Re: [PATCH v2 3/4] fs: unicode: Use strscpy() instead of strncpy()

[Eric Biggers]

On Thu, Mar 18, 2021 at 07:03:04PM +0530, Shreeya Patel wrote:
> Following warning was reported by Kernel Test Robot.
> 
> In function 'utf8_parse_version',
> inlined from 'utf8_load' at fs/unicode/utf8mod.c:195:7:
> >> fs/unicode/utf8mod.c:175:2: warning: 'strncpy' specified bound 12 equals
> destination size [-Wstringop-truncation]
> 175 |  strncpy(version_string, version, sizeof(version_string));
>     |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> The -Wstringop-truncation warning highlights the unintended
> uses of the strncpy function that truncate the terminating NULL
> character from the source string.
> Unlike strncpy(), strscpy() always null-terminates the destination string,
> hence use strscpy() instead of strncpy().
> 
> Signed-off-by: Shreeya Patel <shreeya.patel@xxxxxxxxxxxxx>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> ---
> Changes in v2
>   - Resolve warning of -Wstringop-truncation reported by
>     kernel test robot.
> 
>  fs/unicode/unicode-core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/unicode/unicode-core.c b/fs/unicode/unicode-core.c
> index d5f09e022ac5..287a8a48836c 100644
> --- a/fs/unicode/unicode-core.c
> +++ b/fs/unicode/unicode-core.c
> @@ -179,7 +179,7 @@ static int unicode_parse_version(const char *version, unsigned int *maj,
>  		{0, NULL}
>  	};
>  
> -	strncpy(version_string, version, sizeof(version_string));
> +	strscpy(version_string, version, sizeof(version_string));
>  

Shouldn't unicode_parse_version() return an error if the string gets truncated
here?  I.e. check if strscpy() returns < 0.

Also, this is a "fix" (though one that doesn't currently matter, since 'version'
is currently always shorter than sizeof(version_string)), so it should go first
in the series and have a Fixes tag.

- Eric