[PATCH 7/7] powerpc/kprobes: Allow probing on any ftrace address

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

With KPROBES_ON_FTRACE, kprobe is allowed to be inserted on instructions
that branch to _mcount (referred to as ftrace location). With
-mprofile-kernel, we now include the preceding 'mflr r0' as being part
of the ftrace location.

However, by default, probing on an instruction that is not actually the
branch to _mcount() is prohibited, as that is considered to not be at an
instruction boundary. This is not the case on powerpc, so allow the same
by overriding arch_check_ftrace_location()

In addition, we update kprobe_ftrace_handler() to detect this scenarios
and to pass the proper nip to the pre and post probe handlers.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/kprobes-ftrace.c | 30 ++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
index 972cb28174b2..6a0bd3c16cb6 100644
--- a/arch/powerpc/kernel/kprobes-ftrace.c
+++ b/arch/powerpc/kernel/kprobes-ftrace.c
@@ -12,14 +12,34 @@
 #include <linux/preempt.h>
 #include <linux/ftrace.h>
 
+/*
+ * With -mprofile-kernel, we patch two instructions -- the branch to _mcount
+ * as well as the preceding 'mflr r0'. Both these instructions are claimed
+ * by ftrace and we should allow probing on either instruction.
+ */
+int arch_check_ftrace_location(struct kprobe *p)
+{
+	if (ftrace_location((unsigned long)p->addr))
+		p->flags |= KPROBE_FLAG_FTRACE;
+	return 0;
+}
+
 /* 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;
+	int mflr_kprobe = 0;
 	struct kprobe_ctlblk *kcb;
 
 	p = get_kprobe((kprobe_opcode_t *)nip);
+	if (unlikely(!p)) {
+		p = get_kprobe((kprobe_opcode_t *)(nip - MCOUNT_INSN_SIZE));
+		if (!p)
+			return;
+		mflr_kprobe = 1;
+	}
+
 	if (unlikely(!p) || kprobe_disabled(p))
 		return;
 
@@ -33,6 +53,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 		 */
 		regs->nip -= MCOUNT_INSN_SIZE;
 
+		if (mflr_kprobe)
+			regs->nip -= MCOUNT_INSN_SIZE;
+
 		__this_cpu_write(current_kprobe, p);
 		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 		if (!p->pre_handler || !p->pre_handler(p, regs)) {
@@ -45,6 +68,8 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 				kcb->kprobe_status = KPROBE_HIT_SSDONE;
 				p->post_handler(p, regs, 0);
 			}
+			if (mflr_kprobe)
+				regs->nip += MCOUNT_INSN_SIZE;
 		}
 		/*
 		 * If pre_handler returns !0, it changes regs->nip. We have to
@@ -57,6 +82,11 @@ NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
 int arch_prepare_kprobe_ftrace(struct kprobe *p)
 {
+	if ((unsigned long)p->addr & 0x03) {
+		printk("Attempt to register kprobe at an unaligned address\n");
+		return -EILSEQ;
+	}
+
 	p->ainsn.insn = NULL;
 	p->ainsn.boostable = -1;
 	return 0;
-- 
2.22.0