linux-kernel - Re: [PATCH] tracing: Fix user stack trace "??" output

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Message-ID: <1789efd4-1108-64e8-a6aa-39ca5d5595dc@etsukata.com>
Date:   Thu, 18 Jul 2019 10:23:19 +0900
From:   Eiichi Tsukata <devel@...ukata.com>
To:     rostedt@...dmis.org, linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, peterz@...radead.org, mingo@...hat.com
Subject: Re: [PATCH] tracing: Fix user stack trace "??" output

Hello Steven

Would you review the patch?

On 2019/06/30 17:54, Eiichi Tsukata wrote:
> Commit c5c27a0a5838 ("x86/stacktrace: Remove the pointless ULONG_MAX
> marker") removes ULONG_MAX marker from user stack trace entries but
> trace_user_stack_print() still uses the marker and it outputs unnecessary
> "??".
> 
> For example:
> 
>             less-1911  [001] d..2    34.758944: <user stack trace>
>    =>  <00007f16f2295910>
>    => ??
>    => ??
>    => ??
>    => ??
>    => ??
>    => ??
>    => ??
> 
> The user stack trace code zeroes the storage before saving the stack, so if
> the trace is shorter than the maximum number of entries it can terminate
> the print loop if a zero entry is detected.
> 
> Fixes: 4285f2fcef80 ("tracing: Remove the ULONG_MAX stack trace hackery")
> Signed-off-by: Eiichi Tsukata <devel@...ukata.com>
> ---
>  kernel/trace/trace_output.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index ba751f993c3b..cab4a5398f1d 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -1109,17 +1109,10 @@ static enum print_line_t trace_user_stack_print(struct trace_iterator *iter,
>  	for (i = 0; i < FTRACE_STACK_ENTRIES; i++) {
>  		unsigned long ip = field->caller[i];
>  
> -		if (ip == ULONG_MAX || trace_seq_has_overflowed(s))
> +		if (!ip || trace_seq_has_overflowed(s))
>  			break;
>  
>  		trace_seq_puts(s, " => ");
> -
> -		if (!ip) {
> -			trace_seq_puts(s, "??");
> -			trace_seq_putc(s, '\n');
> -			continue;
> -		}
> -
>  		seq_print_user_ip(s, mm, ip, flags);
>  		trace_seq_putc(s, '\n');
>  	}
>