While doing the checkpoint-restore in the user space one need to determine
whether various kernel objects (like mm_struct-s of file_struct-s) are
shared between tasks and restore this state.

The 2nd step can be solved by using appropriate CLONE_ flags and the
unshare syscall, while there's currently no ways for solving the 1st one.

One of the ways for checking whether two tasks share e.g. mm_struct is to
provide some mm_struct ID of a task to its proc file, but showing such
info considered to be not that good for security reasons.

Thus after some debates we end up in conclusion that using that named
'comparison' syscall might be the best candidate. So here is it --
__NR_kcmp.

It takes up to 5 arguments - the pids of the two tasks (which
characteristics should be compared), the comparison type and (in case of
comparison of files) two file descriptors.

Lookups for pids are done in the caller's PID namespace only.

At moment only x86 is supported and tested.

[akpm@linux-foundation.org: fix up selftests, warnings]
[akpm@linux-foundation.org: include errno.h]
[akpm@linux-foundation.org: tweak comment text]
Signed-off-by: Cyrill Gorcunov
Acked-by: "Eric W. Biederman"
Cc: Pavel Emelyanov
Cc: Andrey Vagin
Cc: KOSAKI Motohiro
Cc: Ingo Molnar
Cc: H. Peter Anvin
Cc: Thomas Gleixner
Cc: Glauber Costa
Cc: Andi Kleen
Cc: Tejun Heo
Cc: Matt Helsley
Cc: Pekka Enberg
Cc: Eric Dumazet
Cc: Vasiliy Kulikov
Cc: Alexey Dobriyan
Cc: Valdis.Kletnieks@vt.edu
Cc: Michal Marek
Cc: Frederic Weisbecker
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds