Re: [PATCH bpf-next] bpf: add BPF_F_SEALED flag to seal BPF links at creation

+Cc Christian since he was involved in the discussion at LSF/MM/BPF.

On Mon Aug 3, 2026 at 4:16 AM CEST, David Windsor wrote:
> Introduce a new BPF_F_SEALED flag for BPF_LINK_CREATE that creates the
> link permanently sealed. A sealed link can never have its program
> replaced via BPF_LINK_UPDATE, can never be detached via BPF_LINK_DETACH,
> and holds an extra self-reference that is never released, so the link and
> its program attachment persist until the machine reboots, even after user
> space closes every fd referring to it. There is no way to unseal a link.
>
> The sealed state is tracked by a new bool field on struct bpf_link.
>
> Signed-off-by: David Windsor <[email protected]>
> ---
>  include/linux/bpf.h            |  2 ++
>  include/uapi/linux/bpf.h       |  1 +
>  kernel/bpf/syscall.c           | 41 +++++++++++++++++++++++++++++++---
>  tools/include/uapi/linux/bpf.h |  1 +
>  4 files changed, 42 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7bfc28673124..a9600a1483ec 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1934,6 +1934,8 @@ struct bpf_link {
>  	 * link's semantics is determined by target attach hook
>  	 */
>  	bool sleepable;
> +	/* set once by BPF_F_SEALED; blocks update/detach, pins link until reboot */
> +	bool sealed;
>  };
>
>  struct bpf_link_ops {
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ffd96e8b920b..8cb30d18fec7 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1251,6 +1251,7 @@ enum bpf_perf_event_type {
>  #define BPF_F_AFTER		(1U << 4)
>  #define BPF_F_ID		(1U << 5)
>  #define BPF_F_PREORDER		(1U << 6)
> +#define BPF_F_SEALED		(1U << 7)
>  #define BPF_F_LINK		BPF_F_LINK /* 1 << 13 */
>
>  /* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 94091130bcc5..e8bd57d5c907 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3411,6 +3411,7 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
>  		seq_printf(m, "link_type:\t<%u>\n", type);
>  	}
>  	seq_printf(m, "link_id:\t%u\n", link->id);
> +	seq_printf(m, "sealed:\t%d\n", READ_ONCE(link->sealed) ? 1 : 0);
>
>  	rcu_read_lock();
>  	prog = READ_ONCE(link->prog);
> @@ -5776,17 +5777,41 @@ static int bpf_map_do_batch(const union bpf_attr *attr,
>  	return err;
>  }
>
> +/* Seal the just-created link: take a self-reference that is never released. */
> +static void link_seal_fd(int fd)
> +{
> +	struct bpf_link *link;
> +
> +	link = bpf_link_get_from_fd(fd);
> +	if (IS_ERR(link))
> +		return;
> +
> +	if (!READ_ONCE(link->sealed)) {
> +		bpf_link_inc(link);
> +		WRITE_ONCE(link->sealed, true);
> +	}
> +
> +	bpf_link_put_direct(link);
> +}
> +
>  #define BPF_LINK_CREATE_LAST_FIELD link_create.uprobe_multi.path_fd
>  static int link_create(union bpf_attr *attr, bpfptr_t uattr)
>  {
>  	struct bpf_prog *prog;
> +	bool seal;
>  	int ret;
>
>  	if (CHECK_ATTR(BPF_LINK_CREATE))
>  		return -EINVAL;
>
> -	if (attr->link_create.attach_type == BPF_STRUCT_OPS)
> -		return bpf_struct_ops_link_create(attr);
> +	/* Strip BPF_F_SEALED before per-type flag validation. */
> +	seal = attr->link_create.flags & BPF_F_SEALED;
> +	attr->link_create.flags &= ~BPF_F_SEALED;
> +
> +	if (attr->link_create.attach_type == BPF_STRUCT_OPS) {
> +		ret = bpf_struct_ops_link_create(attr);
> +		goto out_seal;
> +	}
>
>  	prog = bpf_prog_get(attr->link_create.prog_fd);
>  	if (IS_ERR(prog))
> @@ -5880,6 +5905,9 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
>  out:
>  	if (ret < 0)
>  		bpf_prog_put(prog);
> +out_seal:
> +	if (ret >= 0 && seal)
> +		link_seal_fd(ret);
>  	return ret;
>  }
>
> @@ -5932,6 +5960,11 @@ static int link_update(union bpf_attr *attr)
>  	if (IS_ERR(link))
>  		return PTR_ERR(link);
>
> +	if (READ_ONCE(link->sealed)) {
> +		ret = -EPERM;
> +		goto out_put_link;
> +	}
> +
>  	if (link->ops->update_map) {
>  		ret = link_update_map(link, attr);
>  		goto out_put_link;
> @@ -5984,7 +6017,9 @@ static int link_detach(union bpf_attr *attr)
>  	if (IS_ERR(link))
>  		return PTR_ERR(link);
>
> -	if (link->ops->detach)
> +	if (READ_ONCE(link->sealed))
> +		ret = -EPERM;
> +	else if (link->ops->detach)
>  		ret = link->ops->detach(link);
>  	else
>  		ret = -EOPNOTSUPP;
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index ffd96e8b920b..8cb30d18fec7 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -1251,6 +1251,7 @@ enum bpf_perf_event_type {
>  #define BPF_F_AFTER		(1U << 4)
>  #define BPF_F_ID		(1U << 5)
>  #define BPF_F_PREORDER		(1U << 6)
> +#define BPF_F_SEALED		(1U << 7)
>  #define BPF_F_LINK		BPF_F_LINK /* 1 << 13 */
>
>  /* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
>
> base-commit: 28e911d61d66b92a3bded8b54622ed3cd2795bf6