Re: [PATCH 1/3] powerpc: kprobes: add support for KPROBES_ON_FTRACE

From: Ananth N Mavinakayanahalli
Date: Tue Feb 14 2017 - 23:29:15 EST


On Wed, Feb 15, 2017 at 12:28:34AM +0530, Naveen N. Rao wrote:
> Allow kprobes to be placed on ftrace _mcount() call sites. This
> optimization avoids the use of a trap, by riding on ftrace
> infrastructure.
>
> This depends on HAVE_DYNAMIC_FTRACE_WITH_REGS which depends on
> MPROFILE_KERNEL, which is only currently enabled on powerpc64le with
> newer toolchains.
>
> Based on the x86 code by Masami.
>
> Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> ---
> arch/powerpc/Kconfig | 1 +
> arch/powerpc/include/asm/kprobes.h | 10 ++++
> arch/powerpc/kernel/Makefile | 3 ++
> arch/powerpc/kernel/kprobes-ftrace.c | 100 +++++++++++++++++++++++++++++++++++
> arch/powerpc/kernel/kprobes.c | 4 +-
> arch/powerpc/kernel/optprobes.c | 3 ++
> 6 files changed, 120 insertions(+), 1 deletion(-)
> create mode 100644 arch/powerpc/kernel/kprobes-ftrace.c

You'll also need to update
Documentation/features/debug/kprobes-on-ftrace/arch-support.txt

> +/* Ftrace callback handler for kprobes */
> +void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
> + struct ftrace_ops *ops, struct pt_regs *regs)
> +{
> + struct kprobe *p;
> + struct kprobe_ctlblk *kcb;
> + unsigned long flags;
> +
> + /* Disable irq for emulating a breakpoint and avoiding preempt */
> + local_irq_save(flags);
> + hard_irq_disable();
> +
> + p = get_kprobe((kprobe_opcode_t *)nip);
> + if (unlikely(!p) || kprobe_disabled(p))
> + goto end;
> +
> + kcb = get_kprobe_ctlblk();
> + if (kprobe_running()) {
> + kprobes_inc_nmissed_count(p);
> + } else {
> + unsigned long orig_nip = regs->nip;
> + /* Kprobe handler expects regs->nip = nip + 1 as breakpoint hit */

Can you clarify this? On powerpc, the regs->nip at the time of
breakpoint hit points to the probed instruction, not the one after.

Ananth