Re: [PATCH bpf-next v2] bpf: Log error code on trampoline unlink failure

[Leon Hwang]

On 2026/7/26 23:07, xu.xin16@xxxxxxxxxx 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 functional change intended.

Is it "No change to unlink behavior"?

> 
> Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
> ---
> v1->v2:
> 1) clean the subject name by remove 'syscall' suggested by Leon Hwang
> https://lore.kernel.org/all/20a444b2-aeed-4af8-ba76-e994e2c14087@xxxxxxxxx/
> 2) Add up the missed case in kernel/bpf/trampoline.c:bpf_shim_tramp_link_release()
> 
> kernel/bpf/syscall.c    | 9 ++++++---
>  kernel/bpf/trampoline.c | 5 ++++-
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 6db306d23b47..2348dc33abf4 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3626,10 +3626,13 @@ 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);
> +	if (err)
> +		WARN_ONCE(err, "bpf_trampoline_unlink_prog returns error: %d\n", err);

'if (err)' here is unnecessary.

> 
>  	bpf_trampoline_put(tr_link->trampoline);
> 
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 1a721fc4bef5..dae3c2104ed4 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);
> +	if (err)
> +		WARN_ONCE(err, "bpf_trampoline_unlink_prog returns error: %d\n", err);

Ditto.

>  	bpf_trampoline_put(shim_link->trampoline);
>  }
> 

Would it be better to include this multi_detach() one?

		WARN_ON_ONCE(__bpf_trampoline_unlink_prog(&mnode->node, mnode->trampoline,
					NULL, &trampoline_multi_ops, data));

Thanks,
Leon