uretprobes: return probe entry, prepare_uretprobe()

  When a uprobe with return consumer is hit, prepare_uretprobe function is
invoked. It creates return_instance, hijacks return address and replaces
it with the trampoline.

 N.B. it might be a good idea to introduce get_uprobe() to reflect
put_uprobe() later, but it is not a subject of this patchset.

v6:
 - rework prepare_uretprobe() logic in order to make further unwinding
   in handler_uretprobe() simplier.
v5:
 - switch from hlist to simply linked list for tracking ->*return_uprobes.
 - revert (*) from v4.
 - preallocate first slot xol_area for return probes, see xol_get_area()
   changes.
 - add get_trampoline_vaddr() helper, to emphasize area->vaddr overload.
v4:
 - get rid of area->rp_trampoline_vaddr as it always the same as ->vaddr.
 - cleanup ->return_uprobes list in uprobe_free_utask(), because the
   task can exit from inside the ret-probe'd function(s).
 - in find_active_uprobe(): Once we inserted "int3" we must ensure that
   handle_swbp() will be called even if this uprobe goes away. We have
   the reference but it only protects uprobe itself, it can't protect
   agains delete_uprobe().
   IOW, we must ensure that uprobe_pre_sstep_notifier() can't return 0.
v3:
 - protected uprobe with refcounter. See atomic_inc in prepare_uretprobe()
   and put_uprobe() in a following patch in handle_uretprobe().
v2:
 - get rid of ->return_consumers member from struct uprobe, introduce
   rp_handler() in consumer.

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: invoke return probe handlers

  Uretprobe handlers are invoked when the trampoline is hit, on completion the
trampoline is replaced with the saved return address and the uretprobe instance
deleted.

v6:
 - rework handle_uretprobe()
v5:
 - switch to simply linked list ->return_uprobes
 - rework handle_uretprobe()
v4:
 - check, whether utask is not NULL in handle_uretprobe()
   ? do we want a printk() for this case?
 - get rid of area->rp_trampoline_vaddr
 - minor handle_uretprobe() fixups
v3:
 - protected uprobe with refcounter. See put_uprobe() in handle_uretprobe()
that reflects increment in prepare_uretprobe()
v2:
 - get rid of ->return_consumers member from struct uprobe, introduce
rp_handler() in consumer instead

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: implemented, thus remove -ENOSYS

  enclose return probes implementation

v3 changes: (the patch is introduced in v3)
  - remove 'TODO' as return probes implemented now

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

  Limits the return probes nestedness as a simple remedy for the user
space attacks.

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