netdev - Re: [PATCH bpf-next] bpf: Add LINK_DETACH for iter and perf links

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Message-ID: <f871d538-31b8-437a-b838-900836e13eb8@linux.dev>
Date: Tue, 5 Aug 2025 14:07:20 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Florian Lehner <dev@...-flo.net>, bpf@...r.kernel.org
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, davem@...emloft.net, kuba@...nel.org,
 hawk@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Add LINK_DETACH for iter and perf links



On 8/1/25 5:10 AM, Florian Lehner wrote:
> 73b11c2a introduced LINK_DETACH and implemented it for some link types,
> like xdp, netns and others.
>
> This patch implements LINK_DETACH for perf and iter links, re-using
> existing link release handling code.
>
> Signed-off-by: Florian Lehner <dev@...-flo.net>
> ---
>   kernel/bpf/bpf_iter.c | 7 +++++++
>   kernel/bpf/syscall.c  | 7 +++++++
>   2 files changed, 14 insertions(+)
>
> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
> index 0cbcae727079..823dad09735d 100644
> --- a/kernel/bpf/bpf_iter.c
> +++ b/kernel/bpf/bpf_iter.c
> @@ -397,6 +397,12 @@ static void bpf_iter_link_release(struct bpf_link *link)
>   		iter_link->tinfo->reg_info->detach_target(&iter_link->aux);
>   }
>   
> +static int bpf_iter_link_detach(struct bpf_link *link)
> +{
> +	bpf_iter_link_release(link);
> +	return 0;
> +}
> +
>   static void bpf_iter_link_dealloc(struct bpf_link *link)
>   {
>   	struct bpf_iter_link *iter_link =
> @@ -490,6 +496,7 @@ static int bpf_iter_link_fill_link_info(const struct bpf_link *link,
>   
>   static const struct bpf_link_ops bpf_iter_link_lops = {
>   	.release = bpf_iter_link_release,
> +	.detach = bpf_iter_link_detach,

Not sure how useful for this one. For bpf_iter programs,
the loaded prog will expect certain bpt_iter (e.g., bpf_map_elem, bpf_map, ...).
So even if you have detach, you won't be able to attach to a different
bpf_iter flavor.

Do you have a use case for this one?

>   	.dealloc = bpf_iter_link_dealloc,
>   	.update_prog = bpf_iter_link_replace,
>   	.show_fdinfo = bpf_iter_link_show_fdinfo,
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index e63039817af3..e89694f6874a 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3733,6 +3733,12 @@ static void bpf_perf_link_release(struct bpf_link *link)
>   	fput(perf_link->perf_file);
>   }
>   
> +static int bpf_perf_link_detach(struct bpf_link *link)
> +{
> +	bpf_perf_link_release(link);
> +	return 0;
> +}
> +
>   static void bpf_perf_link_dealloc(struct bpf_link *link)
>   {
>   	struct bpf_perf_link *perf_link = container_of(link, struct bpf_perf_link, link);
> @@ -4027,6 +4033,7 @@ static void bpf_perf_link_show_fdinfo(const struct bpf_link *link,
>   
>   static const struct bpf_link_ops bpf_perf_link_lops = {
>   	.release = bpf_perf_link_release,
> +	.detach = bpf_perf_link_detach,

This one may be possible. You might be able to e.g., try a different bpf_cookie, or
different perf event.

>   	.dealloc = bpf_perf_link_dealloc,
>   	.fill_link_info = bpf_perf_link_fill_link_info,
>   	.show_fdinfo = bpf_perf_link_show_fdinfo,