[PATCH v3 5/9] arm64: kprobes: Make a frame pointer on __kretprobe_trampoline

[Masami Hiramatsu]

Make a frame pointer (make the x29 register points the
address of pt_regs->regs[29]) on __kretprobe_trampoline.

This frame pointer will be used by the stacktracer when it is
called from the kretprobe handlers. In this case, the stack
tracer will unwind stack to trampoline_probe_handler() and
find the next frame pointer in the stack frame of the
__kretprobe_trampoline().

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
---
 arch/arm64/kernel/probes/kprobes_trampoline.S |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/probes/kprobes_trampoline.S b/arch/arm64/kernel/probes/kprobes_trampoline.S
index 520ee8711db1..9a6499bed58b 100644
--- a/arch/arm64/kernel/probes/kprobes_trampoline.S
+++ b/arch/arm64/kernel/probes/kprobes_trampoline.S
@@ -66,6 +66,9 @@ SYM_CODE_START(__kretprobe_trampoline)
 
 	save_all_base_regs
 
+	/* Setup a frame pointer. */
+	add x29, sp, #S_FP
+
 	mov x0, sp
 	bl trampoline_probe_handler
 	/*
@@ -74,6 +77,7 @@ SYM_CODE_START(__kretprobe_trampoline)
 	 */
 	mov lr, x0
 
+	/* The frame pointer (x29) is restored with other registers. */
 	restore_all_base_regs
 
 	add sp, sp, #PT_REGS_SIZE