Re: [PATCH 1/5] string.h: Introduce memtostr() and memtostr_pad()

Andy Shevchenko
2024-04-10T04:08:48Z

On Wed, Apr 10, 2024 at 5:31 AM Kees Cook <keescook@chromium.org> wrote:
>
> Another ambiguous use of strncpy() is to copy from strings that may not
> be NUL-terminated. These cases depend on having the destination buffer
> be explicitly larger than the source buffer's maximum size, having
> the size of the copy exactly match the source buffer's maximum size,
> and for the destination buffer to get explicitly NUL terminated.
>
> This usually happens when parsing protocols or hardware character arrays
> that are not guaranteed to be NUL-terminated. The code pattern is
> effectively this:
>
> char dest[sizeof(src) + 1];
>
> strncpy(dest, src, sizeof(src));
> dest[sizeof(dest) - 1] = '\0';
>
> In practice it usually looks like:
>
> struct from_hardware {
> ...
> char name[HW_NAME_SIZE] __nonstring;
> ...
> };
>
> struct from_hardware *p = ...;
> char name[HW_NAME_SIZE + 1];
>
> strncpy(name, p->name, HW_NAME_SIZE);
> name[NW_NAME_SIZE] = '\0';
>
> This cannot be replaced with:
>
> strscpy(name, p->name, sizeof(name));
>
> because p->name is smaller and not NUL-terminated, so FORTIFY will
> trigger when strnlen(p->name, sizeof(name)) is used. And it cannot be
> replaced with:
>
> strscpy(name, p->name, sizeof(p->name));
>
> because then "name" may contain a 1 character early truncation of
> p->name.
>
> Provide an unambiguous interface for converting a maybe not-NUL-terminated
> string to a NUL-terminated string, with compile-time buffer size checking
> so that it can never fail at runtime: memtostr() and memtostr_pad(). Also
> add KUnit tests for both.

Obvious question, why can't strscpy() be fixed for this corner case?

--
With Best Regards,
Andy Shevchenko