linux-kernel - Re: Update of file offset on write() etc. is non-atomic with I/O

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Message-ID: <20140305000411.GV18016@ZenIV.linux.org.uk>
Date:	Wed, 5 Mar 2014 00:04:11 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	George Spelvin <linux@...izon.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O

On Tue, Mar 04, 2014 at 01:17:50PM -0800, Linus Torvalds wrote:
> On Tue, Mar 4, 2014 at 12:00 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > OK, with the attached set (the first one is essentially unchanged from
> > your first one), it seems to work and produce better code on all targets
> > I've tried.  Comments?
> 
> I'm certainly ok with it. You seem to have left the fput_light()
> function around, though, despite removing fget_[raw_]light(). That
> seems a bit silly, since there is no valid use any more apart from
> net/socket.c that now doesn't balance things properly.

There's also a pile of crap around sockfd_lookup/sockfd_put, related
to that.   Moreover, there's net/compat.c, which probably ought to
have the compat syscalls themselves moved to net/socket.c (under
ifdef CONFIG_COMPAT) and switched to sockfd_lookup_light().
There's l2tp_tunnel_sock_lookup(), which is simply broken - it assumes
that if tunnel->fd still resolves to a socket, that socket must
be l2tp one.  Trivial to drive into BUG_ON(), in queue_work() callback,
no less...  There's bluetooth, assuming that pretty much the same
(that if it got a file descriptor that resolves to a socket, it must
be a bluetooth one).  BTW, I wonder what will happen if one gives
iscsi_sw_tcp_conn_bind() descriptor of a socket of sufficiently
weird sort...

Then there's staging/usbip with its sockfd_to_socket(), which is more or
less parallel to sockfd_lookup().  And open-coded analogs in nbd and
ncpfs...

> > I've also pushed those (on top of old ocfs2 fix) into vfs.git#for-linus,
> > if you prefer to read it that way.  Should propagate in a few...
> 
> Should I pull?
> 
> I also get the feeling that the first patch should likely be marked
> for stable. Hmm?

It should; I'll mark it such when I send a pull request.  I really want
to sort the situation with sockfd_lookup() and friends out, though - at
least to the point where I would understand how painful the fixes
will be.
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