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

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Date:   Tue, 1 Oct 2019 21:58:55 +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 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));

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.

regards,
dan carpenter