Re: [RFC] kasan: include the hashed pointer for an object's location

[Lyude Paul]

On Tue, 2019-10-22 at 04:27 +0200, Dmitry Vyukov wrote:
> On Tue, Oct 22, 2019 at 4:19 AM Lyude Paul <lyude@xxxxxxxxxx> wrote:
> > The vast majority of the kernel that needs to print out pointers as a
> > way to keep track of a specific object in the kernel for debugging
> > purposes does so using hashed pointers, since these are "good enough".
> > Ironically, the one place we don't do this is within kasan. While
> > simply printing a hashed version of where an out of bounds memory access
> > occurred isn't too useful, printing out the hashed address of the object
> > in question usually is since that's the format most of the kernel is
> > likely to be using in debugging output.
> > 
> > Of course this isn't perfect though-having the object's originating
> > address doesn't help users at all that need to do things like printing
> > the address of a struct which is embedded within another struct, but
> > it's certainly better then not printing any hashed addresses. And users
> > which need to handle less trivial cases like that can simply fall back
> > to careful usage of %px.
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > Cc: Sean Paul <sean@xxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: kasan-dev@xxxxxxxxxxxxxxxx
> > ---
> >  mm/kasan/report.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > index 621782100eaa..0a5663fee1f7 100644
> > --- a/mm/kasan/report.c
> > +++ b/mm/kasan/report.c
> > @@ -128,8 +128,9 @@ static void describe_object_addr(struct kmem_cache
> > *cache, void *object,
> >         int rel_bytes;
> > 
> >         pr_err("The buggy address belongs to the object at %px\n"
> > -              " which belongs to the cache %s of size %d\n",
> > -               object, cache->name, cache->object_size);
> > +              " (aka %p) which belongs to the cache\n"
> > +              " %s of size %d\n",
> > +              object, object, cache->name, cache->object_size);
> 
> Hi Lyude,
> 
> This only prints hashed address for heap objects, but
> print_address_description() has 4 different code paths for different
> types of addresses (heap, global, stack, page). Plus there is a case
> for address without shadow.
> Should we print the hashed address at least for all cases in
> print_address_description()?

Yep-this is probably a good idea. Will send a respin in a little bit
> 
> 
> >         if (!addr)
> >                 return;
> > --
> > 2.21.0
> > 
-- 
Cheers,
	Lyude Paul