Re: [PATCH] libselinux: silence -Wstringop-overflow warning from gcc 10.3.1

[Petr Lautrbach <plautrba@redhat.com>]

Nicolas Iooss <nicolas.iooss@m4x.org> writes:

> When building libselinux on Fedora 33 with gcc 10.3.1, the compiler
> reports:
>
>     label_file.c: In function ‘lookup_all.isra’:
>     label_file.c:940:4: error: ‘strncpy’ specified bound depends on the
>     length of the source argument [-Werror=stringop-overflow=]
>       940 |    strncpy(clean_key, key, len - 1);
>           |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     label_file.c:927:8: note: length computed here
>       927 |  len = strlen(key);
>           |        ^~~~~~~~~~~
>     cc1: all warnings being treated as errors
>
> As clean_key is the result of malloc(len), there is no issue here. But
> using strncpy can be considered as strange, because the size of the
> string is already known and the NUL terminator is always added later, in
> function ‘lookup_all.isra.
>
> Replace strncpy with memcpy to silence this gcc false-positive warning.
>
> Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

I wasn't able to reproduce it with gcc-11.0.1-0.7.fc35.x86_64, but it's
indeed reproducible with gcc-10.3.1-1.fc33.x86_64 and this patch fixes
it.

Acked-by: Petr Lautrbach <plautrba@redhat.com>


> ---
>  libselinux/src/label_file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libselinux/src/label_file.c b/libselinux/src/label_file.c
> index 726394ca4332..cfce23e0119e 100644
> --- a/libselinux/src/label_file.c
> +++ b/libselinux/src/label_file.c
> @@ -909,7 +909,7 @@ static const struct spec **lookup_all(struct selabel_handle *rec,
>  			if (!clean_key)
>  				goto finish;
>  
> -			strncpy(clean_key, key, len - 1);
> +			memcpy(clean_key, key, len - 1);
>  		}
>  
>  		clean_key[len - 1] = '\0';
> -- 
> 2.31.0