RE: [PATCH 1/3] Fix backward-ring-buffer.c format-truncation error

[David Laight <David.Laight@ACULAB.COM>]

From: Numfor Mbiziwo-Tiapo
> Sent: 24 July 2019 19:45
>
> Perf does not build with the ubsan (undefined behavior sanitizer)
> and there is an error that says:
> 
> tests/backward-ring-buffer.c:23:45: error: ‘%d’ directive output
> may be truncated writing between 1 and 10 bytes into a region of
> size 8 [-Werror=format-truncation=]
>    snprintf(proc_name, sizeof(proc_name), "p:%d\n", i);
> 
> This can be reproduced by running (from the tip directory):
> make -C tools/perf USE_CLANG=1 EXTRA_CFLAGS="-fsanitize=undefined"
> 
> Th error occurs because they are writing to the 10 byte buffer - the
> index 'i' of the for loop and the 2 byte hardcoded string. If somehow 'i'
> was greater than 8 bytes (10 - 2), then the snprintf function would
> truncate the string. Increasing the size of the buffer fixes the error.

Get the compiler fixed so that it knows the domain of the value can never
exceed the compile-time constant NR_ITERS.

> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@google.com>
> ---
>  tools/perf/tests/backward-ring-buffer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/tests/backward-ring-buffer.c b/tools/perf/tests/backward-ring-buffer.c
> index 6d598cc071ae..1a9c3becf5ff 100644
> --- a/tools/perf/tests/backward-ring-buffer.c
> +++ b/tools/perf/tests/backward-ring-buffer.c
> @@ -18,7 +18,7 @@ static void testcase(void)
>  	int i;
> 
>  	for (i = 0; i < NR_ITERS; i++) {
> -		char proc_name[10];
> +		char proc_name[15];

At least use [16]

> 
>  		snprintf(proc_name, sizeof(proc_name), "p:%d\n", i);
>  		prctl(PR_SET_NAME, proc_name);
> --
> 2.22.0.657.g960e92d24f-goog

	David

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