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From:   Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Date:   Fri, 26 Jul 2019 16:28:59 -0300
To:     Numfor Mbiziwo-Tiapo <nums@google.com>
Cc:     peterz@infradead.org, mingo@redhat.com,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, songliubraving@fb.com, mbd@fb.com,
        linux-kernel@vger.kernel.org, irogers@google.com,
        eranian@google.com
Subject: Re: [PATCH 2/3] Fix annotate.c use of uninitialized value error
Message-ID: <20190726192859.GG20482@kernel.org>
References: <20190724234500.253358-1-nums@google.com>
 <20190724234500.253358-3-nums@google.com>
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Em Wed, Jul 24, 2019 at 04:44:59PM -0700, Numfor Mbiziwo-Tiapo escreveu:
> Our local MSAN (Memory Sanitizer) build of perf throws a warning
> that comes from the "dso__disassemble_filename" function in
> "tools/perf/util/annotate.c" when running perf record.
> 
> The warning stems from the call to readlink, in which "build_id_path"
> was being read into "linkname". Since readlink does not null terminate,
> an uninitialized memory access would later occur when "linkname" is
> passed into the strstr function. This is simply fixed by null-terminating
> "linkname" after the call to readlink.
> 
> 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 record -o - ls / | tools/perf/perf --no-pager annotate\
>  -i - --stdio
> 
> Please see the cover letter for why false positive warnings may be
> generated.
> 
> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@google.com>
> ---
>  tools/perf/util/annotate.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/annotate.c b/tools/perf/util/annotate.c
> index 70de8f6b3aee..d8bfb561bc35 100644
> --- a/tools/perf/util/annotate.c
> +++ b/tools/perf/util/annotate.c
> @@ -1627,6 +1627,7 @@ static int dso__disassemble_filename(struct dso *dso, char *filename, size_t fil
>  	char *build_id_filename;
>  	char *build_id_path = NULL;
>  	char *pos;
> +	int len;
>  
>  	if (dso->symtab_type == DSO_BINARY_TYPE__KALLSYMS &&
>  	    !dso__is_kcore(dso))
> @@ -1655,10 +1656,16 @@ static int dso__disassemble_filename(struct dso *dso, char *filename, size_t fil
>  	if (pos && strlen(pos) < SBUILD_ID_SIZE - 2)
>  		dirname(build_id_path);
>  
> -	if (dso__is_kcore(dso) ||
> -	    readlink(build_id_path, linkname, sizeof(linkname)) < 0 ||
> -	    strstr(linkname, DSO__NAME_KALLSYMS) ||
> -	    access(filename, R_OK)) {
> +	if (dso__is_kcore(dso))
> +		goto fallback;
> +
> +	len = readlink(build_id_path, linkname, sizeof(linkname));
> +	if (len < 0)
> +		goto fallback;

If the buffer has N bytes and the linkname has more than that, then it
will truncate at N bytes and return N, then if we access linkname[N] we
will be accessing one byte after the end of the buffer, no?

> +
> +	linkname[len] = '\0';
> +	if (strstr(linkname, DSO__NAME_KALLSYMS) ||
> +		access(filename, R_OK)) {
>  fallback:
>  		/*
>  		 * If we don't have build-ids or the build-id file isn't in the
> -- 
> 2.22.0.657.g960e92d24f-goog

-- 

- Arnaldo