Re: [PATCH v5 1/3] lib/ucs2_string: Add UCS-2 strscpy function

[Kees Cook]

On Sun, Jul 30, 2023 at 06:19:02PM +0200, Maximilian Luz wrote:
> Add a ucs2_strscpy() function for UCS-2 strings. The behavior is
> equivalent to the standard strscpy() function, just for 16-bit character
> UCS-2 strings.
> 
> Signed-off-by: Maximilian Luz <luzmaximilian@xxxxxxxxx>
> ---
> 
> Changes in v5:
>  - Add ucs2_strscpy() instead of ucs2_strlcpy()
> 
> Patch introduced in v4.
> 
> ---
>  include/linux/ucs2_string.h |  1 +
>  lib/ucs2_string.c           | 35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/include/linux/ucs2_string.h b/include/linux/ucs2_string.h
> index cf3ada3e820e..c499ae809c7d 100644
> --- a/include/linux/ucs2_string.h
> +++ b/include/linux/ucs2_string.h
> @@ -10,6 +10,7 @@ typedef u16 ucs2_char_t;
>  unsigned long ucs2_strnlen(const ucs2_char_t *s, size_t maxlength);
>  unsigned long ucs2_strlen(const ucs2_char_t *s);
>  unsigned long ucs2_strsize(const ucs2_char_t *data, unsigned long maxlength);
> +ssize_t ucs2_strscpy(ucs2_char_t *dst, const ucs2_char_t *src, size_t count);
>  int ucs2_strncmp(const ucs2_char_t *a, const ucs2_char_t *b, size_t len);
>  
>  unsigned long ucs2_utf8size(const ucs2_char_t *src);
> diff --git a/lib/ucs2_string.c b/lib/ucs2_string.c
> index 0a559a42359b..b608129fcbdc 100644
> --- a/lib/ucs2_string.c
> +++ b/lib/ucs2_string.c
> @@ -32,6 +32,41 @@ ucs2_strsize(const ucs2_char_t *data, unsigned long maxlength)
>  }
>  EXPORT_SYMBOL(ucs2_strsize);
>  
> +ssize_t ucs2_strscpy(ucs2_char_t *dst, const ucs2_char_t *src, size_t count)
> +{
> +	long res;
> +
> +	/*
> +	 * Ensure that we have a valid amount of space. We need to store at
> +	 * least one NUL-character.
> +	 */
> +	if (count == 0 || WARN_ON_ONCE(count > INT_MAX))

Is "count" a measure of bytes or characters? It seems to be characters.
can you please add some kern-doc for this function to clarify this.
Also, I wonder if the above check should be "count > INT_MAX / 2" since
the INT_MAX is, generally, done in byte counts.

> +		return -E2BIG;
> +
> +	/*
> +	 * Copy at most 'count' bytes, return early if we find a

If "count" is characters, this comment should not say "bytes". :)

> +	 * NUL-terminator.
> +	 */
> +	for (res = 0; res < count; res++) {
> +		ucs2_char_t c;
> +
> +		c = src[res];
> +		dst[res] = c;
> +
> +		if (!c)
> +			return res;
> +	}
> +
> +	/*
> +	 * The loop above terminated without finding a NUL-terminator,
> +	 * exceeding the 'count': Enforce proper NUL-termination and return
> +	 * error.
> +	 */
> +	dst[count - 1] = 0;
> +	return -E2BIG;
> +}
> +EXPORT_SYMBOL(ucs2_strscpy);
> +
>  int
>  ucs2_strncmp(const ucs2_char_t *a, const ucs2_char_t *b, size_t len)
>  {
> -- 
> 2.41.0
> 

Otherwise looks good to me!

-- 
Kees Cook