linux-kernel - Re: /proc/pid/fd && anon_inode

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Date:	Sun, 25 Aug 2013 06:23:17 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Willy Tarreau <w@....eu>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"security@...nel.org" <security@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: /proc/pid/fd && anon_inode_fops

On Sat, Aug 24, 2013 at 11:24:32PM +0200, Willy Tarreau wrote:

> I doubt it. It seems to me that most such entries are implemented
> for completeness while most valid uses only concern /proc/self/fd.
> Maybe if we had an option so that only /proc/self/fd would actually
> allow to access the fds while all /proc/pid/fd would only show what
> they map to, it would be a good step forward.

How?  The fundamental problem is not visibility of that stuff, it's
new opened file for the same object (Linux behaviour) vs. new descriptor
refering to the same opened file (*BSD and friends).  We can't get
anon_... sanely reopened in the former semantics and they are very
visibly different for regular files, so switching to *BSD one is not
feasible - too high odds of userland breakage.  The difference in
semantics, of course, is that on Linux opening /dev/stdin gives you
a descriptor with independent current IO position; on *BSD you get
a descriptor sharing the current IO position with stdin.  IOW, it's
independent open() of the same file vs. dup().

We are really stuck with the current semantics here - switching to
*BSD one would not only mean serious surgery on descriptor handling
(it's one of the wartier areas in *BSD VFS, in large part because
of magic-open-really-a-dup kludges they have to do), it would change
a long-standing userland API that had been there for nearly 20 years
_and_ one that tends to be used in corner cases of hell knows how many
scripts.
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