Commit 73b11c2a authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
Browse files

bpf: Add support for forced LINK_DETACH command 



Add LINK_DETACH command to force-detach bpf_link without destroying it. It has
the same behavior as auto-detaching of bpf_link due to cgroup dying for
bpf_cgroup_link or net_device being destroyed for bpf_xdp_link. In such case,
bpf_link is still a valid kernel object, but is defuncts and doesn't hold BPF
program attached to corresponding BPF hook. This functionality allows users
with enough access rights to manually force-detach attached bpf_link without
killing respective owner process.

This patch implements LINK_DETACH for cgroup, xdp, and netns links, mostly
re-using existing link release handling code.

Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarSong Liu <songliubraving@fb.com>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200731182830.286260-2-andriin@fb.com
parent 4939b284

include/linux/bpf.h

+1 −0
Original line number Diff line number Diff line
@@ -793,6 +793,7 @@ struct bpf_link { 
struct bpf_link_ops {

	void (*release)(struct bpf_link *link);

	void (*dealloc)(struct bpf_link *link);

	int (*detach)(struct bpf_link *link);

	int (*update_prog)(struct bpf_link *link, struct bpf_prog *new_prog,

			   struct bpf_prog *old_prog);

	void (*show_fdinfo)(const struct bpf_link *link, struct seq_file *seq);

include/uapi/linux/bpf.h

+5 −0
Original line number Diff line number Diff line
@@ -117,6 +117,7 @@ enum bpf_cmd { 
	BPF_LINK_GET_NEXT_ID,

	BPF_ENABLE_STATS,

	BPF_ITER_CREATE,

	BPF_LINK_DETACH,

};



enum bpf_map_type {
@@ -634,6 +635,10 @@ union bpf_attr { 
		__u32		old_prog_fd;

	} link_update;



	struct {

		__u32		link_fd;

	} link_detach;



	struct { /* struct used by BPF_ENABLE_STATS command */

		__u32		type;

	} enable_stats;

kernel/bpf/cgroup.c

+14 −1
Original line number Diff line number Diff line
@@ -814,6 +814,7 @@ static void bpf_cgroup_link_release(struct bpf_link *link) 
{

	struct bpf_cgroup_link *cg_link =

		container_of(link, struct bpf_cgroup_link, link);

	struct cgroup *cg;



	/* link might have been auto-detached by dying cgroup already,

	 * in that case our work is done here
@@ -832,8 +833,12 @@ static void bpf_cgroup_link_release(struct bpf_link *link) 
	WARN_ON(__cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,

				    cg_link->type));



	cg = cg_link->cgroup;

	cg_link->cgroup = NULL;



	mutex_unlock(&cgroup_mutex);

	cgroup_put(cg_link->cgroup);



	cgroup_put(cg);

}



static void bpf_cgroup_link_dealloc(struct bpf_link *link)
@@ -844,6 +849,13 @@ static void bpf_cgroup_link_dealloc(struct bpf_link *link) 
	kfree(cg_link);

}



static int bpf_cgroup_link_detach(struct bpf_link *link)

{

	bpf_cgroup_link_release(link);



	return 0;

}



static void bpf_cgroup_link_show_fdinfo(const struct bpf_link *link,

					struct seq_file *seq)

{
@@ -883,6 +895,7 @@ static int bpf_cgroup_link_fill_link_info(const struct bpf_link *link, 
static const struct bpf_link_ops bpf_cgroup_link_lops = {

	.release = bpf_cgroup_link_release,

	.dealloc = bpf_cgroup_link_dealloc,

	.detach = bpf_cgroup_link_detach,

	.update_prog = cgroup_bpf_replace,

	.show_fdinfo = bpf_cgroup_link_show_fdinfo,

	.fill_link_info = bpf_cgroup_link_fill_link_info,

kernel/bpf/net_namespace.c

+8 −0
Original line number Diff line number Diff line
@@ -142,9 +142,16 @@ static void bpf_netns_link_release(struct bpf_link *link) 
	bpf_prog_array_free(old_array);



out_unlock:

	net_link->net = NULL;

	mutex_unlock(&netns_bpf_mutex);

}



static int bpf_netns_link_detach(struct bpf_link *link)

{

	bpf_netns_link_release(link);

	return 0;

}



static void bpf_netns_link_dealloc(struct bpf_link *link)

{

	struct bpf_netns_link *net_link =
@@ -228,6 +235,7 @@ static void bpf_netns_link_show_fdinfo(const struct bpf_link *link, 
static const struct bpf_link_ops bpf_netns_link_ops = {

	.release = bpf_netns_link_release,

	.dealloc = bpf_netns_link_dealloc,

	.detach = bpf_netns_link_detach,

	.update_prog = bpf_netns_link_update_prog,

	.fill_link_info = bpf_netns_link_fill_info,

	.show_fdinfo = bpf_netns_link_show_fdinfo,

kernel/bpf/syscall.c

+26 −0
Original line number Diff line number Diff line
@@ -3991,6 +3991,29 @@ static int link_update(union bpf_attr *attr) 
	return ret;

}



#define BPF_LINK_DETACH_LAST_FIELD link_detach.link_fd



static int link_detach(union bpf_attr *attr)

{

	struct bpf_link *link;

	int ret;



	if (CHECK_ATTR(BPF_LINK_DETACH))

		return -EINVAL;



	link = bpf_link_get_from_fd(attr->link_detach.link_fd);

	if (IS_ERR(link))

		return PTR_ERR(link);



	if (link->ops->detach)

		ret = link->ops->detach(link);

	else

		ret = -EOPNOTSUPP;



	bpf_link_put(link);

	return ret;

}



static int bpf_link_inc_not_zero(struct bpf_link *link)

{

	return atomic64_fetch_add_unless(&link->refcnt, 1, 0) ? 0 : -ENOENT;
@@ -4240,6 +4263,9 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz 
	case BPF_ITER_CREATE:

		err = bpf_iter_create(&attr);

		break;

	case BPF_LINK_DETACH:

		err = link_detach(&attr);

		break;

	default:

		err = -EINVAL;

		break;