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SubjectRe: [PATCH] lib/strscpy: avoid KASAN false positive
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On 7/18/2017 6:04 PM, Andrew Morton wrote:
> 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?

The word-at-a-time logic was part of the initial commit since I wanted
to ensure that strscpy could be used to replace strlcpy or strncpy without
serious concerns about performance. It seems unfortunate to remove it
unconditionally to support KASAN, but I haven't looked deeply at the
tradeoffs here.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com

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