Re: [PATCH v2] crypto/caam: Avoid GCC constprop bug warning

[Kees Cook]

On Thu, Dec 01, 2022 at 07:18:15PM -0800, Eric Biggers wrote:
> On Thu, Dec 01, 2022 at 05:04:14PM -0800, Kees Cook wrote:
> > GCC 12 appears to perform constant propagation incompletely(?) and can
> > no longer notice that "len" is always 0 when "data" is NULL. Expand the
> > check to avoid warnings about memcpy() having a NULL argument:
> 
> Is there a gcc option to turn off the "memcpy with NULL and len=0 is undefined
> behavior" thing?  It's basically a bug in the C standard.

It's not undefined -- it's just pedantic. __builtin_memcpy is defined
internally to GCC with __attribute__((nonnull (1, 2))), and since it can
find a path from an always-NULL argument, it warns. I think it's a dumb
limitation, given that "zero size to/from NULL" is perfectly valid.

-- 
Kees Cook