Re: Thinko in processing of SHM message size info?

Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Aug 6, 2015 at 1:24 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Thu, Aug 6, 2015 at 9:04 AM, Antonin Houska <ah(at)cybertec(dot)at> wrote:
> >> Can anyone please explain why the following patch shouldn't be applied?
> >>
> >> diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
> >> index 126cb07..4cd52ac 100644
> >> --- a/src/backend/storage/ipc/shm_mq.c
> >> +++ b/src/backend/storage/ipc/shm_mq.c
> >> @@ -584,7 +584,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
> >> if (mqh->mqh_partial_bytes + rb > sizeof(Size))
> >> lengthbytes = sizeof(Size) - mqh->mqh_partial_bytes;
> >> else
> >> - lengthbytes = rb - mqh->mqh_partial_bytes;
> >> + lengthbytes = rb;
> >> memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata,
> >> lengthbytes);
> >> mqh->mqh_partial_bytes += lengthbytes;
> >>
> >>
> >> I'm failing to understand why anything should be subtracted. Note that the
> >> previous iteration must have called shm_mq_inc_bytes_read(), so "rb" should
> >> not include anything of mqh->mqh_partial_bytes. Thanks.
> >

> > Hmm, I think you are correct. This would matter in the case where the
> > message length word was read in more than two chunks. I don't *think*
> > that's possible right now because I believe the only systems where
> > MAXIMUM_ALIGNOF < sizeof(Size) are those with MAXIMUM_ALIGNOF == 4 and
> > sizeof(Size) == 8. However, if we had systems where MAXIMUM_ALIGNOF
> > == 4 and sizeof(Size) == 16, or systems where MAXIMUM_ALIGNOF == 2 and
> > sizeof(Size) == 8, this would be a live bug.

I ought to admit that I didn't think about the specific combinations of
MAXIMUM_ALIGNOF and sizeof(Size), and considered the problem rather rare
(maybe also because it can't happen on my workstation). But the next your
consideration makes sense to me:

> Hmm, actually, maybe it is a live bug anyway, because the if statement
> tests > rather than >=. If we've read 4 bytes and exactly 4 more
> bytes are available, we'd set lengthbytes to 0 instead of 4. Oops.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at