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

[Jason Baron]

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.

Thanks,

-Jason