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Reply-To: efremov@linux.com
Subject: Re: [PATCH] staging: rtl8723bs: hal: Fix memcpy calls
To:     David Laight <David.Laight@ACULAB.COM>,
        'Dan Carpenter' <dan.carpenter@oracle.com>
Cc:     "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>
References: <20190930110141.29271-1-efremov@linux.com>
 <37b195b700394e95aa8329afc9f60431@AcuMS.aculab.com>
 <e4051dcb-10dc-ff17-ec0b-6f51dccdb5bf@linux.com>
 <20191001135649.GH22609@kadam>
 <8d2e8196cae74ec4ae20e9c23e898207@AcuMS.aculab.com>
From:   Denis Efremov <efremov@linux.com>
Message-ID: <a7c002f7-c6f2-a9ed-0100-acfbafea65c5@linux.com>
Date:   Tue, 1 Oct 2019 18:13:21 +0300
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On 10/1/19 5:36 PM, David Laight wrote:
>> From: Dan Carpenter
>> Sent: 01 October 2019 14:57
>> Subject: Re: [PATCH] staging: rtl8723bs: hal: Fix memcpy calls
> ...
>> That's true for glibc memcpy() but not for the kernel memcpy().  In the
>> kernel there are lots of places which do a zero size memcpy().
> 
> And probably from NULL (or even garbage) pointers.
> 
> After all a pointer to the end of an array (a + ARRAY_SIZE(a)) is valid
> but must not be dereferenced - so memcpy() can't dereference it's
> source address when the length is zero.
> 
>> The glibc attitude is "the standard allows us to put knives here" so
>> let's put knives everywhere in the path.  And the GCC attitude is let's
>> silently remove NULL checks instead of just printing a warning that the
>> NULL check isn't required...  It could really make someone despondent.
> 
> gcc is the one that add knives...
> 

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."

But again, I would say that the bug in this code is because the if condition was copy-pasted
and it should be inverted.

Thanks,
Denis