Re: [PATCH] x86, stacktrace: avoid recording save_stack_trace wrappers

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Sep 07, 2017 at 09:57:36AM +0200, Vlastimil Babka wrote:
> The save_stack_trace() and save_stack_trace_tsk() wrappers of
> __save_stack_trace() add themselves to the call stack, and thus appear in the
> recorded stacktraces. This is redundant and wasteful when we have limited space
> to record the useful part of the backtrace with e.g. page_owner functionality.
> 
> Fix this by making sure __save_stack_trace() is noinline (which matches the
> current gcc decision) and bumping the skip in the wrappers. This is similar
> to what was done for arm in 3683f44c42e9 ("ARM: stacktrace: avoid listing
> stacktrace functions in stacktrace") and is pending for arm64.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  arch/x86/kernel/stacktrace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
> index 8dabd7bf1673..4b2fd6092739 100644
> --- a/arch/x86/kernel/stacktrace.c
> +++ b/arch/x86/kernel/stacktrace.c
> @@ -30,7 +30,7 @@ static int save_stack_address(struct stack_trace *trace, unsigned long addr,
>  	return 0;
>  }
>  
> -static void __save_stack_trace(struct stack_trace *trace,
> +static void noinline __save_stack_trace(struct stack_trace *trace,
>  			       struct task_struct *task, struct pt_regs *regs,
>  			       bool nosched)
>  {
> @@ -56,6 +56,7 @@ static void __save_stack_trace(struct stack_trace *trace,
>   */
>  void save_stack_trace(struct stack_trace *trace)
>  {
> +	trace->skip++;
>  	__save_stack_trace(trace, current, NULL, false);
>  }
>  EXPORT_SYMBOL_GPL(save_stack_trace);
> @@ -70,6 +71,7 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
>  	if (!try_get_task_stack(tsk))
>  		return;
>  
> +	trace->skip++;
>  	__save_stack_trace(trace, tsk, NULL, true);
>  
>  	put_task_stack(tsk);

save_stack_trace_tsk() is usually called for other tasks, in which case
these functions aren't on the stack.  So the skip should only be done
when task == current.

Also, I think __save_stack_trace_reliable() should be made
__always_inline (which matches its current GCC behavior) so that it
doesn't accidentally get this problem in the future.

-- 
Josh