linux-kernel - Re: [PATCH 3/3] Fix sched-messaging.c use of uninitialized value errors

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Date:   Fri, 26 Jul 2019 16:52:26 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:     Numfor Mbiziwo-Tiapo <nums@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Song Liu <songliubraving@...com>, mbd@...com,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/3] Fix sched-messaging.c use of uninitialized value errors

On Fri, Jul 26, 2019 at 12:32 PM Arnaldo Carvalho de Melo
<arnaldo.melo@...il.com> wrote:
>
> Em Wed, Jul 24, 2019 at 04:45:00PM -0700, Numfor Mbiziwo-Tiapo escreveu:
> > Our local MSAN (Memory Sanitizer) build of perf throws use of
> > uninitialized value warnings in "tools/perf/bench/sched-messaging.c"
> > when running perf bench.
> >
> > The first warning comes from the "ready" function where the "dummy" char
> > is declared and then passed into "write" without being initialized.
> > Initializing "dummy" to any character silences the warning.
> >
> > The second warning comes from the "sender" function where a "write" call
> > is made to write the contents from the "data" char array when it has not
> > yet been initialized. Calling memset on "data" silences the warning.
>
> So, this is just to silence MSAN, as it doesn't matter what is sent,
> whatever values are in those variables is ok, as it will not be used,
> right?

That's right.

Thanks,
Ian Rogers

> - Arnaldo
>
> > To reproduce this warning, build perf by running:
> > make -C tools/perf CLANG=1 CC=clang EXTRA_CFLAGS="-fsanitize=memory\
> >  -fsanitize-memory-track-origins"
> >
> > (Additionally, llvm might have to be installed and clang might have to
> > be specified as the compiler - export CC=/usr/bin/clang)
> >
> > then running: tools/perf/perf bench sched all
> >
> > Please see the cover letter for why false positive warnings may be
> > generated.
> >
> > Signed-off-by: Numfor Mbiziwo-Tiapo <nums@...gle.com>
> > ---
> >  tools/perf/bench/sched-messaging.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/perf/bench/sched-messaging.c b/tools/perf/bench/sched-messaging.c
> > index f9d7641ae833..d22d7b7b591d 100644
> > --- a/tools/perf/bench/sched-messaging.c
> > +++ b/tools/perf/bench/sched-messaging.c
> > @@ -69,7 +69,7 @@ static void fdpair(int fds[2])
> >  /* Block until we're ready to go */
> >  static void ready(int ready_out, int wakefd)
> >  {
> > -     char dummy;
> > +     char dummy = 'N';
> >       struct pollfd pollfd = { .fd = wakefd, .events = POLLIN };
> >
> >       /* Tell them we're ready. */
> > @@ -87,6 +87,7 @@ static void *sender(struct sender_context *ctx)
> >       char data[DATASIZE];
> >       unsigned int i, j;
> >
> > +     memset(data, 'N', DATASIZE);
> >       ready(ctx->ready_out, ctx->wakefd);
> >
> >       /* Now pump to every receiver. */
> > --
> > 2.22.0.657.g960e92d24f-goog
>
> --
>
> - Arnaldo