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

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Date:   Thu, 1 Dec 2022 19:30:22 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Horia Geantă <horia.geanta@....com>,
        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 Fri, Dec 02, 2022 at 10:50:48AM +0800, Herbert Xu wrote:
> On Thu, Dec 01, 2022 at 05:04:14PM -0800, Kees Cook wrote:
> >
> > diff --git a/drivers/crypto/caam/desc_constr.h b/drivers/crypto/caam/desc_constr.h
> > index 62ce6421bb3f..d9da4173af9d 100644
> > --- a/drivers/crypto/caam/desc_constr.h
> > +++ b/drivers/crypto/caam/desc_constr.h
> > @@ -163,7 +163,8 @@ static inline void append_data(u32 * const desc, const void *data, int len)
> >  {
> >  	u32 *offset = desc_end(desc);
> >  
> > -	if (len) /* avoid sparse warning: memcpy with byte count of 0 */
> > +	/* Avoid GCC warning: memcpy with NULL dest (but byte count of 0). */
> > +	if (data && len)
> >  		memcpy(offset, data, len);
> 
> This makes no sense.  The if clause was added to silence sparse.
> That then in turn caused gcc to barf.  However, sparse has since
> been fixed so that it doesn't warn without the if clause.

It's _GCC_, not sparse, that is enforcing the nonnull argument
attribute.

> The solution is not to keep adding crap to the if clause, but to
> get rid of it once and for all.

Getting rid of the if doesn't solve the warning. I can switch it to just
"if (data)", though. That keeps GCC happy.

-- 
Kees Cook