Re: [Qemu-devel] [PATCH] linux-user: implement pipe2 syscall

[Riku Voipio <riku.voipio@iki.fi>]

On Tue, May 05, 2009 at 11:58:09PM +0100, Jamie Lokier wrote:
> Riku Voipio wrote:
> > implement pipe2 syscall. instead of calling pipe2 directly
> > (which was introduced in 2.6.27), emulate the flag functionality
> > with fcntl.

> This is wrong with multiple threads if the flag contains FD_CLOEXEC.
> If that situation is possible, please don't do this.

> The point of pipe2() with FD_CLOEXEC is to be atomic: make sure
> another thread can never see the file descriptor with FD_CLOEXEC not set.

> If you can't guarantee that, it's better to return ENOSYS as every
> application using pipe2() like this has a fallback to use pipe() and
> FD_CLOEXEC itself, and probably has application logic to protect
> against the race condition.

> If there's only one thread, or if you can arrange to block any
> concurrent clone/fork/execve calls in other threads (in QEMU) during
> the race window, then it's fine to emulate it with fcntl.

We haven't returned from the pipe2 syscall when setting the flag with fcntl.
Before returning from the syscall, the pipe file descriptors could point
to anything (unitialized memory, zeros, ...)