Re: [PATCH 0/5] fuse: handle release synchronously (v4)

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Oct 18, 2014 at 08:40:05AM -0700, Linus Torvalds wrote:
> On Sat, Oct 18, 2014 at 8:35 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Look around for AIO. Look around for the loop driver. Look around for
> > a number of things that do "fget()" and that you completely ignored.
> 
> .. actually, there are more instances of "get_file()" than of
> "fget()", the aio one just happened to be the latter form. Lots and
> lots of ways to get ahold of a file descriptor that keeps it open past
> the "last close".

FWIW, procfs patch touches a very annoying issue: ->show_fdinfo() being
blocking.  I would really like to get rid of that particular get_file()
and even more so - of get_files_struct() in there.

I certainly agree that anyone who expects that close() means the end of IO
is completely misguided.  Mappings don't disappear on close(), neither does
a descriptor returned by dup(), or one that child got over fork(),
or something sent over in SCM_RIGHTS datagram, or, as you suggested, made
backing store for /dev/loop, etc.

What's more, in the example given upthread, somebody might've spotted that
file in /proc/<pid>/fd/* and *opened* it.  At which point umount would
have to fail with EBUSY.  And the same lsof(8) might've done just that.

It's not a matter of correctness or security, especially since somebody who
could do that, could've stopped your process, PTRACE_POKEd a fairly short
series of syscalls that would connect to AF_UNIX socket, send the file
over to them and clean after itself, then single-stepped through all of that,
restored the original state and resumed your process.  

It is a QoI matter, though.  And get_files_struct() in there is a lot more
annoying than get_file()/fput().  Suppose you catch the process during
exit().  All of a sudden, read from /proc/<pid>/fdinfo/<n> ends up doing
shitloads of filp_close().  It would be nice to avoid that.

Folks, how much pain would it be to make ->show_fdinfo() non-blocking?