path: root/Documentation/libtracefs-kprobes.txt

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

NAME
----
tracefs_kprobe_alloc, tracefs_kretprobe_alloc, tracefs_kprobe_raw, tracefs_kretprobe_raw -
Allocate, get, and create kprobes

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

struct tracefs_dynevent pass:[*]
*tracefs_kprobe_alloc*(const char pass:[*]_system_, const char pass:[*]_event_,
		       const char pass:[*]_addr_, const char pass:[*]_format_);
struct tracefs_dynevent pass:[*]
*tracefs_kretprobe_alloc*(const char pass:[*]_system_, const char pass:[*]_event_,
			  const char pass:[*]_addr_, const char pass:[*]_format_, unsigned int _max_);
int *tracefs_kprobe_raw*(const char pass:[*]_system_, const char pass:[*]_event_,
			 const char pass:[*]_addr_, const char pass:[*]_format_);
int *tracefs_kretprobe_raw*(const char pass:[*]_system_, const char pass:[*]_event_,
			    const char pass:[*]_addr_, const char pass:[*]_format_);
--

DESCRIPTION
-----------
*tracefs_kprobe_alloc*() allocates a new kprobe context. The kbrobe is not configured in the system.
The new kprobe will be in the _system_ group (or kprobes if _system_ is NULL) and have the name of
_event_ (or _addr_ if _event_ is NULL). The kprobe will be inserted to _addr_ (function name, with
or without offset, or a address), and the _format_ will define the format of the kprobe. See the
Linux documentation file under: Documentation/trace/kprobetrace.rst

*tracefs_kretprobe_alloc*() is the same as *tracefs_kprobe_alloc*, but allocates context for
kretprobe. It has one additional parameter, which is optional, _max_ - maxactive count.
See description of kretprobes in the Documentation/trace/kprobetrace.rst file.

*tracefs_kprobe_raw*() will create a kprobe event. If _system_ is NULL, then
the default "kprobes" is used for the group (event system). Otherwise if _system_
is specified then the kprobe will be created under the group by that name. The
_event_ is the name of the kprobe event to create. The _addr_ can be a function,
a function and offset, or a kernel address. This is where the location of the
kprobe will be inserted in the kernel. The _format_ is the kprobe format as
specified as FETCHARGS in the Linux kernel source in the Documentation/trace/kprobetrace.rst
document.

*tracefs_kretprobe_raw*() is the same as *tracefs_kprobe_raw()*, except that it
creates a kretprobe instead of a kprobe. The difference is also described
in the Linux kernel source in the Documentation/trace/kprobetrace.rst file.

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

*tracefs_kprobe_raw*() and *tracefs_kretprobe_raw*() return 0 on success, or -1 on error.
If a parsing error occurs on *tracefs_kprobe_raw*() or *tracefs_kretprobe_raw*() then
*tracefs_error_last*(3) may be used to retrieve the error message explaining the parsing issue.

The *tracefs_kprobe_alloc*() and *tracefs_kretprobe_alloc*() APIs return a pointer to an allocated
tracefs_dynevent structure, describing the probe. This pointer must be freed by
*tracefs_dynevent_free*(3). Note, this only allocates a descriptor representing the kprobe. It does
not modify the running system.

ERRORS
------
The following errors are for all the above calls:

*EPERM* Not run as root user

*ENODEV* Kprobe events are not configured for the running kernel.

*ENOMEM* Memory allocation error.

*tracefs_kprobe_raw*(), *tracefs_kretprobe_raw*(), *tracefs_kprobe_alloc*(),
and *tracefs_kretprobe_alloc*() can fail with the following errors:

*EBADMSG* if _addr_ is NULL.

*EINVAL*  Most likely a parsing error occurred (use *tracefs_error_last*(3) to possibly
          see what that error was).

Other errors may also happen caused by internal system calls.

EXAMPLE
-------
[source,c]
--
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

#include <tracefs.h>

static struct tep_event *open_event;
static struct tep_format_field *file_field;

static struct tep_event *openret_event;
static struct tep_format_field *ret_field;

static int callback(struct tep_event *event, struct tep_record *record,
		    int cpu, void *data)
{
	struct trace_seq seq;

	trace_seq_init(&seq);
	tep_print_event(event->tep, &seq, record, "%d-%s: ", TEP_PRINT_PID, TEP_PRINT_COMM);

	if (event->id == open_event->id) {
		trace_seq_puts(&seq, "open file='");
		tep_print_field(&seq, record->data, file_field);
		trace_seq_puts(&seq, "'\n");
	} else if (event->id == openret_event->id) {
		unsigned long long ret;
		tep_read_number_field(ret_field, record->data, &ret);
		trace_seq_printf(&seq, "open ret=%lld\n", ret);
	} else {
		goto out;
	}

	trace_seq_terminate(&seq);
	trace_seq_do_printf(&seq);
out:
	trace_seq_destroy(&seq);

	return 0;
}

static pid_t run_exec(char **argv, char **env)
{
	pid_t pid;

	pid = fork();
	if (pid)
		return pid;

	execve(argv[0], argv, env);
	perror("exec");
	exit(-1);
}

const char *mykprobe = "my_kprobes";

enum kprobe_type {
	KPROBE,
	KRETPROBE,
};

static void __kprobe_create(enum kprobe_type type, const char *event,
			    const char *addr, const char *fmt)
{
	char *err;
	int r;

	if (type == KPROBE)
		r = tracefs_kprobe_raw(mykprobe, event, addr, fmt);
	else
		r = tracefs_kretprobe_raw(mykprobe, event, addr, fmt);
	if (r < 0) {
		err = tracefs_error_last(NULL);
		perror("Failed to create kprobe:");
		if (err && strlen(err))
			fprintf(stderr, "%s\n", err);
	}
}

static void kprobe_create(const char *event, const char *addr,
			  const char *fmt)
{
	__kprobe_create(KPROBE, event, addr, fmt);
}

static void kretprobe_create(const char *event, const char *addr,
			     const char *fmt)
{
	__kprobe_create(KRETPROBE, event, addr, fmt);
}

int main (int argc, char **argv, char **env)
{
	struct tracefs_instance *instance;
	struct tep_handle *tep;
	const char *sysnames[] = { mykprobe, NULL };
	pid_t pid;

	if (argc < 2) {
		printf("usage: %s command\n", argv[0]);
		exit(-1);
	}

	instance = tracefs_instance_create("exec_open");
	if (!instance) {
		perror("creating instance");
		exit(-1);
	}

	tracefs_dynevent_destroy_all(TRACEFS_DYNEVENT_KPROBE | TRACEFS_DYNEVENT_KRETPROBE, true);

	kprobe_create("open", "do_sys_openat2",
		      "file=+0($arg2):ustring flags=+0($arg3):x64 mode=+8($arg3):x64\n");

	kretprobe_create("openret", "do_sys_openat2", "ret=%ax");

	tep = tracefs_local_events_system(NULL, sysnames);
	if (!tep) {
		perror("reading events");
		exit(-1);
	}
	open_event = tep_find_event_by_name(tep, mykprobe, "open");
	file_field = tep_find_field(open_event, "file");

	openret_event = tep_find_event_by_name(tep, mykprobe, "openret");
	ret_field = tep_find_field(openret_event, "ret");

	tracefs_event_enable(instance, mykprobe, NULL);
	pid = run_exec(&argv[1], env);

	/* Let the child start to run */
	sched_yield();

	do {
		tracefs_load_cmdlines(NULL, tep);
		tracefs_iterate_raw_events(tep, instance, NULL, 0, callback, NULL);
	} while (waitpid(pid, NULL, WNOHANG) != pid);

	/* Will disable the events */
	tracefs_dynevent_destroy_all(TRACEFS_DYNEVENT_KPROBE | TRACEFS_DYNEVENT_KRETPROBE, true);
	tracefs_instance_destroy(instance);
	tep_free(tep);

	return 0;
}
--

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) 2021 VMware, Inc. Free use of this software is granted under
the terms of the GNU Public License (GPL).