[PATCH bpf-next v4] bpf: log error code on trampoline unlink failure

[xu.xin16]

From: Xu Xin <xu.xin16@xxxxxxxxxx>

Replace silent WARN_ON_ONCE with WARN_ONCE that prints the actual error
code from bpf_trampoline_unlink_prog(). This aids debugging of race
conditions during link teardown, while keeping the warning rate limited
to avoid log flooding.

This will be very helpful for speeding up trouble-shooting of some crash
UAF due to bpf_trampoline_unlink_prog failures.

No change to unlink behavior.

Signed-off-by: Xu Xin <xu.xin16@xxxxxxxxxx>
Acked-by: Leon Hwang <leon.hwang@xxxxxxxxx>
---
Change log:
- Small fixes according to the v3 disccusion:
  https://lore.kernel.org/all/82926caa-e16a-457a-8d75-61dfc8dac102@xxxxxxxxx/

 kernel/bpf/syscall.c    |  8 +++++---
 kernel/bpf/trampoline.c | 12 ++++++++----
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 6db306d23b47..52c57f8beb37 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3626,10 +3626,12 @@ static void bpf_tracing_link_release(struct bpf_link *link)
 {
 	struct bpf_tracing_link *tr_link =
 		container_of(link, struct bpf_tracing_link, link.link);
+	int err;

-	WARN_ON_ONCE(bpf_trampoline_unlink_prog(&tr_link->link.node,
-						tr_link->trampoline,
-						tr_link->tgt_prog));
+	err = bpf_trampoline_unlink_prog(&tr_link->link.node,
+					tr_link->trampoline,
+					tr_link->tgt_prog);
+	WARN_ONCE(err, "bpf_trampoline_unlink_prog returns error: %d\n", err);

 	bpf_trampoline_put(tr_link->trampoline);

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 1a721fc4bef5..49dceea3d51e 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -997,12 +997,15 @@ static void bpf_shim_tramp_link_release(struct bpf_link *link)
 {
 	struct bpf_shim_tramp_link *shim_link =
 		container_of(link, struct bpf_shim_tramp_link, link.link);
+	int err;

 	/* paired with 'shim_link->trampoline = tr' in bpf_trampoline_link_cgroup_shim */
 	if (!shim_link->trampoline)
 		return;

-	WARN_ON_ONCE(bpf_trampoline_unlink_prog(&shim_link->link.node, shim_link->trampoline, NULL));
+	err = bpf_trampoline_unlink_prog(&shim_link->link.node, shim_link->trampoline, NULL);
+	WARN_ONCE(err, "bpf_trampoline_unlink_prog returns error: %d\n", err);
+
 	bpf_trampoline_put(shim_link->trampoline);
 }

@@ -1712,15 +1715,16 @@ int bpf_trampoline_multi_detach(struct bpf_prog *prog, struct bpf_tracing_multi_
 {
 	struct bpf_tracing_multi_data *data = &link->data;
 	struct bpf_tracing_multi_node *mnode;
-	int i;
+	int i, err;

 	trampoline_lock_all();

 	for_each_mnode(mnode, link) {
 		data->entry = &mnode->entry;
 		bpf_trampoline_multi_attach_init(mnode->trampoline);
-		WARN_ON_ONCE(__bpf_trampoline_unlink_prog(&mnode->node, mnode->trampoline,
-					NULL, &trampoline_multi_ops, data));
+		err = __bpf_trampoline_unlink_prog(&mnode->node, mnode->trampoline, NULL,
+					&trampoline_multi_ops, data);
+		WARN_ONCE(err, "__bpf_trampoline_unlink_prog returns error: %d\n", err);
 	}

 	if (ftrace_hash_count(data->unreg))
-- 
2.25.1