bpf: Fix WARNING in bpf_tracing_link_release

The trampoline could be corrupted by the blindly
'tr->flags = BPF_TRAMP_F_TAIL_CALL_CTX' in verifier.

1. A fexit attached to a tail_call_reachable prog. 'tr->flags' became
   'BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_TAIL_CALL_CTX'. And, the
   trampoline would poke the target prog's nop insn using jmp insn instead
   of call insn.
2. Another fexit loaded with the same tail_call_reachable prog target.
   'tr->flags' became 'BPF_TRAMP_F_TAIL_CALL_CTX'.
3. Close the first fexit link. Due to no BPF_TRAMP_F_CALL_ORIG in
   'tr->flags', the trampoline will fail to restore the prog's nop insn
   using call insn.

[    3.410719] WARNING: kernel/bpf/syscall.c:3551 at bpf_tracing_link_release+0x53/0x60, CPU#1: test_progs/98
...
[    3.428793]  bpf_link_free+0x58/0x130
[    3.429293]  bpf_link_release+0x23/0x30

Fix the warning by updating 'tr->flags' with '|=' and lock.

Fixes: 2b5dcb31a19a ("bpf, x64: Fix tailcall infinite loop")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20260722151909.69142-2-leon.hwang@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

3 files changed, 10 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e066f44a9c05..7bfc28673124 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1523,6 +1523,7 @@ int bpf_trampoline_multi_attach(struct bpf_prog *prog, u32 *ids,
 				struct bpf_tracing_multi_link *link);
 int bpf_trampoline_multi_detach(struct bpf_prog *prog,
 				struct bpf_tracing_multi_link *link);
+void bpf_trampoline_set_flags(struct bpf_trampoline *tr, u32 flags);
 
 /*
  * When the architecture supports STATIC_CALL replace the bpf_dispatcher_fn
@@ -1646,6 +1647,7 @@ static inline int bpf_trampoline_multi_detach(struct bpf_prog *prog,
 {
 	return -ENOTSUPP;
 }
+static inline void bpf_trampoline_set_flags(struct bpf_trampoline *tr, u32 flags) {}
 #endif
 
 struct bpf_func_info_aux {
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 6eadf64f7ec9..129d07db117e 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -670,6 +670,13 @@ out:
 	return ERR_PTR(err);
 }
 
+void bpf_trampoline_set_flags(struct bpf_trampoline *tr, u32 flags)
+{
+	trampoline_lock(tr);
+	tr->flags |= flags;
+	trampoline_unlock(tr);
+}
+
 static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mutex,
 				 const struct bpf_trampoline_ops *ops, void *data)
 {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 52be0a118cce..66d8d9eaec05 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19523,7 +19523,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 		return -ENOMEM;
 
 	if (tgt_prog && tgt_prog->aux->tail_call_reachable)
-		tr->flags = BPF_TRAMP_F_TAIL_CALL_CTX;
+		bpf_trampoline_set_flags(tr, BPF_TRAMP_F_TAIL_CALL_CTX);
 
 	prog->aux->dst_trampoline = tr;
 	return 0;