[PATCH] kprobes: Print an error if probe is rejected

[Naveen N. Rao]

When probing at different locations in the kernel, it is not always
evident if the location can be probed or not. As an example:

    $ perf probe __radix__flush_tlb_range:35
    Failed to write event: Invalid argument
      Error: Failed to add events.

The probed line above is:
     35         if (!mmu_has_feature(MMU_FTR_GTSE) && type == FLUSH_TYPE_GLOBAL) {

This ends up trying to probe on BUILD_BUG_ON(), which is rejected.
However, the user receives no indication at all as to why the probe
failed. Print an error in such cases so that it is clear that the probe
was rejected.

Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
---
 kernel/kprobes.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index e41385afe79dc5..7c8929165924ed 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1542,8 +1542,10 @@ static int check_kprobe_address_safe(struct kprobe *p,
 	int ret;
 
 	ret = arch_check_ftrace_location(p);
-	if (ret)
+	if (ret) {
+		pr_err("kprobes: can't probe at the provided ftrace location\n");
 		return ret;
+	}
 	jump_label_lock();
 	preempt_disable();
 
@@ -1552,6 +1554,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
 	    within_kprobe_blacklist((unsigned long) p->addr) ||
 	    jump_label_text_reserved(p->addr, p->addr) ||
 	    find_bug((unsigned long)p->addr)) {
+		pr_err("kprobes: can't probe at address in reject list\n");
 		ret = -EINVAL;
 		goto out;
 	}
@@ -1976,8 +1979,10 @@ int register_kretprobe(struct kretprobe *rp)
 	void *addr;
 
 	ret = kprobe_on_func_entry(rp->kp.addr, rp->kp.symbol_name, rp->kp.offset);
-	if (ret)
+	if (ret) {
+		pr_err("kretprobes: can't probe at address outside function entry\n");
 		return ret;
+	}
 
 	/* If only rp->kp.addr is specified, check reregistering kprobes */
 	if (rp->kp.addr && warn_kprobe_rereg(&rp->kp))
@@ -1989,8 +1994,10 @@ int register_kretprobe(struct kretprobe *rp)
 			return PTR_ERR(addr);
 
 		for (i = 0; kretprobe_blacklist[i].name != NULL; i++) {
-			if (kretprobe_blacklist[i].addr == addr)
+			if (kretprobe_blacklist[i].addr == addr) {
+				pr_err("kretprobes: can't probe at address in reject list\n");
 				return -EINVAL;
+			}
 		}
 	}
 

base-commit: 2e38eb04c95e5546b71bb86ee699a891c7d212b5
-- 
2.31.1