linux-kernel - Re: [PATCH] staging: rtl8723bs: hal: Fix memcpy calls

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Message-ID: <20191009143855.GE13286@kadam>
Date:   Wed, 9 Oct 2019 17:38:55 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Denis Efremov <efremov@...ux.com>
Cc:     David Laight <David.Laight@...LAB.COM>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        Jes Sorensen <jes.sorensen@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Bastien Nocera <hadess@...ess.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Larry Finger <Larry.Finger@...inger.net>
Subject: Re: [PATCH] staging: rtl8723bs: hal: Fix memcpy calls

On Tue, Oct 01, 2019 at 09:58:55PM +0300, Dan Carpenter wrote:
> On Tue, Oct 01, 2019 at 06:13:21PM +0300, Denis Efremov wrote:
> > Just found an official documentation to this issue:
> > https://gcc.gnu.org/gcc-4.9/porting_to.html
> > "Null pointer checks may be optimized away more aggressively
> > ...
> > The pointers passed to memmove (and similar functions in <string.h>) must be non-null
> > even when nbytes==0, so GCC can use that information to remove the check after the
> > memmove call. Calling copy(p, NULL, 0) can therefore deference a null pointer and crash."
> > 
> 
> Correct.  In glibc those functions are annotated as non-NULL.
> 
> extern void *memcpy (void *__restrict __dest, const void *__restrict __src,
>                      size_t __n) __THROW __nonnull ((1, 2));

I was wrong on this.  It's built into GCC so it doesn't matter how it's
annotated.

> 
> We aren't going to do that in the kernel.  A second difference is that
> in the kernel we use -fno-delete-null-pointer-checks so it doesn't
> delete the NULL checks.

But it's true that the kernel has -fno-delete-null-pointer-checks so I
don't think this is worth patching.

regards,
dan carpenter