[PATCH] kretprobes: reject registration if a symbol offset is specified

[Naveen N. Rao]

Users shouldn't be able to specify an offset with kretprobes, as we always
want to probe at function entry. Otherwise, we won't be able to capture
the proper return address resulting in the kretprobe never firing.

With samples/kprobes/kretprobe_example.c including an offset:
	my_kretprobe.kp.offset = 40;

Before this patch, the probe gets planted but never fires.

After this patch:
	$ sudo insmod samples/kprobes/kretprobe_example.ko
	[sudo] password for naveen: 
	insmod: ERROR: could not insert module samples/kprobes/kretprobe_example.ko: Operation not permitted

And dmesg:
	[48253.757629] register_kretprobe failed, returned -22

Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
---
 kernel/kprobes.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 60a702a05684..83ad7e440417 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1847,6 +1847,9 @@ int register_kretprobe(struct kretprobe *rp)
 	int i;
 	void *addr;
 
+	if (rp->kp.offset)
+		return -EINVAL;
+
 	if (kretprobe_blacklist_size) {
 		addr = kprobe_addr(&rp->kp);
 		if (IS_ERR(addr))
-- 
2.11.0