[PATCH 6/6][RFC] kprobes: Allow probe on ftrace reserved text (but move it)

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

If a probe is placed on a ftrace nop (or ftrace_caller), simply move the
probe to the next instruction instead of rejecting it. This will allow
kprobes not to be affected by ftrace using the -mfentry gcc option which
will put the ftrace nop at the beginning of the function. As a very common
case for kprobes is to add a probe to the very beginning of a function
we need a way to handle the case when ftrace takes the first instruction.

Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/kprobes.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index c62b854..560d80e 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1319,10 +1319,20 @@ int __kprobes register_kprobe(struct kprobe *p)
 	struct kprobe *old_p;
 	struct module *probed_mod;
 	kprobe_opcode_t *addr;
+	unsigned long ftrace_addr;
 
 	addr = kprobe_addr(p);
 	if (IS_ERR(addr))
 		return PTR_ERR(addr);
+
+	/*
+	 * If the address is located on a ftrace nop, set the
+	 * breakpoint to the following instruction.
+	 */
+	ftrace_addr = ftrace_location((unsigned long)addr);
+	if (unlikely(ftrace_addr))
+		addr = (kprobe_opcode_t *)(ftrace_addr + MCOUNT_INSN_SIZE);
+
 	p->addr = addr;
 
 	ret = check_kprobe_rereg(p);
@@ -1333,7 +1343,6 @@ int __kprobes register_kprobe(struct kprobe *p)
 	preempt_disable();
 	if (!kernel_text_address((unsigned long) p->addr) ||
 	    in_kprobes_functions((unsigned long) p->addr) ||
-	    ftrace_text_reserved(p->addr, p->addr) ||
 	    jump_label_text_reserved(p->addr, p->addr)) {
 		ret = -EINVAL;
 		goto cannot_probe;
-- 
1.7.9.5

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