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

[Emil Tsalapatis]

On Wed Jul 29, 2026 at 2:11 AM EDT, xu.xin16 wrote:
> 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>

Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>

The Sashiko concern seems valid, please let me know if you are
planning on looking into it otherwise I can take a look.

> ---
> 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))