linux-kernel - Re: [PATCH 05/44] perf data: Fix 'strncat may truncate' build failure with recent gcc

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Date:   Tue, 28 May 2019 11:59:10 -0500
From:   Shawn Landden <slandden@...il.com>
To:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Clark Williams <williams@...hat.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Adrian Hunter <adrian.hunter@...el.com>,
        Jiri Olsa <jolsa@...hat.com>, Wang Nan <wangnan0@...wei.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 05/44] perf data: Fix 'strncat may truncate' build failure
 with recent gcc

On Tue, May 28, 2019 at 8:04 AM Arnaldo Carvalho de Melo
<arnaldo.melo@...il.com> wrote:
>
> Em Mon, May 27, 2019 at 05:46:26PM -0500, Shawn Landden escreveu:
> > On Mon, May 27, 2019 at 5:38 PM Arnaldo Carvalho de Melo
> > <acme@...nel.org> wrote:
> > >
> > > From: Shawn Landden <shawn@....icu>
> > >
> > > This strncat() is safe because the buffer was allocated with zalloc(),
> > > however gcc doesn't know that. Since the string always has 4 non-null
> > > bytes, just use memcpy() here.
> > >
> > >     CC       /home/shawn/linux/tools/perf/util/data-convert-bt.o
> > >   In file included from /usr/include/string.h:494,
> > >                    from /home/shawn/linux/tools/lib/traceevent/event-parse.h:27,
> > >                    from util/data-convert-bt.c:22:
> > >   In function ‘strncat’,
> > >       inlined from ‘string_set_value’ at util/data-convert-bt.c:274:4:
> > >   /usr/include/powerpc64le-linux-gnu/bits/string_fortified.h:136:10: error: ‘__builtin_strncat’ output may be truncated copying 4 bytes from a string of length 4 [-Werror=stringop-truncation]
> > >     136 |   return __builtin___strncat_chk (__dest, __src, __len, __bos (__dest));
> > >         |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >
> > > Signed-off-by: Shawn Landden <shawn@....icu>
> > > Cc: Adrian Hunter <adrian.hunter@...el.com>
> > > Cc: Jiri Olsa <jolsa@...hat.com>
> > > Cc: Namhyung Kim <namhyung@...nel.org>
> > > Cc: Wang Nan <wangnan0@...wei.com>
> > > LPU-Reference: 20190518183238.10954-1-shawn@....icu
> > > Link: https://lkml.kernel.org/n/tip-289f1jice17ta7tr3tstm9jm@git.kernel.org
> > > Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> > > ---
> > >  tools/perf/util/data-convert-bt.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/tools/perf/util/data-convert-bt.c b/tools/perf/util/data-convert-bt.c
> > > index e0311c9750ad..9097543a818b 100644
> > > --- a/tools/perf/util/data-convert-bt.c
> > > +++ b/tools/perf/util/data-convert-bt.c
> > > @@ -271,7 +271,7 @@ static int string_set_value(struct bt_ctf_field *field, const char *string)
> > >                                 if (i > 0)
> > >                                         strncpy(buffer, string, i);
> > >                         }
> > > -                       strncat(buffer + p, numstr, 4);
> > > +                       memcpy(buffer + p, numstr, 4);
> > I took care to have enough context in my patch that you could see what
> > was going on. I wonder if there is a way to make that care
> > propate when people add Signed-off-by: lines.
>
> I just checked and the patch is the same, the description I only changed
> the subject line, so that when one uses:
Functionally, yes. However look at how my version has enough context
that you can immediately know that the patch is correct (instead of
the default of 5 lines):
https://www.spinics.net/lists/linux-perf-users/msg08563.html

>
>    git log --oneline
>
> we can know what is the component and what kind of build failure was
> that.
>
> - Arnaldo
>
> > >                         p += 3;
> > >                 }
> > >         }
> > > --
> > > 2.20.1
> > >
>
> --
>
> - Arnaldo