Re: [mm/slub] 555b8c8cb3: WARNING:at_lib/stackdepot.c:#stack_depot_fetch

[Marco Elver <elver@google.com>]

On Mon, 4 Apr 2022 at 16:20, Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 4/4/22 10:10, Marco Elver wrote:
> > On Mon, Apr 04, 2022 at 12:05PM +0900, Hyeonggon Yoo wrote:
> > (Maybe CONFIG_KCSAN_STRICT=y is going to yield something? I still doubt
> > it thought, this bug is related to corrupted stackdepot handle
> > somewhere...)
> >
> >> I noticed that it is not reproduced when KASAN=y and KFENCE=n (reproduced 0 of 181).
> >> and it was reproduced 56 of 196 when KASAN=n and KFENCE=y
> >>
> >> maybe this issue is related to kfence?
>
> Hmm kfence seems to be a good lead. If I understand kfence_guarded_alloc()
> correctly, it tries to set up something that really looks like a normal slab
> page? Especially the part with comment /* Set required slab fields. */
> But it doesn't seem to cover the debugging parts that SLUB sets up with
> alloc_debug_processing(). This includes alloc stack saving, thus, after
> commit 555b8c8cb3, a stackdepot handle setting. It probably normally doesn't
> matter as is_kfence_address() redirects processing of kfence-allocated
> objects so we don't hit any slub code that expects the debugging parts to be
> properly initialized.
>
> But here we are in mem_dump_obj() -> kmem_dump_obj() -> kmem_obj_info().
> Because kmem_valid_obj() returned true, fooled by folio_test_slab()
> returning true because of the /* Set required slab fields. */ code.
> Yet the illusion is not perfect and we read garbage instead of a valid
> stackdepot handle.
>
> IMHO we should e.g. add the appropriate is_kfence_address() test into
> kmem_valid_obj(), to exclude kfence-allocated objects? Sounds much simpler
> than trying to extend the illusion further to make kmem_dump_obj() work?
> Instead kfence could add its own specific handler to mem_dump_obj() to print
> its debugging data?

I think this explanation makes sense!  Indeed, KFENCE already records
allocation stacks internally anyway, so it should be straightforward
to convince it to just print that.

Thanks,
-- Marco