[PATCH v3 8/8] Documentation: tracing: Add entry argument access at function exit

Masami Hiramatsu (Google)
2024-03-04T03:41:19Z

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

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v3:
- Add usage example in fprobetrace.rst.
---
Documentation/trace/fprobetrace.rst | 31 +++++++++++++++++++++++++++++++
Documentation/trace/kprobetrace.rst | 9 +++++++++
2 files changed, 40 insertions(+)

diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
index e35e6b18df40..0f187e3796e4 100644
--- a/Documentation/trace/fprobetrace.rst
+++ b/Documentation/trace/fprobetrace.rst
@@ -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()`.
diff --git a/Documentation/trace/kprobetrace.rst b/Documentation/trace/kprobetrace.rst
index bf9cecb69fc9..a49662ccd53c 100644
--- a/Documentation/trace/kprobetrace.rst
+++ b/Documentation/trace/kprobetrace.rst
@@ -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