From: Masami Hiramatsu (Google) mhiramat@kernel.org
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