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Date:   Mon, 14 Nov 2022 15:54:30 +0000
To:     Thomas Gleixner <tglx@linutronix.de>
From:   =?utf-8?Q?Barnab=C3=A1s_P=C5=91cze?= <pobrn@protonmail.com>
Cc:     linux-kernel@vger.kernel.org, John Stultz <jstultz@google.com>
Subject: Re: [PATCH v1] timerqueue: use rb_entry_safe() in timerqueue_getnext()
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Hi


2022. november 14., h=C3=A9tf=C5=91 1:17 keltez=C3=A9ssel, Thomas Gleixner =
=C3=ADrta:

> On Thu, Oct 27 2022 at 21:37, Barnab=C3=A1s P=C5=91cze wrote:
>=20
> > When `timerqueue_getnext()` is called on an empty timerqueue
> > the returned rb_node pointer will be NULL. Using `rb_entry()`
> > on a potentially NULL pointer will only - coincidentally - work
> > if the offset of the rb_node member is 0. This is currently the
> > case for `timerqueue_node`, but should this ever change,
> > `timerqueue_getnext()` will no longer work correctly on empty
> > timerqueues. To avoid this, switch to using `rb_entry_safe()`.
>=20
> I agree with the change but not with the argumentation above.
>=20
> Whatever the current offset of node is does not matter at all,
> really. This is a blantant missing NULL pointer check which works by
> chance.
>=20
> So there is no weasel wording justfied about "coincidentally" and "might
> not longer work correctly".
>=20
> Just spell it out that this is a blantant bug and nothing else.

I agree, I was just trying to describe why it has not caused any issues yet=
.

Would this be better?

  When `timerqueue_getnext()` is called on an empty timer queue, it will
  use `rb_entry()` on a NULL pointer, which is invalid. Fix that by using
  `rb_entry_safe()` which handles NULL pointers.
 =20
  This has not caused any issues so far because the offset of the `rb_node`
  member in `timerqueue_node` is 0, so `rb_entry()` is essentially a no-op.


>=20
> Back then when that code got introduced rb_entry_safe() did not exist at
> all so it's even more obvious that this is simply a missing NULL pointer
> check, right?

As far as I can tell it did exist and it was actually used
when the offending change was committed (511885d7061e).


>=20
> This also requires a Fixes: tag which flags the commit which introduces
> this bug.
>=20
> Thanks,
>=20
>         tglx
>=20


Regards,
Barnab=C3=A1s P=C5=91cze