Re: [PATCH v2] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Steven Rostedt]

On Fri,  7 Jul 2023 23:03:19 +0900
"Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Ensure running fprobe_exit_handler() has finished before
> calling rethook_free() in the unregister_fprobe() so that caller can free
> the fprobe right after unregister_fprobe().
> 
> unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
> have finished by calling unregister_ftrace_function() which synchronizes
> RCU. But commit 5f81018753df ("fprobe: Release rethook after the ftrace_ops
> is unregistered") changed to call rethook_free() after
> unregister_ftrace_function(). So call rethook_stop() to make rethook
> disabled before unregister_ftrace_function() and ensure it again.
> 
> Here is the possible code flow that can call the exit handler after
> unregister_fprobe().
> 
> ------
>  CPU1                              CPU2
>  call unregister_fprobe(fp)
>  ...
>                                    __fprobe_handler()
>                                    rethook_hook() on probed function
>  unregister_ftrace_function()
>                                    return from probed function
>                                    rethook hooks
>                                    find rh->handler == fprobe_exit_handler
>                                    call fprobe_exit_handler()
>  rethook_free():
>    set rh->handler = NULL;
>  return from unreigster_fprobe;
>                                    call fp->exit_handler() <- (*)
> ------
> 
> (*) At this point, the exit handler is called after returning from
> unregister_fprobe().
> 
> This fixes it as following;
> ------
>  CPU1                              CPU2
>  call unregister_fprobe()
>  ...
>  rethook_stop():
>    set rh->handler = NULL;
>                                    __fprobe_handler()
>                                    rethook_hook() on probed function
>  unregister_ftrace_function()
>                                    return from probed function
>                                    rethook hooks
>                                    find rh->handler == NULL
>                                    return from rethook
>  rethook_free()
>  return from unreigster_fprobe;
> ------
> 
> 
> Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is unregistered")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Looks good.

Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>


> ---
>  Changes in v2:
>  - Update changelog to add a problematic code flow.

Nit, for making forensic analysis easier in the future, I now always add a
link to the previous version. That is:

Changes since v1: https://lore.kernel.org/linux-trace-kernel/168796344232.46347.7947681068822514750.stgit@devnote2/
- Update changelog to add a problematic code flow.

-- Steve


> ---
>  include/linux/rethook.h |    1 +
>  kernel/trace/fprobe.c   |    3 +++
>  kernel/trace/rethook.c  |   13 +++++++++++++
>  3 files changed, 17 insertions(+)
>