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    SubjectRe: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe instance
    On Thu, Oct 14, 2021 at 05:04:05PM +0900, Masami Hiramatsu wrote:
    > On Wed, 13 Oct 2021 11:01:39 +0100
    > Mark Rutland <mark.rutland@arm.com> wrote:
    >
    > > On Fri, Oct 08, 2021 at 09:28:39PM +0900, Masami Hiramatsu wrote:
    > > > 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.

    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.

    > > 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.
    >
    > >
    > > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
    > >
    > > Regardless of the above:
    > >
    > > Acked-by: Mark Rutland <mark.rutland@arm.com>
    >
    > Thank you!
    >
    > >
    > > Mark.
    > >
    > > > ---
    > > > 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 <mhiramat@kernel.org>

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