libtracefs(3)
=============

NAME
----
tracefs_function_filter, tracefs_function_notrace, tracefs_filter_functions
- Functions to modify the the function trace filters

SYNOPSIS
--------
[verse]
--
*#include <tracefs.h>*

int *tracefs_function_filter*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_filter_, const char pass:[*]_module_, int _flags_);
int *tracefs_function_notrace*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_filter_, const char pass:[*]_module_, int _flags_);
int *tracefs_filter_functions*(const char pass:[*]_filter_, const char pass:[*]_module_, char pass:[*]pass:[*]pass:[*]_list_);
--

DESCRIPTION
-----------
*tracefs_function_filter* and *tracefs_function_notrace* can be used to limit the
Linux kernel functions that would be traced by the function and function-graph tracers.
The *tracefs_function_filter* defines a list of functions that can be traced.
The *tracefs_function_notrace* defines a list of functions that will not be traced.
If a function is in both lists, it will not be traced.

They take an _instance_ , that can be NULL for the top level tracing,
_filter_, a string that represents a filter that should
be applied to define what functions are to be traced,
_module_, to limit the filtering on a specific module (or NULL to filter on all functions),
_flags_ which holds control knobs on how the filters will be handled (see *FLAGS*)
section below.

The *tracefs_filter_functions* returns a list of functions that can be filtered on
via the _filter_ and _module_ that are supplied. If both _filter_ and _module_ are
NULL then, all available functions that can be filtered is returned.
On success, _list_ must be freed with *tracefs_list_free()*(3).

The _filter_ may be either a straight match of a
function, a glob or regex(3). A glob is where 'pass:[*]' matches zero or more
characters, '?' will match zero or one character, and '.' only matches a
period. If the _filter_ is determined to be a regex (where it contains
anything other than alpha numeric characters, or '.', 'pass:[*]', '?') the _filter_
will be processed as a regex(3) following the rules of regex(3), and '.' is
not a period, but will match any one character. To force a regular
expression, either prefix _filter_ with a '^' or append it with a '$' as
the _filter_ does complete matches of the functions anyway.

If _module_ is set and _filter_ is NULL, this will imply the same as _filter_ being
equal to "pass:[*]". Which will enable all functions for a given _module_. Otherwise
the _filter_ may be NULL if a previous call to *tracefs_function_filter()* with
the same _instance_ had *TRACEFS_FL_CONTINUE* set and this call does not. This is
useful to simply commit the previous filters. It may also be NULL
if *TRACEFS_FL_RESET* is set and the previous call did not have the same _instance_
and *TRACEFS_FL_CONTINUE* set. This is useful to just clear the filter.

FLAGS
-----

The _flags_ parameter may have the following set, or be zero.

*TRACEFS_FL_RESET* :
If _flags_ contains *TRACEFS_FL_RESET*, then it will clear the filters that
are currently set before applying _filter_. Otherwise, _filter_ is added to
the current set of filters already enabled. If this flag is set and the
previous call to tracefs_function_filter() had the same _instance_ and the
*TRACEFS_FL_CONTINUE* flag was set, then the function will fail with a
return of -1 and errno set to EBUSY.

*TRACEFS_FL_CONTINUE* :
If _flags_ contains *TRACEFS_FL_CONTINUE*, then _filter_ will not take
effect after a successful call to tracefs_function_filter(). This allows for
multiple calls to tracefs_function_filter() to update the filter function
and then a single call (one without the *TRACEFS_FL_CONTINUE* flag set) to
commit all the filters.
It can be called multiple times to add more filters. A call without this
flag set will commit the changes before returning (if the _filter_ passed in
successfully matched). A tracefs_function_filter() call after one that had
the *TRACEFS_FL_CONTINUE* flag set for the same instance will fail if
*TRACEFS_FL_RESET* flag is set, as the reset flag is only applicable for the
first filter to be added before committing.

*TRACEFS_FL_FUTURE* :
If _flags_ contains *TRACEFS_FL_FUTURE* and _module_ holds a string of a module,
then if the module is not loaded it will attemp to write the filter with the module
in the filter file. Starting in Linux v4.13 module functions could be added to the
filter before they are loaded. The filter will be cached, and when the module is
loaded, the filter will be set before the module executes, allowing to trace
init functions of a module. This will only work if the _filter_ is not a
regular expression.

RETURN VALUE
------------

For *tracefs_function_filter()* and *tracefs_function_notrace()* a
return of 0 means success. If the there is an error but the filtering was not
started, then 1 is returned. If filtering was started but an error occurs,
then -1 is returned. The state of the filtering may be in an unknown state.

If *TRACEFS_FL_CONTINUE* was set, and 0 or -1 was returned, then another call
to *tracefs_function_filter()* must be done without *TRACEFS_FL_CONTINUE* set
in order to commit (and close) the filtering.

For *tracefs_filter_functions()*, a return of 0 means success, and the _list_
parameter is filled with a list of function names that matched _filter_ and
_module_. _list_ is a string array, where the last string pointer in the
array is NULL. The _list_ must be freed with *tracefs_list_free()*.
On failure, a negative is returned, and _list_ is ignored.

ERRORS
------

*tracefs_function_filter*() can fail with the following errors:

*EINVAL* The filter is invalid or did not match any functions.

*EBUSY* The previous call of *tracefs_function_filter*() was called
with the same instance and *TRACEFS_FL_CONTINUE* set and the current call
had *TRACEFS_FL_RESET* set.

Other errors may also happen caused by internal system calls.

EXAMPLE
-------
[source,c]
--
#include <stdio.h>
#include <errno.h>
#include <tracefs.h>

#define INST "dummy"

static const char *filters[] = { "run_init_process", "try_to_run_init_process", "dummy1", NULL };

int main(int argc, char *argv[])
{
	struct tracefs_instance *inst = tracefs_instance_create(INST);
	char **func_list;
	int ret;
	int i;

	if (!inst) {
		/* Error creating new trace instance */
	}

	if (tracefs_filter_functions("*lock*", NULL, &func_list) < 0) {
		printf("Failed to read filter functions\n");
		goto out;
	}
	printf("Ignoring the following functions:\n");
	for (i = 0; func_list[i]; i++)
		printf("  %s\n", func_list[i]);
	tracefs_list_free(func_list);

	/* Do not trace any function with the word "lock" in it */
	ret = tracefs_function_notrace(inst, "*lock*", NULL, TRACEFS_FL_RESET);
	if (ret) {
		printf("Failed to set the notrace filter\n");
		goto out;
	}

	/* First reset the filter */
	ret = tracefs_function_filter(inst, NULL, NULL,
				      TRACEFS_FL_RESET | TRACEFS_FL_CONTINUE);
	if (ret) {
		printf("Failed to reset the filter\n");
		/* Make sure it is closed, -1 means filter was started */
		if (ret < 0)
			tracefs_function_filter(inst, NULL, NULL, 0);
	}

	for (i = 0; filters[i]; i++) {
		ret = tracefs_function_filter(inst, filters[i], NULL,
					      TRACEFS_FL_CONTINUE);

		if (ret) {
			if (errno == EINVAL)
				printf("Filter %s did not match\n", filters[i]);
			else
				printf("Failed writing %s\n", filters[i]);
		}
	}

	ret = tracefs_function_filter(inst, "*", "ext4", 0);
	if (ret) {
		printf("Failed to set filters for ext4\n");
		/* Force the function to commit previous filters */
		tracefs_function_filter(inst, NULL, NULL, 0);
	}

 out:
	tracefs_instance_destroy(inst);
	return ret;
}
--

FILES
-----
[verse]
--
*tracefs.h*
	Header file to include in order to have access to the library APIs.
*-ltracefs*
	Linker switch to add when building a program that uses the library.
--

SEE ALSO
--------
*libtracefs*(3),
*libtraceevent*(3),
*trace-cmd*(1)

AUTHOR
------
[verse]
--
*Steven Rostedt* <rostedt@goodmis.org>
*Tzvetomir Stoyanov* <tz.stoyanov@gmail.com>
*sameeruddin shaik* <sameeruddin.shaik8@gmail.com>
--
REPORTING BUGS
--------------
Report bugs to  <linux-trace-devel@vger.kernel.org>

LICENSE
-------
libtracefs is Free Software licensed under the GNU LGPL 2.1

RESOURCES
---------
https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/

COPYING
-------
Copyright \(C) 2020 VMware, Inc. Free use of this software is granted under
the terms of the GNU Public License (GPL).