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

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Message-ID: <CAP-5=fUOpiJypozewCSi6Avg2pQnjmvNcFMM5iUsbTR1P_47zQ@mail.gmail.com>
Date: Fri, 3 Oct 2025 14:25:39 -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 Wed, Sep 17, 2025 at 11:05 AM Ian Rogers <irogers@...gle.com> wrote:
>
> 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.

It'd be nice to land this for the v6.18 pull. I don't think it has any
visible implications and just makes stuff better.

Thanks,
Ian


> 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
> >