Re: [PATCH v2] ftrace: Hide a extra entry in stack trace

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 26 Sep 2024 15:13:07 +0900
Tatsuya S <tatsuya.s2862@gmail.com> wrote:

> A extra entry is shown on stack trace(CONFIG_UNWINDER_ORC=y).
> 
>     [003] .....   110.171589: vfs_write <-__x64_sys_write
>     [003] .....   110.171600: <stack trace>
>  => XXXXXXXXX (Wrong function name)
>  => vfs_write
>  => __x64_sys_write
>  => do_syscall_64
>  => entry_SYSCALL_64_after_hwframe

OK, I confirmed it;

------
echo 1 > options/func_stack_trace 
echo "vfs_write" >> set_ftrace_filter
echo "function" > current_tracer
echo > /dev/null
cat trace
              sh-136     [005] .....   266.884180: vfs_write <-ksys_write
              sh-136     [005] .....   266.884188: <stack trace>
 => 0xffffffffa0004099
 => vfs_write
 => ksys_write
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe
------

> 
> To resolve this, increment skip in __ftrace_trace_stack().
> The reason why skip is incremented in __ftrace_trace_stack()
> is because __ftrace_trace_stack() in stack trace is the only function
> that wasn't skipped from anywhere.
> 
> Signed-off-by: Tatsuya S <tatsuya.s2862@gmail.com>
> ---
>  kernel/trace/trace.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c3b2c7dfadef..0f2e255f563c 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2916,10 +2916,8 @@ static void __ftrace_trace_stack(struct trace_buffer *buffer,
>  	 * Add one, for this function and the call to save_stack_trace()
>  	 * If regs is set, then these functions will not be in the way.
>  	 */

Hmm, with this change, the above comment should also be updated.


> -#ifndef CONFIG_UNWINDER_ORC
> -	if (!regs)
> +	if (IS_ENABLED(CONFIG_UNWINDER_ORC) || !regs)
>  		skip++;
> -#endif

Also, this solves just one pattern (only enable function tracer) but if
there are fprobes (or kprobes) on the same function, it introduces another issue.
e.g. (with this fix)

------
echo 1 > options/func_stack_trace
echo 1 > options/stacktrace
echo "vfs_write" >> set_ftrace_filter
echo "function" > current_tracer
echo "f:myevent vfs_write" > dynamic_events
echo 1 > events/fprobes/myevent/enable
echo > /dev/null
cat trace
...
              sh-140     [001] ...1.    18.352601: myevent: (vfs_write+0x4/0x560)
              sh-140     [001] ...1.    18.352602: <stack trace>
 => ksys_write
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe
              sh-140     [001] ...1.    18.352602: vfs_write <-ksys_write
              sh-140     [001] ...1.    18.352604: <stack trace>
 => ftrace_regs_call
 => vfs_write
 => ksys_write
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe
------
As you can see, myevent skips "vfs_write".
(and function tracer still have ftrace_regs_call() )

Thank you,

>  
>  	preempt_disable_notrace();
>  
> -- 
> 2.46.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>