RE: [PATCH bpf-next] selftests/bpf: Fix strncpy() fortify warning

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

From: Rong Tao
> Sent: 29 October 2022 04:00
> 
> From: Rong Tao <rongtao@cestc.cn>
> 
> Replace strncpy() with strncat(), strncat() leaves the dst string zero
> terminated. Compile samples/bpf warning:

This only makes a difference because tests for strncat() have not been added.
srtncat() really is never the string function you are looking for.

	David


> 
> $ cd samples/bpf
> $ make
> ...
> In function ‘__enable_controllers’:
> samples/bpf/../../tools/testing/selftests/bpf/cgroup_helpers.c:80:17: warning: ‘strncpy’ specified
> bound 4097 equals destination size [-Wstringop-truncation]
>    80 |                 strncpy(enable, controllers, sizeof(enable));
>       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
>  tools/testing/selftests/bpf/cgroup_helpers.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c
> b/tools/testing/selftests/bpf/cgroup_helpers.c
> index a70e873b267e..912e6522c7c5 100644
> --- a/tools/testing/selftests/bpf/cgroup_helpers.c
> +++ b/tools/testing/selftests/bpf/cgroup_helpers.c
> @@ -77,7 +77,8 @@ static int __enable_controllers(const char *cgroup_path, const char *controllers
>  		enable[len] = 0;
>  		close(fd);
>  	} else {
> -		strncpy(enable, controllers, sizeof(enable) - 1);
> +		enable[0] = '\0';
> +		strncat(enable, controllers, sizeof(enable) - 1);
>  	}
> 
>  	snprintf(path, sizeof(path), "%s/cgroup.subtree_control", cgroup_path);
> --
> 2.31.1

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