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

Masami Hiramatsu wrote:

On Tue, 18 Jun 2019 20:17:06 +0530
"Naveen N. Rao" [off-list ref] wrote:

quoted

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 <redacted>
---
 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)) {

Hmm, is this really unlikely? If we put a kprobe on the second instruction address,
we will see p == NULL always.

quoted

+		p = get_kprobe((kprobe_opcode_t *)(nip - MCOUNT_INSN_SIZE));
+		if (!p)

Here will be unlikely, because we can not find kprobe at both of nip and
nip - MCOUNT_INSN_SIZE.

quoted

+			return;
+		mflr_kprobe = 1;
+	}
+
 	if (unlikely(!p) || kprobe_disabled(p))

"unlikely(!p)" is not needed here.

...

Joe Perches wrote:

On Fri, 2019-06-21 at 23:50 +0900, Masami Hiramatsu wrote:

quoted

On Tue, 18 Jun 2019 20:17:06 +0530
"Naveen N. Rao" [off-list ref] wrote:

trivia:

quoted

quoted

diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c

[]

quoted

quoted

@@ -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");

Please use the appropriate KERN_<LEVEL> or pr_<level>

All good points. Thanks for the review.


- Naveen