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

[ebiederm@xmission.com (Eric W. Biederman)]

Al Viro <viro@ZenIV.linux.org.uk> writes:

> 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?

I took a quick look and there are a couple of instances in tun,
eventpoll, and fanotify/inotify that take a spinlock while traversing
the data that needs to be printed.

So it would take a good hard stare at those pieces of code to understand
the locking, and potentially rewrite those routines.

The only one I am particularly familiar with tun did not look
fundamentally hard to change but it also isn't something I would
casually do either, as it would be easy to introduce nasty races by
accident.

Eric