linux-kernel - Re: 2.6.28, rlimits, performance and debian etch

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Message-ID: <87fxj74pol.fsf@mid.deneb.enyo.de>
Date:	Sun, 25 Jan 2009 11:59:06 +0100
From:	Florian Weimer <fw@...eb.enyo.de>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	linux-kernel@...r.kernel.org, DSA <debian-admin@...ts.debian.org>,
	team@...urity.debian.org, libpam-modules@...kages.debian.org
Subject: Re: 2.6.28, rlimits, performance and debian etch

* David Daney:

> One problem is that for values of RLIMIT_NOFILE less than something
> like 4096, it is much faster to call sys_close() on all possible
> values than iterate through a handful of open files from /proc/self/fd
> using opendir(3)/readdir(3).

Really?  Yuck.

> The real solution is to convert your user space programs to use the
> new syscalls that allow for race-free setting of close-on-exec.
> Then you no longer need to mess around with iterating over these
> things.

These system calls are too recent to use in the next two Debian
releases.  In addition, we can't really be sure that all libraries use
the new calls.

I find the design of the CLOEXEC business somewhat revulsive, by the
way.  It reminds me of those DoSomethingEx APIs in another platform.
The *_at system calls are a similar wart.  Even with this stuff, I
still can't safely open a file with a different effective user ID in a
multithreaded application, or create a AF_UNIX socket with specific
file system permissions.  Some thread-specific context with what have
been traditionally considered per-process attributes might have been
better. 8-(
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