Commit 2d602c8c authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
Browse files

bpf: Support GET_FD_BY_ID and GET_NEXT_ID for bpf_link 



Add support to look up bpf_link by ID and iterate over all existing bpf_links
in the system. GET_FD_BY_ID code handles not-yet-ready bpf_link by checking
that its ID hasn't been set to non-zero value yet. Setting bpf_link's ID is
done as the very last step in finalizing bpf_link, together with installing
FD. This approach allows users of bpf_link in kernel code to not worry about
races between user-space and kernel code that hasn't finished attaching and
initializing bpf_link.

Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429001614.1544-4-andriin@fb.com
parent a3b80e10

include/uapi/linux/bpf.h

+2 −0
Original line number Diff line number Diff line
@@ -113,6 +113,8 @@ enum bpf_cmd { 
	BPF_MAP_DELETE_BATCH,

	BPF_LINK_CREATE,

	BPF_LINK_UPDATE,

	BPF_LINK_GET_FD_BY_ID,

	BPF_LINK_GET_NEXT_ID,

};



enum bpf_map_type {

kernel/bpf/syscall.c

+49 −0
Original line number Diff line number Diff line
@@ -3713,6 +3713,48 @@ out_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;

}



#define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id



static int bpf_link_get_fd_by_id(const union bpf_attr *attr)

{

	struct bpf_link *link;

	u32 id = attr->link_id;

	int fd, err;



	if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID))

		return -EINVAL;



	if (!capable(CAP_SYS_ADMIN))

		return -EPERM;



	spin_lock_bh(&link_idr_lock);

	link = idr_find(&link_idr, id);

	/* before link is "settled", ID is 0, pretend it doesn't exist yet */

	if (link) {

		if (link->id)

			err = bpf_link_inc_not_zero(link);

		else

			err = -EAGAIN;

	} else {

		err = -ENOENT;

	}

	spin_unlock_bh(&link_idr_lock);



	if (err)

		return err;



	fd = bpf_link_new_fd(link);

	if (fd < 0)

		bpf_link_put(link);



	return fd;

}



SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)

{

	union bpf_attr attr;
@@ -3830,6 +3872,13 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz 
	case BPF_LINK_UPDATE:

		err = link_update(&attr);

		break;

	case BPF_LINK_GET_FD_BY_ID:

		err = bpf_link_get_fd_by_id(&attr);

		break;

	case BPF_LINK_GET_NEXT_ID:

		err = bpf_obj_get_next_id(&attr, uattr,

					  &link_idr, &link_idr_lock);

		break;

	default:

		err = -EINVAL;

		break;

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