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Re: [Qemu-devel] [PATCH] Don't leak file descriptors

From: Jamie Lokier
Subject: Re: [Qemu-devel] [PATCH] Don't leak file descriptors
Date: Mon, 16 Nov 2009 02:15:39 +0000
User-agent: Mutt/1.5.13 (2006-08-11)

Scott Tsai wrote:
> On Fri, Nov 13, 2009 at 11:17 PM, Kevin Wolf <address@hidden> wrote:
> > We're leaking file descriptors to child processes. Set FD_CLOEXEC on file
> > descriptors that don't need to be passed to children to stop this 
> > misbehaviour.
> Since qemu is a multi threaded program, how about opening those file
> descriptors with the equivalent of O_CLOEXEC set to avoid the race
> condition when a fork comes between the 'open/socket/accept' operation
> and the 'fcntl'?

For qemu-system this shoudn't matter, as long as all the forking and
opening is done in the same thread, with other threads only used for
virtual CPUs.

For qemu-user, maybe it's relevant?

> We could create helper functions like 'qemu_socket_cloexec'.
> The implementation of qemu_socket_cloexec would use the new system
> calls and flags listed in:
> http://udrepper.livejournal.com/20407.html
> if available and fall back to separate 'open' and 'fcnt' operations
> when not building with a new enough glibc.

To do it thoroughly, it's possible to emulate O_CLOEXEC's
thread-safety, by blocking fork operations in other threads during an
open+fcntl sequence using an rwlock, or without locking by keeping
track of "possibly open" file descriptors and closing them
unconditionally in fork children.

That would help with qemu-user emulation of O_CLOEXEC (et al) too.

-- Jamie

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