[PATCH -tip v9 10/14] kprobes: Enable stacktrace from pt_regs in kretprobe handler

[Masami Hiramatsu]

Since the ORC unwinder from pt_regs requires setting up regs->ip
correctly, set the correct return address to the regs->ip before
calling user kretprobe handler.

This allows the kretrprobe handler to trace stack from the
kretprobe's pt_regs by stack_trace_save_regs() (eBPF will do
this), instead of stack tracing from the handler context by
stack_trace_save() (ftrace will do this).

Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Tested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 Changes in v9:
  - Update comment to explain specifically why this is necessary.
 Changes in v8:
  - Update comment to clarify why this is needed.
 Changes in v3:
  - Cast the correct_ret_addr to unsigned long.
---
 kernel/kprobes.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 8f7d659eb277..e7c75725934b 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1924,6 +1924,13 @@ unsigned long __kretprobe_trampoline_handler(struct pt_regs *regs,
 		BUG_ON(1);
 	}
 
+	/*
+	 * Set the return address as the instruction pointer, because if the
+	 * user handler calls stack_trace_save_regs() with this 'regs',
+	 * the stack trace will start from the instruction pointer.
+	 */
+	instruction_pointer_set(regs, (unsigned long)correct_ret_addr);
+
 	/* Run the user handler of the nodes. */
 	first = current->kretprobe_instances.first;
 	while (first) {