linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Date:	Sun, 30 Sep 2007 01:31:49 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: F_DUPFD_CLOEXEC implementation

Hi Ulrich,

On Friday 28 September 2007 18:34, Ulrich Drepper wrote:
> One more small change to extend the availability of creation of
> file descriptors with FD_CLOEXEC set.  Adding a new command to
> fcntl() requires no new system call and the overall impact on
> code size if minimal.

Tangential question: do you have any idea how userspace can
safely do nonblocking read or write on a potentially-shared fd?

IIUC, currently it cannot be done without races:

old_flags = fcntl(fd, F_GETFL);
...other process may change flags!...
fcntl(fd, F_SETFL, old_flags | O_NONBLOCK);
read(fd, ...)
...other process may see flags changed under its feet!...
fcntl(fd, F_SETFL, old_flags);

Can this be fixed?
--
vda
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