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Date: Tue, 18 Jul 2017 15:04:41 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Jones <davej@codemonkey.org.uk>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>, kasan-dev@googlegroups.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Chris Metcalf <cmetcalf@ezchip.com>
Subject: Re: [PATCH] lib/strscpy: avoid KASAN false positive
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On Wed, 19 Jul 2017 00:31:36 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

> On 07/18/2017 11:26 PM, Linus Torvalds wrote:
> > On Tue, Jul 18, 2017 at 1:15 PM, Andrey Ryabinin
> > <aryabinin@virtuozzo.com> wrote:
> >>
> >> No, it does warn about valid users. The report that Dave posted wasn't about wrong strscpy() usage
> >> it was about reading 8-bytes from 5-bytes source string. It wasn't about buggy 'count' at all.
> >> So KASAN will warn for perfectly valid code like this:
> >>         char dest[16];
> >>         strscpy(dest, "12345", sizeof(dest)):
> > 
> > Ugh, ok, yes.
> > 
> >> For strscpy() that would mean making the *whole* read from 'src' buffer unchecked by KASAN.
> > 
> > So we do have that READ_ONCE_NOCHECK(), but could we perhaps have
> > something that doesn't do a NOCHECK but a partial check and is simply
> > ok with "this is an optimistc longer access"
> > 
> 
> This can be dont, I think.
> 
> Something like this:
> static inline unsigned long read_partial_nocheck(unsigned long *x)
> {
> 	unsigned long ret = READ_ONCE_NOCHECK(x);
> 	kasan_check_partial(x, sizeof(unsigned long));
> 	return ret;
> }
> 

(Cc Chris)

We could just remove all that word-at-a-time logic.  Do we have any
evidence that this would harm anything?