netdev - Fwd: Cross thread shutdown of connected UDP socket, unexpected recvfrom behavior

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Message-ID: <CAApbN=K8j+XQG+pQF2fxBSrDEPcjD242e9uP72azD9XWBzHcGQ@mail.gmail.com>
Date:	Wed, 13 Nov 2013 12:07:20 -0800
From:	mpb <mpb.mail@...il.com>
To:	netdev@...r.kernel.org
Subject: Fwd: Cross thread shutdown of connected UDP socket, unexpected
 recvfrom behavior

Hi netdev@...r.kernel.org,

Someone on LKML suggested that I forward this issue to the netdev list.

In a nutshell, recvfrom returns ambiguous results, and it is
impossible to distinguish between these two situations:

a) calling recvfrom and receiving a zero length UDP message
b) calling recvfrom and blocking until the socket is shutdown (locally)

More details below.

Thanks!

-mpb

---------- Forwarded message ----------
From: mpb <mpb.mail@...il.com>
Date: Sat, Nov 9, 2013 at 5:32 PM
Subject: Cross thread shutdown of connected UDP socket, unexpected
recvfrom behavior
To: linux-kernel@...r.kernel.org

Hi LKML,

I have a C/pthreads program with two threads ("main" and "thread").
"thread" calls recvfrom on a connected UDP socket and blocks, waiting
for input.  "main" then calls shutdown SHUT_RD on the socket.  In
"thread", recvfrom returns, apparently successfully, returning a zero
length string, and setting src_addr to a bogus(?) address family,
port, and source address.

Is the above the correct behavior for recvfrom?

Here is output from my program.  "main" sends "hello\0", then "" (the
empty string), then calls shutdown.  "thread" receives "hello\0", "",
and then finally receives an empty string that was never sent!

thread  recvfrom: Success
  rv 6  addrlen 16  fam 2  port 8000  addr 100007f

thread  recvfrom: Success
  rv 0  addrlen 16  fam 2  port 8000  addr 100007f

main    shutdown: Success
  rv 0

thread  recvfrom: Success
  rv 0  addrlen 16  fam 59060  port 44237  addr deaadef0

The source code (2k) for the porgram is attached.  I'm running Ubuntu
13.04, kernel 3.8.0-19-generic #30-Ubuntu, on 32-bit i686.

For reference, this June 2000 LKML thread discusses calling close in a
similar situation.
http://www.gossamer-threads.com/lists/linux/kernel/144379

Please CC me if you have questions, otherwise I'll try to watch for
answers in the list archives.

Thanks!

-mpb

View attachment "test.c" of type "text/x-csrc" (1861 bytes)