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

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 <redacted>
---
 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