Re: [RFC PATCH v2 11/15] khwasan, mm: perform untagged pointers comparison in krealloc

[Ingo Molnar <mingo@kernel.org>]

* Andrey Konovalov <andreyknvl@google.com> wrote:

> On Sat, Mar 24, 2018 at 9:29 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andrey Konovalov <andreyknvl@google.com> wrote:
> >
> >> The krealloc function checks where the same buffer was reused or a new one
> >> allocated by comparing kernel pointers. KHWASAN changes memory tag on the
> >> krealloc'ed chunk of memory and therefore also changes the pointer tag of
> >> the returned pointer. Therefore we need to perform comparison on untagged
> >> (with tags reset) pointers to check whether it's the same memory region or
> >> not.
> >>
> >> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> >> ---
> >>  mm/slab_common.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/slab_common.c b/mm/slab_common.c
> >> index a33e61315ca6..5911f2194cf7 100644
> >> --- a/mm/slab_common.c
> >> +++ b/mm/slab_common.c
> >> @@ -1494,7 +1494,7 @@ void *krealloc(const void *p, size_t new_size, gfp_t flags)
> >>       }
> >>
> >>       ret = __do_krealloc(p, new_size, flags);
> >> -     if (ret && p != ret)
> >> +     if (ret && khwasan_reset_tag(p) != khwasan_reset_tag(ret))
> >>               kfree(p);
> >
> > Small nit:
> >
> > If 'reset' here means an all zeroes tag (upper byte) then khwasan_clear_tag()
> > might be a slightly easier to read primitive?
> 
> 'Reset' means to set the upper byte to the value that is native for
> kernel pointers, and that is 0xFF. So it sets the tag to all ones, not
> all zeroes. I can still rename it to khwasan_clear_tag(), if you think
> that makes sense in this case as well.

Ok, if it's not 0 then I agree that 'reset' is the better name. 'clear' would in 
fact be actively confusing.

Thanks,

	Ingo