Re: Dereference before NULL check (src/backend/storage/ipc/latch.c)

At Mon, 2 Nov 2020 16:22:09 +1300, Thomas Munro <thomas.munro@gmail.com> wrote in 
> On Mon, Nov 2, 2020 at 1:49 PM Kyotaro Horiguchi
> <horikyota.ntt@gmail.com> wrote:
> > At Sat, 31 Oct 2020 11:40:53 -0300, Ranier Vilela <ranier.vf@gmail.com> wrote in
> > > Per Coverity.
> > >
> > > If test set->latch against NULL, is why it can be NULL.
> > > ResetEvent can dereference NULL.
> >
> > If the returned event is WL_LATCH_SET, set->latch cannot be NULL. We
> > shouldn't inadvertently ignore the unexpected or broken situation.We
> > could put Assert instead, but I think that we don't need do something
> > here at all since SIGSEGV would be raised at the right location.
> 
> Hmm.  I changed that to support set->latch == NULL, so that you can
> use the long lived WES in the rare code paths that call WaitLatch()
> without a latch (for example the code I proposed at [1]).  The Windows

Ooo.  We don't update epoll events in that case. Ok, I understand
WL_LATCH_SET can fire while set->latch == NULL.

(I was confused by WaitEventAdjust* asserts set->latch != NULL for
 WL_LATCH_SET. Shouldn't we move the check from ModifyWaitEvent() to
 WaitEventAdjust*()?))

> version leaves the event handle of the most recently used latch in
> set->handles[n] (because AFAICS there is no way to have a "hole" in
> the handles array).  The event can fire while you are waiting on "no
> latch".  Perhaps it should be changed to
> ResetEvent(set->handles[cur_event->pos + 1])?
> 
> [1] https://www.postgresql.org/message-id/flat/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com

Seems right. Just doing that *seems* to work fine, but somehow I
cannot build on Windows for now...

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center