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

[Andrew Morton <akpm@linux-foundation.org>]

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?