERN case, then
standard globbing is performed to find all matching paths. In this
case, the $PATH environment variable is not used.
If systemtap was invoked with the -c or -x options, then process
probes are restricted to the process hierarchy associated with the
target process.
JAVA
Support for probing Java methods is available using Byteman as a
backend. Byteman is an instrumentation tool from the JBoss project
which systemtap can use to monitor invocations for a specific method
or line in a Java program.
Systemtap does so by generating a Byteman script listing the probes
to instrument and then invoking the Byteman bminstall utility.
This Java instrumentation support is currently a prototype feature
with major limitations. Moreover, Java probing currently does not
work across users; the stap script must run (with appropriate permis‐
sions) under the same user that the Java process being probed. (Thus
a stap script under root currently cannot probe Java methods in a
non-root-user Java process.)
The first probe type refers to Java processes by the name of the Java
process:
java("PNAME").class("CLASSNAME").method("PATTERN")
java("PNAME").class("CLASSNAME").method("PATTERN").return
The PNAME argument must be a pre-existing jvm pid, and be identifi‐
able via a jps listing.
The PATTERN parameter specifies the signature of the Java method to
probe. The signature must consist of the exact name of the method,
followed by a bracketed list of the types of the arguments, for in‐
stance "myMethod(int,double,Foo)". Wildcards are not supported.
The probe can be set to trigger at a specific line within the method
by appending a line number with colon, just as in other types of
probes: "myMethod(int,double,Foo):245".
The CLASSNAME parameter identifies the Java class the method belongs
to, either with or without the package qualification. By default, the
probe only triggers on descendants of the class that do not override
the method definition of the original class. However, CLASSNAME can
take an optional caret prefix, as in ^org.my.MyClass, which specifies
that the probe should also trigger on all descendants of MyClass that
override the original method. For instance, every method with signa‐
ture foo(int) in program org.my.MyApp can be probed at once using
java("org.my.MyApp").class("^java.lang.Object").method("foo(int)")
The second probe type works analogously, but refers to Java processes
by PID:
java(PID).class("CLASSNAME").method("PATTERN")
java(PID).class("CLASSNAME").method("PATTERN").return
(PIDs for an already running process can be obtained using the jps(1)
utility.)
Context variables defined within java probes include $arg1 through
$arg10 (for up to the first 10 arguments of a method), represented as
character-pointers for the toString() form of each actual argument.
The arg1 through arg10 script variables provide access to these as
ordinary strings, fetched via user_string_warn().
Prior to systemtap version 3.1, $arg1 through $arg10 could contain
either integers or character pointers, depending on the types of the
objects being passed to each particular java method. This previous
behaviour may be invoked with the stap --compatible=3.0 flag.
PROCFS
These probe points allow procfs "files" in /proc/systemtap/MODNAME to
be created, read and written using a permission that may be modified
using the proper umask value. Default permissions are 0400 for read
probes, and 0200 for write probes. If both a read and write probe are
being used on the same file, a default permission of 0600 will be
used. Using procfs.umask(0040).read would result in a 0404 permis‐
sion set for the file. (MODNAME is the name of the systemtap mod‐
ule). The proc filesystem is a pseudo-filesystem which is used as an
interface to kernel data structures. There are several probe point
variants supported by the translator:
procfs("PATH").read
procfs("PATH").umask(UMASK).read
procfs("PATH").read.maxsize(MAXSIZE)
procfs("PATH").umask(UMASK).maxsize(MAXSIZE)
procfs("PATH").write
procfs("PATH").umask(UMASK).write
procfs.read
procfs.umask(UMASK).read
procfs.read.maxsize(MAXSIZE)
procfs.umask(UMASK).read.maxsize(MAXSIZE)
procfs.write
procfs.umask(UMASK).write
PATH is the file name (relative to /proc/systemtap/MODNAME) to be
created. If no PATH is specified (as in the last two variants
above), PATH defaults to "command". The file name "__stdin" is used
internally by systemtap for input probes and should not be used as a
PATH for procfs probes; see the input probe section below.
When a user reads /proc/systemtap/MODNAME/PATH, the corresponding
procfs read probe is triggered. The string data to be read should be
assigned to a variable named $value, like this:
procfs("PATH").read { $value = "100\n" }
When a user writes into /proc/systemtap/MODNAME/PATH, the correspond‐
ing procfs write probe is triggered. The data the user wrote is
available in the string variable named $value, like this:
procfs("PATH").write { printf("user wrote: %s", $value) }
MAXSIZE is the size of the procfs read buffer. Specifying MAXSIZE
allows larger procfs output. If no MAXSIZE is specified, the procfs
read buffer defaults to STP_PROCFS_BUFSIZE (which defaults to
MAXSTRINGLEN, the maximum length of a string). If setting the procfs
read buffers for more than one file is needed, it may be easiest to
override the STP_PROCFS_BUFSIZE definition. Here's an example of us‐
ing MAXSIZE:
procfs.read.maxsize(1024) {
$value = "long string..."
$value .= "another long string..."
$value .= "another long string..."
$value .= "another long string..."
}
INPUT
These probe points make input from stdin available to the script dur‐
ing runtime. input.char is the only variant of this family that is
currently supported. This probe is triggered each time a character
is read from stdin. The current character is available in the string
variable named $value. There is no newline buffering, the next char‐
acter is read from stdin as soon as it becomes available.
Input probes are aliases for procfs("__stdin").write. Systemtap re‐
configures stdin if the precence of this procfs probe is detected,
therefore "__stdin" should not be used as an argument for procfs
probes. Additionally, input probes will not work with the -F and
--remote options.
NETFILTER HOOKS
These probe points allow observation of network packets using the
netfilter mechanism. A netfilter probe in systemtap corresponds to a
netfilter hook function in the original netfilter probes API. It is
probably more convenient to use tapset::netfilter(3stap), which wraps
the primitive netfilter hooks and does the work of extracting useful
information from the context variables.
There are several probe point variants supported by the translator:
netfilter.hook("HOOKNAME").pf("PROTOCOL_F")
netfilter.pf("PROTOCOL_F").hook("HOOKNAME")
netfilter.hook("HOOKNAME").pf("PROTOCOL_F").priority("PRIORITY")
netfilter.pf("PROTOCOL_F").hook("HOOKNAME").priority("PRIORITY")
PROTOCOL_F is the protocol family to listen for, currently one of NF‐
PROTO_IPV4, NFPROTO_IPV6, NFPROTO_ARP, or NFPROTO_BRIDGE.
HOOKNAME is the point, or 'hook', in the protocol stack at which to
intercept the packet. The available hook names for each protocol fam‐
ily are taken from the kernel header files <linux/netfilter_ipv4.h>,
<linux/netfilter_ipv6.h>, <linux/netfilter_arp.h> and <linux/netfil‐
ter_bridge.h>. For instance, allowable hook names for NFPROTO_IPV4
are NF_INET_PRE_ROUTING, NF_INET_LOCAL_IN, NF_INET_FORWARD, NF_IN‐
ET_LOCAL_OUT, and NF_INET_POST_ROUTING.
PRIORITY is an integer priority giving the order in which the probe
point should be triggered relative to any other netfilter hook func‐
tions which trigger on the same packet. Hook functions execute on
each packet in order from smallest priority number to largest priori‐
ty number. If no PRIORITY is specified (as in the first two probe
point variants above), PRIORITY defaults to "0".
There are a number of predefined priority names of the form
NF_IP_PRI_* and NF_IP6_PRI_* which are defined in the kernel header
files <linux/netfilter_ipv4.h> and <linux/netfilter_ipv6.h> respec‐
tively. The script is permitted to use these instead of specifying an
integer priority. (The probe points for NFPROTO_ARP and NFPRO‐
TO_BRIDGE currently do not expose any named hook priorities to the
script writer.) Thus, allowable ways to specify the priority in‐
clude:
priority("255")
priority("NF_IP_PRI_SELINUX_LAST")
A script using guru mode is permitted to specify any identifier or
number as the parameter for hook, pf, and priority. This feature
should be used with caution, as the parameter is inserted verbatim
into the C code generated by systemtap.
The netfilter probe points define the following context variables:
$hooknum
The hook number.
$skb The address of the sk_buff struct representing the packet. See
<linux/skbuff.h> for details on how to use this struct, or al‐
ternatively use the tapset tapset::netfilter(3stap) for easy
access to key information.
$in The address of the net_device struct representing the network
device on which the packet was received (if any). May be 0 if
the device is unknown or undefined at that stage in the proto‐
col stack.
$out The address of the net_device struct representing the network
device on which the packet will be sent (if any). May be 0 if
the device is unknown or undefined at that stage in the proto‐
col stack.
$verdict
(Guru mode only.) Assigning one of the verdict values defined
in <linux/netfilter.h> to this variable alters the further
progress of the packet through the protocol stack. For in‐
stance, the following guru mode script forces all ipv6 network
packets to be dropped:
probe netfilter.pf("NFPROTO_IPV6").hook("NF_IP6_PRE_ROUTING") {
$verdict = 0 /* nf_drop */
}
For convenience, unlike the primitive probe points discussed
here, the probes defined in tapset::netfilter(3stap) export
the lowercase names of the verdict constants (e.g. NF_DROP be‐
comes nf_drop) as local variables.
KERNEL TRACEPOINTS
This family of probe points hooks up to static probing tracepoints
inserted into the kernel or modules. As with markers, these trace‐
points are special macro calls inserted by kernel developers to make
probing faster and more reliable than with DWARF-based probes, and
DWARF debugging information is not required to probe tracepoints.
Tracepoints have an extra advantage of more strongly-typed parameters
than markers.
Tracepoint probes look like: kernel.trace("name"). The tracepoint
name string, which may contain the usual wildcard characters, is
matched against the names defined by the kernel developers in the
tracepoint header files. To restrict the search to specific subsys‐
tems (e.g. sched, ext3, etc...), the following syntax can be used:
kernel.trace("system:name"). The tracepoint system string may also
contain the usual wildcard characters.
The handler associated with a tracepoint-based probe may read the op‐
tional parameters specified at the macro call site. These are named
according to the declaration by the tracepoint author. For example,
the tracepoint probe kernel.trace("sched:sched_switch") provides the
parameters $prev and $next. If the parameter is a complex type, as
in a struct pointer, then a script can access fields with the same
syntax as DWARF $target variables. Also, tracepoint parameters can‐
not be modified, but in guru-mode a script may modify fields of pa‐
rameters.
The subsystem and name of the tracepoint are available in $$system
and $$name and a string of name=value pairs for all parameters of the
tracepoint is available in $$vars or $$parms.
KERNEL MARKERS (OBSOLETE)
This family of probe points hooks up to an older style of static
probing markers inserted into older kernels or modules. These mark‐
ers are special STAP_MARK macro calls inserted by kernel developers
to make probing faster and more reliable than with DWARF-based
probes. Further, DWARF debugging information is not required to
probe markers.
Marker probe points begin with kernel. The next part names the mark‐
er itself: mark("name"). The marker name string, which may contain
the usual wildcard characters, is matched against the names given to
the marker macros when the kernel and/or module was compiled. Op‐
tionally, you can specify format("format"). Specifying the marker
format string allows differentiation between two markers with the
same name but different marker format strings.
The handler associated with a marker-based probe may read the option‐
al parameters specified at the macro call site. These are named
$arg1 through $argNN, where NN is the number of parameters supplied
by the macro. Number and string parameters are passed in a type-safe
manner.
The marker format string associated with a marker is available in
$format. And also the marker name string is available in $name.
HARDWARE BREAKPOINTS
This family of probes is used to set hardware watchpoints for a given
(global) kernel symbol. The probes take three components as inputs :
1. The virtual address / name of the kernel symbol to be traced is
supplied as argument to this class of probes. ( Probes for only data
segment variables are supported. Probing local variables of a func‐
tion cannot be done.)
2. Nature of access to be probed : a. .write probe gets triggered
when a write happens at the specified address/symbol name. b. rw
probe is triggered when either a read or write happens.
3. .length (optional) Users have the option of specifying the ad‐
dress interval to be probed using "length" constructs. The user-spec‐
ified length gets approximated to the closest possible address length
that the architecture can support. If the specified length exceeds
the limits imposed by architecture, an error message is flagged and
probe registration fails. Wherever 'length' is not specified, the
translator requests a hardware breakpoint probe of length 1. It
should be noted that the "length" construct is not valid with symbol
names.
Following constructs are supported :
probe kernel.data(ADDRESS).write
probe kernel.data(ADDRESS).rw
probe kernel.data(ADDRESS).length(LEN).write
probe kernel.data(ADDRESS).length(LEN).rw
probe kernel.data("SYMBOL_NAME").write
probe kernel.data("SYMBOL_NAME").rw
This set of probes make use of the debug registers of the processor,
which is a scarce resource. (4 on x86 , 1 on powerpc ) The script
translation flags a warning if a user requests more hardware break‐
point probes than the limits set by architecture. For example,a
pass-2 warning is flashed when an input script requests 5 hardware
breakpoint probes on an x86 system while x86 architecture supports a
maximum of 4 breakpoints. Users are cautioned to set probes judi‐
ciously.
PERF
This family of probe points interfaces to the kernel "perf event" in‐
frastructure for controlling hardware performance counters. The
events being attached to are described by the "type", "config" fields
of the perf_event_attr structure, and are sampled at an interval gov‐
erned by the "sample_period" and "sample_freq" fields.
These fields are made available to systemtap scripts using the fol‐
lowing syntax:
probe perf.type(NN).config(MM).sample(XX)
probe perf.type(NN).config(MM).hz(XX)
probe perf.type(NN).config(MM)
probe perf.type(NN).config(MM).process("PROC")
probe perf.type(NN).config(MM).counter("COUNTER")
probe perf.type(NN).config(MM).process("PROC").counter("COUNTER")
The systemtap probe handler is called once per XX increments of the
underlying performance counter when using the .sample field or at a
frequency in hertz when using the .hz field. When not specified, the
default behavior is to sample at a count of 1000000. The range of
valid type/config is described by the perf_event_open(2) system call,
and/or the linux/perf_event.h file. Invalid combinations or exhaust‐
ed hardware counter resources result in errors during systemtap
script startup. Systemtap does not sanity-check the values: it mere‐
ly passes them through to the kernel for error- and safety-checking.
By default the perf event probe is systemwide unless .process is
specified, which will bind the probe to a specific task. If the name
is omitted then it is inferred from the stap -c argument. A perf
event can be read on demand using .counter. The body of the perf
probe handler will not be invoked for a .counter probe; instead, the
counter is read in a user space probe via:
process("PROCESS").statement("func@file") {stat <<< @perf("NAME")}
PYTHON
Support for probing python 2 and python 3 function is available with
the help of an extra python support module. Note that the debuginfo
for the version of python being probed is required. To run a python
script with the extra python support module you'd add the '-m
HelperSDT' option to your python command, like this:
stap foo.stp -c "python -m HelperSDT foo.py"
Python probes look like the following:
python2.module("MPATTERN").function("PATTERN")
python2.module("MPATTERN").function("PATTERN").call
python2.module("MPATTERN").function("PATTERN").return
python3.module("MPATTERN").function("PATTERN")
python3.module("MPATTERN").function("PATTERN").call
python3.module("MPATTERN").function("PATTERN").return
The list above includes multiple variants and modifiers which provide
additional functionality or filters. They are:
.function
Places a probe at the beginning of the named function
by default, unless modified by PATTERN. Parameters are
available as context variables.
.call Places a probe at the beginning of the named function.
Parameters are available as context variables.
.return
Places a probe at the moment before the return from the
named function. Parameters and local/global python
variables are available as context variables.
PATTERN stands for a string literal that aims to identify a point in
the python program. It is made up of three parts:
· The first part is the name of a function (e.g. "foo") or class
method (e.g. "bar.baz"). This part may use the "*" and "?" wild‐
carding operators to match multiple names.
· The second part is optional and begins with the "@" character.
It is followed by the path to the source file containing the
function, which may include a wildcard pattern. The python path
is searched for a matching filename.
· Finally, the third part is optional if the file name part was
given, and identifies the line number in the source file preceded
by a ":" or a "+". The line number is assumed to be an absolute
line number if preceded by a ":", or relative to the declaration
line of the function if preceded by a "+". All the lines in the
function can be matched with ":*". A range of lines x through y
can be matched with ":x-y". Ranges and specific lines can be
mixed using commas, e.g. ":x,y-z".
In the above list of probe points, MPATTERN stands for a python mod‐
ule or script name that names the python module of interest. This
part may use the "*" and "?" wildcarding operators to match multiple
names. The python path is searched for a matching filename.
Here are some example probe points, defining the associated events.
begin, end, end
refers to the startup and normal shutdown of the session. In
this case, the handler would run once during startup and twice
during shutdown.
timer.jiffies(1000).randomize(200)
refers to a periodic interrupt, every 1000 +/- 200 jiffies.
kernel.function("*init*"), kernel.function("*exit*")
refers to all kernel functions with "init" or "exit" in the
name.
kernel.function("*@kernel/time.c:240")
refers to any functions within the "kernel/time.c" file that
span line 240. Note that this is not a probe at the
statement at that line number. Use the kernel.statement probe
instead.
kernel.trace("sched_*")
refers to all scheduler-related (really, prefixed) tracepoints
in the kernel.
kernel.mark("getuid")
refers to an obsolete STAP_MARK(getuid, ...) macro call in the
kernel.
module("usb*").function("*sync*").return
refers to the moment of return from all functions with "sync"
in the name in any of the USB drivers.
kernel.statement(0xc0044852)
refers to the first byte of the statement whose compiled
instructions include the given address in the kernel.
kernel.statement("*@kernel/time.c:296")
refers to the statement of line 296 within "kernel/time.c".
kernel.statement("bio_init@fs/bio.c+3")
refers to the statement at line bio_init+3 within "fs/bio.c".
kernel.data("pid_max").write
refers to a hardware breakpoint of type "write" set on pid_max
syscall.*.return
refers to the group of probe aliases with any name in the
third position
stap(1),
probe::*(3stap),
tapset::*(3stap)
This page is part of the systemtap (a tracing and live-system
analysis tool) project. Information about the project can be found
at ⟨https://sourceware.org/systemtap/⟩. If you have a bug report for
this manual page, send it to systemtap@sourceware.org. This page was
obtained from the project's upstream Git repository
⟨git://sourceware.org/git/systemtap.git⟩ on 2017-07-05. If you dis‐
cover any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
STAPPROBES(3stap)
Pages that refer to this page: stap(1), stap-merge(1), stapex(3stap), error::buildid(7stap), error::pass2(7stap), error::pass3(7stap), error::sdt(7stap), stappaths(7), stapdyn(8), staprun(8), stap-server(8)