Re: [PATCH] events: add a flag to perf_event_open() to set O_CLOEXEC

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 30, 2013 at 10:35:50PM +0100, Yann Droneaud wrote:
> This patch adds PERF_FLAG_FD_CLOEXEC flag for
> perf_event_open() syscall.
> 
> perf_event_open() creates a new file descriptor,
> but unlike open() syscall, it lack a flag to atomicaly
> set close-on-exec (O_CLOEXEC).
> 
> Not using O_CLOEXEC by default and not letting userspace
> provide the "open" flags should be avoided: in most case
> O_CLOEXEC must be used to not leak file descriptor across
> exec().
> 
> Using O_CLOEXEC when creating a file descriptor allows
> userspace to set latter, using fcntl(), without any risk
> of race, if the file descriptor is going to be inherited
> or not across exec().
> 
> Link: http://lkml.kernel.org/r/94b641a81a06ba4943cf77e80bc271c8@meuh.org
> Link: http://lkml.kernel.org/r/cover.1383121137.git.ydroneaud@opteya.com
> Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
> ---
> 
> Hi Peter,
> 
> This patch should replaces
> "[PATCH v4 6/7] events: use get_unused_fd_flags(0) instead of get_unused_fd()"
> 
> Please have a look.

I'm still terminally confused as to all of this... Why does it matter
what the default is if you can change it with fcntl() ? Also, how can
you tell nobody relies on the current behaviour and its therefore safe
to change?