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Subject[2.4][2.2] Bug: accept discards socket options/O_NONBLOCK
Please mind the To: and Cc: headers. If there are relevant followups,
please send me a Cc: as I'm only subscribed to the linux-kernel and dns
mailing lists.

In some debugging, Pavel Kankovsky and Daniel J. Bernstein have tracked
down a Linux Kernel bug that I can confirm for 2.2.17 and 2.4.0-test8 (I
did not try 2.0.38, don't have gcc 2.7.2.3 installed). I have copies of
their mail (without Received: headers) available (see below).

BUG DESCRIPTION:
(This is for IPv4, someone would have to check IPv6 as well).
The socket flag O_NONBLOCK is _NOT_ properly inherited through an
accept(2) call, in spite of what socket(7) documents. This is a bug.
accept(2) must copy the file descriptor's flag of the socket.

SOCKET(7) EXCERPT: (from man pages 1.31)
It is possible to do non-blocking IO on sockets by setting the
O_NONBLOCK flag on a socket file descriptor using fcntl(2).
O_NONBLOCK is inherited through an accept. Then all operations
that would normally block will (usu­ ally) return with EAGAIN;
connect(2) returns an EIN­ PROGRESS error in this case. The user
can then wait for various events via poll(2) or select(2).

EXAMPLE:
At http://home.pages.de/~mandree/socktest.c, there is a small test
program (3 kB) that is not really portable, and that relies on
dnscache for library functions (it's a quick hack).

Build it on Linux 2.2 or Linux 2.4.0-test8 and see this: (I'm running
./socktest 1234 & telnet localhost 1234)

options on socket (fd=3): O_NONBLOCK
options after accept (fd=6):
accepted from 127.0.0.1:1068

It SHOULD be: (FreeBSD 4.0 gets it right):
options on socket (fd=3): O_NONBLOCK
options after accept (fd=4): O_NONBLOCK
accepted from 127.0.0.1:1053

To compile the program on FreeBSD 4, comment out the line with O_SYNC
and insert #include <poll.h>; compilation instructions are at the head
of that file.

REFERENCES:
test program is at
http://home.pages.de/~mandree/socktest.c
dnscache source code is at
http://cr.yp.to/djbdns/dnscache-1.00.tar.gz (78 kB).
Pavel's mail is at
http://home.pages.de/~mandree/20000904102753.2572.0%40argo.troja.mff.cuni.cz
Dan's mail is at
http://home.pages.de/~mandree/20000904193940.20671.qmail%40cr.yp.to

--
Matthias Andree
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