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

On Fri, Feb 17, 2017 at 12:30 AM, Cyrill Gorcunov [off-list ref] wrote:

When we checkpoint a process we look into /proc/<pid>/fdinfo/<fd> of eventpoll
file and parse target files list from there. In most situations this is fine
because target file is present in the /proc/<pid>/fd/ list. But in case if file
descriptor was dup'ed or transferred via unix socket and closed after,
it might not be in the list and we can't figure out which file descriptor
to pass into epoll_ctl call.

To resolve this tie lets add EPOLL_CTL_DUP operation which simply takes
target file descriptor number and installs it into a caller's file table,
thus we can use kcmp() syscall and figure out which exactly file to be
added into eventpoll on restore procedure.

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.