On Mon, 4 Apr 2022 at 16:20, Vlastimil Babka <vbabka@xxxxxxx> 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
