Re: [PATCH bpf-next 04/13] bpf: Add tracing_multi link support for bpf progs

[Jiri Olsa <olsajiri@gmail.com>]

On Sun, Aug 09, 2026 at 11:01:02PM +0800, Leon Hwang wrote:

SNIP

> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index eddd259d3776..fc51ea2428be 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -1572,12 +1572,23 @@ static int update_fentry_multi(struct bpf_trampoline *tr, u32 orig_flags,
>  			       struct bpf_tramp_image *im, struct ftrace_hash *hash,
>  			       struct bpf_tracing_multi_data *data)
>  {
> -	unsigned long addr = (unsigned long)(im ? im->image : tr->cur_image->image);
> +	if (tr->func.ftrace_managed) {
> +		unsigned long addr = (unsigned long)(im ? im->image : tr->cur_image->image);
>  
> -	if (bpf_trampoline_use_jmp(tr->flags))
> -		addr = ftrace_jmp_set(addr);
> +		if (bpf_trampoline_use_jmp(tr->flags))
> +			addr = ftrace_jmp_set(addr);
> +
> +		ftrace_hash_add(hash, data->entry, tr->ip, addr);
> +	} else {
> +		void *old_addr = tr->cur_image ? tr->cur_image->image : NULL;
> +		void *new_addr = im ? im->image : NULL;
> +		int ret;
> +
> +		ret = bpf_trampoline_update_fentry(tr, orig_flags, old_addr, new_addr);
> +		if (ret)
> +			return ret;
> +	}

hum, so IIUC this sequentially attaches to target bpf program as it
would with current API, so there's no attachent speedup, right?

>  
> -	ftrace_hash_add(hash, data->entry, tr->ip, addr);
>  	tr->cur_image = im;
>  	return 0;
>  }
> @@ -1627,6 +1638,18 @@ static void bpf_trampoline_multi_attach_free(struct bpf_trampoline *tr)
>  
>  static void bpf_trampoline_multi_attach_rollback(struct bpf_trampoline *tr)
>  {
> +	if (!tr->func.ftrace_managed) {
> +		void *failed_addr = tr->cur_image ? tr->cur_image->image : NULL;
> +		void *old_addr = tr->multi_attach.old_image ?
> +				 tr->multi_attach.old_image->image : NULL;
> +		u32 orig_flags = tr->flags;
> +		int ret;
> +
> +		tr->flags = tr->multi_attach.old_flags;
> +		ret = bpf_trampoline_update_fentry(tr, orig_flags, failed_addr, old_addr);
> +		WARN_ONCE(ret, "bpf_trampoline_update_fentry failed: %d\n", ret);
> +	}
> +
>  	if (tr->cur_image)
>  		bpf_tramp_image_put(tr->cur_image);
>  	tr->cur_image = tr->multi_attach.old_image;
> @@ -1643,6 +1666,7 @@ static void bpf_trampoline_multi_attach_rollback(struct bpf_trampoline *tr)
>  	for_each_mnode_cnt(mnode, link, link->nodes_cnt)
>  
>  int bpf_trampoline_multi_attach(struct bpf_prog *prog, u32 *ids,
> +				u64 *keys, struct bpf_prog **progs,
>  				struct bpf_tracing_multi_link *link)
>  {
>  	struct bpf_tracing_multi_data *data = &link->data;
> @@ -1651,18 +1675,18 @@ int bpf_trampoline_multi_attach(struct bpf_prog *prog, u32 *ids,
>  	struct bpf_tracing_multi_node *mnode;
>  	struct bpf_trampoline *tr;
>  	int i, err, rollback_cnt;
> -	u64 key;
>  
>  	for_each_mnode(mnode, link) {
>  		rollback_cnt = i;
>  
> -		err = bpf_check_attach_btf_id_multi(btf, prog, ids[i], &tgt_info);
> +		if (progs)
> +			err = bpf_check_attach_target(NULL, prog, progs[i], ids[i], &tgt_info);
> +		else
> +			err = bpf_check_attach_btf_id_multi(btf, prog, ids[i], &tgt_info);
>  		if (err)
>  			goto rollback_put;
>  
> -		key = bpf_trampoline_compute_key(NULL, btf, ids[i]);
> -
> -		tr = bpf_trampoline_get(key, &tgt_info);
> +		tr = bpf_trampoline_get(keys[i], &tgt_info);
>  		if (!tr) {
>  			err = -ENOMEM;
>  			goto rollback_put;
> @@ -1691,6 +1715,9 @@ int bpf_trampoline_multi_attach(struct bpf_prog *prog, u32 *ids,
>  	for_each_mnode(mnode, link) {
>  		bpf_trampoline_multi_attach_init(mnode->trampoline);
>  
> +		if (progs && progs[i]->aux->tail_call_reachable)
> +			mnode->trampoline->flags |= BPF_TRAMP_F_TAIL_CALL_CTX;
> +
>  		data->entry = &mnode->entry;
>  		err = __bpf_trampoline_link_prog(&mnode->node, mnode->trampoline, NULL,
>  						 &trampoline_multi_ops, data);

IIUC for bpf_program targets the actuall attachment is happening in
here, right?

the rest of the function logic won't execute, because there won't
be any data in the reg/noreg/mod hashes..?

you seem to use the tracing_multi API to ease up attachment to multiple
bpf programs and end up with just single link fd for all attachments

I think it'd be cleaner to have separate api or code paths for that

jirka