Messages in this thread | | | Date | Thu, 1 Dec 2022 19:23:49 -0800 | From | Kees Cook <> | Subject | Re: [PATCH v2] crypto/caam: Avoid GCC constprop bug warning |
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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
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