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

[Jamie Lokier <jamie@shareable.org>]

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