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

[Linus Torvalds]

On Tue, Jul 18, 2017 at 1:15 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> 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"

We have that for the dcache case too, although there the code does
that odd kasan_unpoison_shadow() instead.

                 Linus