On Thu, 2023-11-16 at 15:55 +0100, Alexander Potapenko wrote: > On Wed, Nov 15, 2023 at 9:34 PM Ilya Leoshkevich <iii@xxxxxxxxxxxxx> > wrote: > > > > Avoid false KMSAN negatives with SLUB_DEBUG by allowing > > kmsan_slab_free() to poison the freed memory, and by preventing > > init_object() from unpoisoning new allocations. > > > > Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx> > > --- > > mm/kmsan/hooks.c | 2 +- > > mm/slub.c | 3 ++- > > 2 files changed, 3 insertions(+), 2 deletions(-) > > > > diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c > > index 7b5814412e9f..7a30274b893c 100644 > > --- a/mm/kmsan/hooks.c > > +++ b/mm/kmsan/hooks.c > > @@ -76,7 +76,7 @@ void kmsan_slab_free(struct kmem_cache *s, void > > *object) > > return; > > > > /* RCU slabs could be legally used after free within the > > RCU period */ > > - if (unlikely(s->flags & (SLAB_TYPESAFE_BY_RCU | > > SLAB_POISON))) > > + if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU)) > > return; > > /* > > * If there's a constructor, freed memory must remain in > > the same state > > diff --git a/mm/slub.c b/mm/slub.c > > index 63d281dfacdb..8d9aa4d7cb7e 100644 > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -1024,7 +1024,8 @@ static __printf(3, 4) void slab_err(struct > > kmem_cache *s, struct slab *slab, > > add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE); > > } > > > > -static void init_object(struct kmem_cache *s, void *object, u8 > > val) > > +__no_sanitize_memory static void > > __no_sanitize_memory should be used with great care, because it drops > all instrumentation from the function, and any shadow writes will be > lost. > Won't it be better to add kmsan_poison() to init_object() if you want > it to stay uninitialized? I wanted to avoid a ping-pong here, in which we already have properly poisoned memory, then memset() incorrectly unpoisons it, and then we undo the damage. My first attempt involved using __memset() instead, but this resulted in worse assembly code. I wish there were something like memset_noinstr(). Right now init_object() doesn't seem to be doing anything besides these memset()s, but this can of course change in the future. So I don't mind using kmsan_poison() instead of __no_sanitize_memory here too much, since it results in better maintainability.
