Re: [PATCH bpf-next 1/2] bpf: Fix WARNING in bpf_tracing_link_release

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Jul 21, 2026 at 09:30:34PM +0800, Leon Hwang wrote:
> 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.
> 2. Another fexit loaded with the same tail_call_reachable prog target.
> 3. Close the first fexit link.

had to ask ai for more details on how the warning was trigered ;-)
would be nice to have that info in the changelog

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> [    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>
> ---
>  include/linux/bpf.h     | 2 ++
>  kernel/bpf/trampoline.c | 7 +++++++
>  kernel/bpf/verifier.c   | 2 +-
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index d9542127dfdf..fc84f39967ae 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 @@ static struct bpf_tramp_image *bpf_tramp_image_alloc(u64 key, int size)
>  	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;
> -- 
> 2.55.0
>