linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Date:	Sun, 30 Sep 2007 16:11:32 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Denys Vlasenko <vda.linux@...glemail.com>
cc:	Ulrich Drepper <drepper@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: F_DUPFD_CLOEXEC implementation

On Sun, 30 Sep 2007, Denys Vlasenko wrote:

> 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?

I'm not sure I understood correctly your use case. But, if you have two 
processes/threads randomly switching O_NONBLOCK on/off, your problems 
arise not only the F_SETFL time.
If one of the tasks is not expecting an fd to be O_NONBLOCK, that will 
likely end up not handling correctly read/write-miss situations.
In that case it'd be better to keep the fd as O_NONBLOCK, and manually 
create blocking behaviour (when needed) with poll+read/write.



- Davide


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