linux-kernel - Re: [PATCH] utimensat() non-conformances and fixes [v3]

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Message-ID: <cfd18e0f0806030427p22a274dev746e1656662f34f6@mail.gmail.com>
Date:	Tue, 3 Jun 2008 13:27:42 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Al Viro" <viro@...iv.linux.org.uk>
Cc:	"Miklos Szeredi" <miklos@...redi.hu>, drepper@...hat.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-man@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] utimensat() non-conformances and fixes [v3]

On Tue, Jun 3, 2008 at 1:22 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Jun 03, 2008 at 01:13:00PM +0200, Miklos Szeredi wrote:
>> > > I'm not sure of the correct way to get the required nameidata (to do a
>> > > vfs_permission() call) from the file descriptor.  Can you give me a
>> > > tip there?
>> >
>> > Could you point me at the right way of doing this?
>>
>> You don't need nameidata for this at all.  Just call permission() with
>> a NULL nameidata.
>>
>> Ugly API?  Yes, will be cleaned up if we manage to find some common
>> ground with the VFS maintainers.
>
> As soon as I'm done with sysctls...
>
> FWIW, I very much doubt that you are right wrt required permissions, though.
> AFAICS, intent here is "if you can write to file, you can touch the timestamps
> anyway" and having descriptor opened for write gives that, current permissions
> be damned.

The standard is pretty clear on this point:

[[
Only a process with the effective user ID equal to the user ID of the
file, or with write access to the file, or with appropriate privileges
may use futimens( ) or utimensat( ) with a null pointer as the times
argument or with both tv_nsec fields set to the special value
UTIME_NOW.
]]

The crucial words here are "a process ... with write access to the
file" -- in other words, the permissions are determined by the
process's credentials, not by the access mode of the file descriptor.
I was not 100% sure on that to start with, so I did check it out with
one of the folk at The Open Group, to make sure of my understanding.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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