[PATCH -tip v10 12/16] kprobes: Enable stacktrace from pt_regs in kretprobe handler

Masami Hiramatsu
2021-07-29T14:10:53Z

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@redhat.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
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 833f07f33115..ebc587b9a346 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1937,6 +1937,13 @@ 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) {