linux-kernel - Re: [RFC PATCH] fs: allow open(dir, O

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Date:	Thu, 30 Oct 2014 17:57:24 -0700
From:	Eric Rannaud <e@...ocritical.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

On Thu, Oct 30, 2014 at 3:58 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Oct 30, 2014 at 3:48 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> Agreed. Will apply and add the stable cc.
>
> Ho humm. Thinking about this some more, I'm starting to wonder. Not
> about this patch per se (open on a newly created file should indeed
> succeed regardless), but about the horrible glibc behavior of screwing
> up the third argument.
>
> If you want to do O_TMPFILE + linkat() (or some eventual future
> flink()), the mode really matters. So this idiotic glibc behavior of
> only forwarding the third argument if O_CREAT is set seems to be a
> bug.

Yes, there definitely is a glibc bug: a fix is being worked on and it
looks like it will go in. The change replaces the test for O_CREAT by
a test for either O_CREAT or O_TMPFILE.

> Why the hell does glibc think it's a good idea to intersect system
> call semantics? It's not a good idea - it's just stupid in the
> extreme. And in this case it seems to actively breaks things.

What Andy said: it's the most portable, because of the optional last
argument, if you have a bunch of wrappers for open and openat written
in C, which they do.

Some of these wrappers are for statically checking that the mode
argument is present when necessary (using __builtin_constant_p,
__builtin_va_arg_pack, and friends), and to "fortify" the code against
a malicious (?) injection of O_CREAT in a code path that didn't have
it at compile time (where flags was a build constant). Other wrappers
add O_LARGEFILE. One wrapper for openat checks that dirfd is a
directory (no idea why, as the kernel does the same check -- without a
race; maybe to try to guarantee ENOTDIR on some old kernel version?
It's been here since 2005, when openat was added to glibc).

-- 
Eric Rannaud <e@...ocritical.com>
Nanocritical, CEO
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