uretprobes/x86: Hijack return address

  - Hijack the return address and replace it with a trampoline address.

v1 changes:
  - use force_sig_info()
  - rework and simplify logic

RFCv5 changes:
  - change the fail return code, because orig_ret_vaddr=0 is possible
  - style fixup
RFCv2 changes:
  - remove ->doomed flag, kill task immediately

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: Limit the depth of return probe nestedness

  Unlike the kretprobes we can't trust userspace, thus must have
protection from user space attacks, this patch limits the return
probes nestedness as a simple remedy for it. Plus the user-space
have the "unlimited" stack, it looks simply impossible to handle
the deep recursion correctly.

  The intention is to have KISS and bare minimum solution for the
initial implementation in order to not complicate the uretprobes
code.

  In the future we may come up with more sophisticated solution that
should remove this depth limitation, however it is not easy task
and lays beyond this patchset. It should consider things like: breakpoint
address lays outside the stack and stack growth direction, longjmp,
sigaltstack... be able to clean up return instances.

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: Remove -ENOSYS as return probes implemented

  Enclose return probes implementation.

Signed-off-by: Anton Arapov <anton@redhat.com>