linux-kernel - Re: [PATCH v1] perf parse-events: Fix parsing of >30kb event strings

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Message-ID: <CAP-5=fUSuh+hKVg_1owmzPAZka3DGmhr6FzN__n_Cb2XUET3=A@mail.gmail.com>
Date: Wed, 17 Sep 2025 11:05:22 -0700
From: Ian Rogers <irogers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] perf parse-events: Fix parsing of >30kb event strings

On Thu, Sep 4, 2025 at 9:26 PM Ian Rogers <irogers@...gle.com> wrote:
>
> Metrics may generate many particularly uncore event references. The
> resulting event string may then be >32kb. The parse events lex is
> using "%option reject" which stores backtracking state in a buffer
> sized at roughtly 30kb. If the event string is larger than this then a
> buffer overflow and typically a crash happens.
>
> The need for "%option reject" was for BPF events which were removed in
> commit 3d6dfae88917 ("perf parse-events: Remove BPF event
> support"). As "%option reject" is both a memory and performance cost
> let's remove it and fix the parsing case for event strings being over
> ~30kb.
>
> Whilst cleaning up "%option reject" make the header files accurately
> reflect functions used in the code and tidy up not requiring yywrap.
>
> Measuring on the "PMU JSON event tests" a modest reduction of 0.41%
> user time and 0.27% max resident size was observed. More importantly
> this change fixes parsing large metrics and event strings.
>
> Signed-off-by: Ian Rogers <irogers@...gle.com>

Ping. I think this may have gotten lost in noise about things like
hardware json and the python metrics. It is a small change, bug fix
and performance win. It can land independently of anything else. PTAL.

Thanks,
Ian

> ---
>  tools/perf/util/parse-events.l | 17 +++--------------
>  1 file changed, 3 insertions(+), 14 deletions(-)
>
> diff --git a/tools/perf/util/parse-events.l b/tools/perf/util/parse-events.l
> index 2034590eb789..1eaa8dbc26d8 100644
> --- a/tools/perf/util/parse-events.l
> +++ b/tools/perf/util/parse-events.l
> @@ -5,16 +5,14 @@
>  %option stack
>  %option bison-locations
>  %option yylineno
> -%option reject
> +%option noyywrap
>
>  %{
>  #include <errno.h>
> -#include <sys/types.h>
> -#include <sys/stat.h>
> -#include <unistd.h>
> +#include <stdlib.h>
> +#include <stdio.h>
>  #include "parse-events.h"
>  #include "parse-events-bison.h"
> -#include "evsel.h"
>
>  char *parse_events_get_text(yyscan_t yyscanner);
>  YYSTYPE *parse_events_get_lval(yyscan_t yyscanner);
> @@ -222,10 +220,6 @@ do {                                                       \
>         yycolumn += yyleng;                             \
>  } while (0);
>
> -#define USER_REJECT            \
> -       yycolumn -= yyleng;     \
> -       REJECT
> -
>  %}
>
>  %x mem
> @@ -423,8 +417,3 @@ r{num_raw_hex}              { return str(yyscanner, PE_RAW); }
>  .                      { }
>
>  %%
> -
> -int parse_events_wrap(void *scanner __maybe_unused)
> -{
> -       return 1;
> -}
> --
> 2.51.0.355.g5224444f11-goog
>