Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number

From: Jason Baron
Date: Fri Feb 17 2017 - 13:34:16 EST

On 02/17/2017 01:01 PM, Cyrill Gorcunov wrote:
On Fri, Feb 17, 2017 at 09:20:34AM -0800, Andy Lutomirski wrote:

This is a scary thing to let an unprivileged process do.

I'm wondering if there might be a nicer way to address this using a
better interface in /proc.

Well, I tend to agree. Need to add security checking if the target
file is accessable by a caller. As to better interface to procfs
nothing comes to mind immediately. Another potential problem is that
since it is never guaranteed that target file number listed in fdinfo
matching existing /proc/pid/fd/N, so that I think we will have to
use this dup functionality for every target file, which of course
not that fast. Probably need to think more if I manage to invent
some better and faster interface to find where exactly target file
belong in the whole process tree of a container.

i was imagining some proc or proc-like interface that lets you inspect
an open file without needing to know what process has the fd.

We will have to find out which process opened the target fd (currently
in criu we simply assume that target file is always in process which
created epoll and other scenarios are not supported. in most situations
that's enough but unfortunately we find a testcase where it's not
true and have to find a way to support migrated targets too).

What if you introduced a new type of fd that's an "fd reference". You
could add a kcmp mode that tells you whether an fd reference refers to
the same thing as a real fd, but you'd arrange for fd references to be
otherwise useless.

Alternatively, you could simply have an interface like kcmp (maybe a
new kcmp mode) that lets you compare an epoll set entry to an actual
fd. Then you could figure out what it is but only if you already have
the fd by some other means. Of course, if there are no references
left, you still have a problem. Hmm.

kcmp over target set seems to be preferred since it gonna be a way
faster. But I think about fd-ref idea too. Thanks a huge!

I sort of like kcmp() for this as well maybe the epoll fd argument can be a pointer to something like:

struct epoll_slot {
int epfd;
int slot;

Where 'epfd' is the epoll fd, and slot is just an index into the target fds associated with the epoll fd. That is, you would start from 0 and go up to n. When you reach the last one you get in -EBADSLT or something.

Regarding Andy's point of of not having a reference by some other means, I think you have to have it basically otherwise it would be removed from the epfd. I guess it could be being passed via a unix socket? But there must be a way to get at those anyways, in order to restore them.

Also, in the initial patch this function can not restore all target epoll files:

'ep_find_tfd(struct eventpoll *ep, int tfd)'

Because the 'target' file for an epfd is the pair: {fd, file}. Thus, you could have say two different 'target' files with the same fd number.