linux-kernel - Re: [PATCH] lib/strscpy: avoid KASAN false positive

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Date:   Wed, 19 Jul 2017 00:32:29 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dave Jones <davej@...emonkey.org.uk>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/strscpy: avoid KASAN false positive



On 07/19/2017 12:31 AM, Andrey Ryabinin wrote:
> On 07/18/2017 11:26 PM, Linus Torvalds wrote:
>> On Tue, Jul 18, 2017 at 1:15 PM, Andrey Ryabinin
>> <aryabinin@...tuozzo.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.
s/dont/done