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

[Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>]

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@linux.vnet.ibm.com>
> ---
>  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