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Documentation: tracing: Add entry argument access at function exit 

Add a notes about the entry argument access at function exit probes for
kprobes and fprobe trace event.

Link: https://lore.kernel.org/all/170952367549.229804.8843506960483577062.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
This commit is contained in:
Masami Hiramatsu (Google) 2024-03-04 12:41:15 +09:00
parent f6e2253a61
commit e8c32f2476
2 changed files with 40 additions and 0 deletions
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31
Documentation/trace/fprobetrace.rst
View File

@ -70,6 +70,14 @@ Synopsis of fprobe-events
For the details of TYPE, see :ref:`kprobetrace documentation <kprobetrace_types>`.
Function arguments at exit
--------------------------
Function arguments can be accessed at exit probe using $arg<N> fetcharg. This
is useful to record the function parameter and return value at once, and
trace the difference of structure fields (for debuging a function whether it
correctly updates the given data structure or not)
See the :ref:`sample<fprobetrace_exit_args_sample>` below for how it works.
BTF arguments
-------------
BTF (BPF Type Format) argument allows user to trace function and tracepoint
@ -218,3 +226,26 @@ traceprobe event, you can trace that field as below.
<idle>-0 [000] d..3. 5606.690317: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="kworker/0:1" usage=1 start_time=137000000
kworker/0:1-14 [000] d..3. 5606.690339: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="swapper/0" usage=2 start_time=0
<idle>-0 [000] d..3. 5606.692368: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="kworker/0:1" usage=1 start_time=137000000
.. _fprobetrace_exit_args_sample:
The return probe allows us to access the results of some functions, which returns
the error code and its results are passed via function parameter, such as an
structure-initialization function.
For example, vfs_open() will link the file structure to the inode and update
mode. You can trace that changes with return probe.
::
# echo 'f vfs_open mode=file->f_mode:x32 inode=file->f_inode:x64' >> dynamic_events
# echo 'f vfs_open%%return mode=file->f_mode:x32 inode=file->f_inode:x64' >> dynamic_events
# echo 1 > events/fprobes/enable
# cat trace
sh-131 [006] ...1. 1945.714346: vfs_open__entry: (vfs_open+0x4/0x40) mode=0x2 inode=0x0
sh-131 [006] ...1. 1945.714358: vfs_open__exit: (do_open+0x274/0x3d0 <- vfs_open) mode=0x4d801e inode=0xffff888008470168
cat-143 [007] ...1. 1945.717949: vfs_open__entry: (vfs_open+0x4/0x40) mode=0x1 inode=0x0
cat-143 [007] ...1. 1945.717956: vfs_open__exit: (do_open+0x274/0x3d0 <- vfs_open) mode=0x4a801d inode=0xffff888005f78d28
cat-143 [007] ...1. 1945.720616: vfs_open__entry: (vfs_open+0x4/0x40) mode=0x1 inode=0x0
cat-143 [007] ...1. 1945.728263: vfs_open__exit: (do_open+0x274/0x3d0 <- vfs_open) mode=0xa800d inode=0xffff888004ada8d8
You can see the `file::f_mode` and `file::f_inode` are upated in `vfs_open()`.

9
Documentation/trace/kprobetrace.rst
View File

@ -70,6 +70,15 @@ Synopsis of kprobe_events
(\*3) this is useful for fetching a field of data structures.
(\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
Function arguments at kretprobe
-------------------------------
Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This
is useful to record the function parameter and return value at once, and
trace the difference of structure fields (for debuging a function whether it
correctly updates the given data structure or not).
See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how
it works.
.. _kprobetrace_types:
Types