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

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

[Arnaldo Carvalho de Melo]

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@linaro.org>
Link: https://lore.kernel.org/lkml/bcf50648-3c7e-4513-8717-0d14492c53b9@linaro.org
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 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, }, }, },
 };
 
-- 
2.47.0

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@linaro.org>
> Link: https://lore.kernel.org/lkml/bcf50648-3c7e-4513-8717-0d14492c53b9@linaro.org
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Alan Maguire <alan.maguire@oracle.com>
> Cc: Howard Chu <howardchu95@gmail.com>
> Cc: Ian Rogers <irogers@google.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Kan Liang <kan.liang@linux.intel.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> ---
>   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@linaro.org>