Re: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe instance

On Thu, 14 Oct 2021 10:13:32 +0100
Mark Rutland [off-list ref] wrote:

On Thu, Oct 14, 2021 at 05:04:05PM +0900, Masami Hiramatsu wrote:

quoted

On Wed, 13 Oct 2021 11:01:39 +0100
Mark Rutland [off-list ref] wrote:

quoted

On Fri, Oct 08, 2021 at 09:28:39PM +0900, Masami Hiramatsu wrote:

quoted

Record the frame pointer instead of stack address with kretprobe
instance as the identifier on the instance list.
Since arm64 always enable CONFIG_FRAME_POINTER, we can use the
actual frame pointer (x29).

Just to check, why do we need to use the FP rather than SP? It wasn't
clear to me if that's necessary later in the series, or if I'm missing
something here.

Actually, this is for finding correct return address from the per-task
kretprobe instruction list (suppose it as a shadow stack) when it will
be searched in stack-backtracing. At that point, the framepointer will
be a reliable key.

Sure, my question was more "why isn't the SP a reliable key?", because both
the SP and FP should be balanced at function-entry and function-return
time. I'm asking because I think I'm missing a subtlety.

Ah, because SP is not used while unwinding frame. For the kretprobe,
either FP or SP is OK. But for the stacktrace.c, I can not use SP
and is easy to change to use FP. :)

So, when we introduce ORC unwinder on arm64, I think I have to reconsider
using SP based on the configuration.

Thank you,

I'm perfectly happy to use the FP even if they're equivalent; I just
want to make sure there's not some issue I'm unaware of that could
affect unwinding.

Thanks,
Mark.

quoted

quoted

FWIW, I plan to rework arm64's ftrace bits to use FP for
HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, so I'm happy to do likewise here.

Yes, I think you can use FP for that too.

quoted

quoted

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>

Regardless of the above:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thank you!

quoted

Mark.

quoted

---
 arch/arm64/kernel/probes/kprobes.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index e7ad6da980e8..d9dfa82c1f18 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c

@@ -401,14 +401,14 @@ int __init arch_populate_kprobe_blacklist(void)
 
 void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
 {
-	return (void *)kretprobe_trampoline_handler(regs, (void *)kernel_stack_pointer(regs));
+	return (void *)kretprobe_trampoline_handler(regs, (void *)regs->regs[29]);
 }
 
 void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
 				      struct pt_regs *regs)
 {
 	ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
-	ri->fp = (void *)kernel_stack_pointer(regs);
+	ri->fp = (void *)regs->regs[29];
 
 	/* replace return addr (x30) with trampoline */
 	regs->regs[30] = (long)&__kretprobe_trampoline;

-- 
Masami Hiramatsu [off-list ref]

-- 
Masami Hiramatsu [off-list ref]

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