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Date:   Tue, 1 Oct 2019 21:58:55 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     Denis Efremov <efremov@linux.com>
Cc:     David Laight <David.Laight@ACULAB.COM>,
        "devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
        Jes Sorensen <jes.sorensen@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Bastien Nocera <hadess@hadess.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Larry Finger <Larry.Finger@lwfinger.net>
Subject: Re: [PATCH] staging: rtl8723bs: hal: Fix memcpy calls
Message-ID: <20191001185730.GM29696@kadam>
References: <20190930110141.29271-1-efremov@linux.com>
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 <e4051dcb-10dc-ff17-ec0b-6f51dccdb5bf@linux.com>
 <20191001135649.GH22609@kadam>
 <8d2e8196cae74ec4ae20e9c23e898207@AcuMS.aculab.com>
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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