[RFC PATCH bpf-next 4/8] bpf: support GET_FD_BY_ID and GET_NEXT_ID for bpf_link

[Andrii Nakryiko <andriin@fb.com>]

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.

Further, it's critical that BPF_LINK_GET_FD_BY_ID only ever allows to create
bpf_link FD that's O_RDONLY. This is to protect processes owning bpf_link and
thus allowed to perform modifications on them (like LINK_UPDATE), from other
processes that got bpf_link ID from GET_NEXT_ID API. In the latter case, only
querying bpf_link information (implemented later in the series) will be
allowed.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 include/uapi/linux/bpf.h |  2 ++
 kernel/bpf/syscall.c     | 56 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index eccfd1dea951..407c086bc9e4 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -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 {
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8b3a7d5814ae..527ec16702be 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3719,6 +3719,55 @@ static int link_update(union bpf_attr *attr)
 	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 open_flags
+
+static int bpf_link_get_fd_by_id(const union bpf_attr *attr)
+{
+	struct bpf_link *link;
+	u32 id = attr->link_id;
+	int f_flags;
+	int fd, err;
+
+	if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID) ||
+	    /* links are not allowed to be open by ID as writable */
+	    attr->open_flags & ~BPF_F_RDONLY)
+		return -EINVAL;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	f_flags = bpf_get_file_flag(attr->open_flags);
+	if (f_flags < 0)
+		return f_flags;
+
+	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, f_flags);
+	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;
@@ -3836,6 +3885,13 @@ 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;
-- 
2.24.1