Re: [PATCH rt-tests v1 v1 1/4] pmqtest: Increase buffer to avoid overflow

[John Kacur <jkacur@redhat.com>]

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On Mon, 19 Aug 2019, Daniel Wagner wrote:

> Increase the size of the char buffer. gcc 9.1.1 reports:
> 
> src/pmqtest/pmqtest.c: In function ‘main’:
> src/pmqtest/pmqtest.c:46:21: warning: ‘%d’ directive writing between 1 and 10 bytes into a region of size 8 [-Wformat-overflow=]
>    46 | #define SYNCMQ_NAME "/syncmsg%d"
>       |                     ^~~~~~~~~~~~
> 
> src/pmqtest/pmqtest.c:445:3: note: ‘sprintf’ output between 10 and 19 bytes into a destination of size 16
>   445 |   sprintf(mqname, SYNCMQ_NAME, i);
>       |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Signed-off-by: Daniel Wagner <wagi@monom.org>
> ---
>  src/pmqtest/pmqtest.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/src/pmqtest/pmqtest.c b/src/pmqtest/pmqtest.c
> index a04fc49872bf..3ce799bd6319 100644
> --- a/src/pmqtest/pmqtest.c
> +++ b/src/pmqtest/pmqtest.c
> @@ -440,7 +440,7 @@ int main(int argc, char *argv[])
>  		goto nomem;
>  
>  	for (i = 0; i < num_threads; i++) {
> -		char mqname[16];
> +		char mqname[19];
>  
>  		sprintf(mqname, SYNCMQ_NAME, i);
>  		receiver[i].syncmq = mq_open(mqname, oflag, 0777, &mqstat);
> @@ -567,7 +567,7 @@ int main(int argc, char *argv[])
>  	}
>  	nanosleep(&maindelay, NULL);
>  	for (i = 0; i < num_threads; i++) {
> -		char mqname[16];
> +		char mqname[19];
>  
>  		mq_close(receiver[i].syncmq);
>  		sprintf(mqname, SYNCMQ_NAME, i);
> -- 
> 2.21.0
> 

I don't love the use of "magic numbers". Also the compiler considers the 
signed integers to be −2147483648 to 2147483647 so including the sign that 
is potentially up to 11 chars, plus our string is "/syncmsg" is 8 chars
so that's where the 19 comes from. However we are using the int to 
represent threads, so we know we can't have a negative number. However 
sprintf also adds '\n' which brings us back to 19 again anyway.

This is better than what we have, so 

Signed-off-by: John Kacur <jkacur@redhat.com>