linux-kernel - Re: [RFC][PATCH 8/9] vfs: Implement generic revoked file operations

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Message-ID: <20090412203107.GH4394@shareable.org>
Date:	Sun, 12 Apr 2009 21:31:07 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Hugh Dickins <hugh@...itas.com>, Tejun Heo <tj@...nel.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [RFC][PATCH 8/9] vfs: Implement generic revoked file operations

Eric W. Biederman wrote:
> >> revoked_file_ops return 0 from reads (aka EOF). Tell poll the file is
> >> always ready for I/O and return -EIO from all other operations.
> >
> > I think read should return -EIO too.  If a program is reading from a
> > /proc file (say), and the thing it's reading suddenly disappears, EOF
> > gives the false impression that it's read to the end of formatted data
> > from that file and it can process the data as if it's complete, which
> > is wrong.
> 
> Good point EIO is the current read return value for a removed proc file.
> 
> For closed pipes, and hung up ttys the read return value is 0, and from
> my reading that is what bsd returns after a sys_revoke.

A few suggestions below.  Feel free to ignore them on account of the
basic revoking functionality being more important :-)

I'm not sure a revoked pipe should look like a normally closed one.
ECONNRESET?

For hung up ttys, I agree.  But where's the SIGHUP :-) You probably do
want the process using it to die if it's not handling SIGHUP, because
terminal-using processes don't always terminate themselves on EOF.

For things writing to a pipe or file, SIGPIPE may be appropriate in
addition to EIO, to avoid runaway processes.  Looks odd I know.  For
writing to a terminal, SIGHUP again.

> The reason I have f_op settable is because I never expected complete
> agreement on the return codes, and because it makes auditing and spotting
> this kind of thing easier.
>
> I guess I should make two variations on revoked_file_ops then.  Say
> eof_file_ops, eio_file_ops.  Identical except for their treatment of
> reads.

Fair enough.  It's good to have good defaults.  I'm not convinced
eof_file_ops is ever a good default.  sighup_file_ops and
sigpipe_file_ops maybe :-)

-- Jamie
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