uprobes/x86: Reimplement arch_uretprobe_is_alive()

Add the x86 specific version of arch_uretprobe_is_alive()
helper. It returns true if the stack frame mangled by
prepare_uretprobe() is still on stack. So if it returns false,
we know that the probed function has already returned.

We add the new return_instance->stack member and change the
generic code to initialize it in prepare_uretprobe, but it
should be equally useful for other architectures.

TODO: this assumes that the probed application can't use
      multiple stacks (say sigaltstack). We will try to improve
      this logic later.

Tested-by: Pratyush Anand <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Srikar Dronamraju <[email protected]>
Acked-by: Anton Arapov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: oleg-nesterov

arch/x86/kernel/uprobes.c

@@ -985,3 +985,8 @@ arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr, struct pt_regs
 
 	return -1;
 }
+
+bool arch_uretprobe_is_alive(struct return_instance *ret, struct pt_regs *regs)
+{
+	return regs->sp <= ret->stack;
+}

include/linux/uprobes.h

@@ -95,6 +95,7 @@ struct uprobe_task {
 struct return_instance {
 	struct uprobe		*uprobe;
 	unsigned long		func;
+	unsigned long		stack;		/* stack pointer */
 	unsigned long		orig_ret_vaddr; /* original return address */
 	bool			chained;	/* true, if instance is nested */
 

kernel/events/uprobes.c

@@ -1562,6 +1562,7 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 
 	ri->uprobe = get_uprobe(uprobe);
 	ri->func = instruction_pointer(regs);
+	ri->stack = user_stack_pointer(regs);
 	ri->orig_ret_vaddr = orig_ret_vaddr;
 	ri->chained = chained;