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

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Message-ID: <20190726193227.GH20482@kernel.org>
Date:   Fri, 26 Jul 2019 16:32:27 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Numfor Mbiziwo-Tiapo <nums@...gle.com>
Cc:     peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, songliubraving@...com, mbd@...com,
        linux-kernel@...r.kernel.org, irogers@...gle.com,
        eranian@...gle.com
Subject: Re: [PATCH 3/3] Fix sched-messaging.c use of uninitialized value
 errors

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?

- 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