linux-kernel - Re: [PATCH 15/15] kasan: use stack_depot

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Message-ID: <CANpmjNPVu10Y+gO=r3eaU9GP8VL_dqmch3QQXYX8g9D-+HjVPg@mail.gmail.com>
Date:   Mon, 4 Sep 2023 20:58:50 +0200
From:   Marco Elver <elver@...gle.com>
To:     Andrey Konovalov <andreyknvl@...il.com>
Cc:     andrey.konovalov@...ux.dev,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        Evgenii Stepanov <eugenis@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 15/15] kasan: use stack_depot_evict for tag-based modes

On Mon, 4 Sept 2023 at 20:48, Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> On Wed, Aug 30, 2023 at 11:38 AM Marco Elver <elver@...gle.com> wrote:
> >
> > > --- a/mm/kasan/tags.c
> > > +++ b/mm/kasan/tags.c
> > > @@ -96,7 +96,7 @@ static void save_stack_info(struct kmem_cache *cache, void *object,
> > >                       gfp_t gfp_flags, bool is_free)
> > >  {
> > >       unsigned long flags;
> > > -     depot_stack_handle_t stack;
> > > +     depot_stack_handle_t stack, old_stack;
> > >       u64 pos;
> > >       struct kasan_stack_ring_entry *entry;
> > >       void *old_ptr;
> > > @@ -120,6 +120,8 @@ static void save_stack_info(struct kmem_cache *cache, void *object,
> > >       if (!try_cmpxchg(&entry->ptr, &old_ptr, STACK_RING_BUSY_PTR))
> > >               goto next; /* Busy slot. */
> > >
> > > +     old_stack = READ_ONCE(entry->stack);
> >
> > Why READ_ONCE? Is it possible that there is a concurrent writer once the
> > slot has been "locked" with STACK_RING_BUSY_PTR?
> >
> > If there is no concurrency, it would be clearer to leave it unmarked and
> > add a comment to that effect. (I also think a comment would be good to
> > say what the WRITE_ONCE below pair with, because at this point I've
> > forgotten.)
>
> Hm, I actually suspect we don't need these READ/WRITE_ONCE to entry
> fields at all. This seems to be a leftover from the initial series
> when I didn't yet have the rwlock. The rwlock prevents the entries
> from being read (in kasan_complete_mode_report_info) while being
> written and the try_cmpxchg prevents the same entry from being
> rewritten (in the unlikely case of wrapping during writing).
>
> Marco, do you think we can drop these READ/WRITE_ONCE?

Yes, I think they can be dropped.