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

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Date:   Thu, 1 Dec 2022 19:23:49 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Horia Geantă <horia.geanta@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Pankaj Gupta <pankaj.gupta@....com>,
        Gaurav Jain <gaurav.jain@....com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org, kernel test robot <lkp@...el.com>,
        Anders Roxell <anders.roxell@...aro.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] crypto/caam: Avoid GCC constprop bug warning

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