[PATCH v2 07/11] uprobes/x86: Reimplement arch_uretprobe_is_alive()

[Oleg Nesterov <oleg@redhat.com>]

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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/uprobes.c |    5 +++++
 include/linux/uprobes.h   |    1 +
 kernel/events/uprobes.c   |    1 +
 3 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 0b81ad6..9d5f570 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -993,3 +993,8 @@ 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;
+}
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 50d2764..7ab6d2c 100644
--- a/include/linux/uprobes.h
+++ b/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 */
 
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 1c71b62..c5f316e 100644
--- a/kernel/events/uprobes.c
+++ b/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;
 
-- 
1.5.5.1