Re: [PATCH 1/1 perf-tools] perf trace: The return from 'write' isn't a pid

[James Clark]

On 11/10/2024 8:21 pm, Arnaldo Carvalho de Melo wrote:
> When adding a explicit beautifier for the 'write' syscall when the BPF
> based buffer collector was introduced there was a cut'n'paste error that
> carried the syscall_fmt->errpid setting from a nearby syscall (waitid)
> that returns a pid.
>
> So the write return was being suppressed by the return pretty printer,
> remove that field, reverting it back to the default return handler, that
> prints positive numbers as-is and interpret negative values as errnos.
>
> I actually introduced the problem while making Howard's original patch
> work just with the 'write' syscall, as we couldn't just look for any
> buffers, the ones that are filled in by the kernel couldn't use the same
> sys_enter BPF collector.
>
> Fixes: b257fac12f38d7f5 ("perf trace: Pretty print buffer data")
> Reported-by: James Clark <james.clark@xxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/bcf50648-3c7e-4513-8717-0d14492c53b9@xxxxxxxxxx
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Cc: Howard Chu <howardchu95@xxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>   tools/perf/builtin-trace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index f6e8475290739a10..d3f11b90d0255c7e 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -1399,7 +1399,7 @@ static const struct syscall_fmt syscall_fmts[] = {
>   	  .arg = { [2] = { .scnprintf = SCA_WAITID_OPTIONS, /* options */ }, }, },
>   	{ .name	    = "waitid",	    .errpid = true,
>   	  .arg = { [3] = { .scnprintf = SCA_WAITID_OPTIONS, /* options */ }, }, },
> -	{ .name	    = "write",	    .errpid = true,
> +	{ .name	    = "write",
>   	  .arg = { [1] = { .scnprintf = SCA_BUF /* buf */, .from_user = true, }, }, },
>   };
>   

Fixes the return value:

  0.000 ( 0.010 ms): echo/100910 write(fd: 1, buf: , count: 6) 

                = 6

I still see the missing address for buf, but I suppose that's a separate 
issue that's introduced in the same commit. Also whether the address was 
even useful I don't know.

Reviewed-by: James Clark <james.clark@xxxxxxxxxx>