Re: UDP recvmsg blocks after select(), 2.6 bug?

John Pearson
2004-10-19T01:26:26Z

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This has probably been said before, but just in case I missed
it; there are many viewpoints in this thread, but the two main camps
appear to be arguing at cross-purposes.

As far as I can see:

- YES, Linux select 'lies' and violates POSIX wrt checksums:
a call to recvmsg() might well have blocked when select()
said data was ready, as a result of a currupt UDP packet;

- NO, 'fixing' select() won't guarantee that recvmsg()
will not block/return EAGAIN, because select() only
guarantees that a call to recvmsg() would not have blocked
at that time - as others have observed, it cannot guarantee
that 'valid' data won't subsequently be discarded; any
subsequent call to recvmsg() is only 'immediate' in a fuzzy,
imprecise and inadequate sense.

Can we get back to arguing about something less repetitive
(or at least, make the circle larger and more scenic)?

John.

On Sun, Oct 17, 2004 at 07:22:44PM +0200, Lars Marowsky-Bree wrote
> On 2004-10-17T16:17:06, Buddy Lucas <buddy.lucas@gmail.com> wrote:
>
> > > The SuV spec is actually quite detailed about the options here:
> > >
> > > A descriptor shall be considered ready for reading when a call
> > > to an input function with O_NONBLOCK clear would not block,
> > > whether or not the function would transfer data successfully.
> > > (The function might return data, an end-of-file indication, or
> > > an error other than one indicating that it is blocked, and in
> > > each of these cases the descriptor shall be considered ready for
> > > reading.)
> > But it says nowhere that the select()/recvmsg() operation is atomic, right?
>
> See, Buddy, the point here is that Linux _does_ violate the
> specification. You can try weaseling out of it, but it's not going to
> work.
>
> This isn't per se the same as saying that it's not a sensible violation,
> but very clearly the specs disagree with the current Linux behaviour.
>
> It's impossible to claim that you are allowed by the spec to block on a
> recvmsg directly following a successful select. You are not. You could
> claim that, but you'd be wrong.
>
> If the packet has been dropped in between, which _could_ have happened
> because UDP is allowed to be dropped basically anywhere, EIO may be
> returned. But blocking or returning EAGAIN/EWOULDBLOCK is verboten. The
> spec is very clearly on that.
>
> (Now I'd claim that returning EIO after a succesful select is also
> slightly suboptimal - the performance optimizations should be turned off
> for blocking sockets, IMHO, and the data which caused the select() to
> return should be considered comitted - but it would be allowed.)
>
> I'm not so sure what's so hard to accept about that. It may be well that
> Linux is following the de-facto industry standard (or even setting it)
> here, and I'd agree that if you don't want blocking use O_NONBLOCK, but
> in no way can Linux claim POSIX/SuV spec compliance for this behaviour.
>
> I'm not getting why people argue so much to try and weasel the words so
> that it comes out as compliant. It's not. It may make sense due to
> practical reasons, but it's not compliant.
>
>
> Sincerely,
> Lars Marowsky-Brée <lmb@suse.de>
>
> --
> High Availability & Clustering
> SUSE Labs, Research and Development
> SUSE LINUX AG - A Novell company
>
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