Re: NetBSD Security Advisory 1999-001: select(2)/accept(2) race

[richardk () CHIARK GREENEND ORG UK (Richard Kettlewell)]

Luke Mewburn <lukem () GOANNA CS RMIT EDU AU> writes:

> Many TCP servers open a TCP socket in the default blocking mode, use
> select(2) to wait for connections, and then accept(2) connections in
> blocking mode.  Under some circumstances, the accept(2) may hang
> waiting for another connection, denying service to clients trying to
> connect to other ports.
[...]
> Two solutions are possible:
>
> 1) Modify all TCP servers to use non-blocking listening sockets.
>    Unfortunately, this requires changing a large amount of code, much
>    of it maintained by third parties.

This has been in the UNIX Sockets FAQ for quite some time:

    http://kipper.york.ac.uk/~vic/sock-faq/sfaq.html#faq16

| On some other implementations, accept seemed to be capable of
| blocking if this occured. This is important, since if select() said
| the listening socket was readable, then you would normally expect
| not to block in the accept() call. The fix is, of course, to set
| nonblocking mode on the listening socket if you are going to use
| select() on it.

ttfn/rjk