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

[Maximilian Luz]

On 8/4/23 10:18, Kees Cook wrote:
> 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.

Count is a measure of characters. I'll add a doc-comment.

Regarding INT_MAX / 2: I'm fine with either. I'd change it to
INT_MAX / sizeof(*dst) if you say it's generally enforced in bytes.

> > +		return -E2BIG;
> > +
> > +	/*
> > +	 * Copy at most 'count' bytes, return early if we find a
>
> If "count" is characters, this comment should not say "bytes". :)

Correct. Will fix this.
 
> > +	 * 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!

Thanks!

Regards
Max