libbpf: Add bpf_link detach APIs

Add low-level bpf_link_detach() API. Also add higher-level bpf_link__detach()
one.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200731182830.286260-3-andriin@fb.com

tools/include/uapi/linux/bpf.h

@@ -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;

tools/lib/bpf/bpf.c

@@ -603,6 +603,16 @@ int bpf_link_create(int prog_fd, int target_fd,
 	return sys_bpf(BPF_LINK_CREATE, &attr, sizeof(attr));
 }
 
+int bpf_link_detach(int link_fd)
+{
+	union bpf_attr attr;
+
+	memset(&attr, 0, sizeof(attr));
+	attr.link_detach.link_fd = link_fd;
+
+	return sys_bpf(BPF_LINK_DETACH, &attr, sizeof(attr));
+}
+
 int bpf_link_update(int link_fd, int new_prog_fd,
 		    const struct bpf_link_update_opts *opts)
 {

tools/lib/bpf/bpf.h

@@ -178,6 +178,8 @@ LIBBPF_API int bpf_link_create(int prog_fd, int target_fd,
 			       enum bpf_attach_type attach_type,
 			       const struct bpf_link_create_opts *opts);
 
+LIBBPF_API int bpf_link_detach(int link_fd);
+
 struct bpf_link_update_opts {
 	size_t sz; /* size of this struct for forward/backward compatibility */
 	__u32 flags;	   /* extra flags */

tools/lib/bpf/libbpf.c

@@ -7748,6 +7748,11 @@ struct bpf_link *bpf_link__open(const char *path)
 	return link;
 }
 
+int bpf_link__detach(struct bpf_link *link)
+{
+	return bpf_link_detach(link->fd) ? -errno : 0;
+}
+
 int bpf_link__pin(struct bpf_link *link, const char *path)
 {
 	int err;

tools/lib/bpf/libbpf.h

@@ -229,6 +229,7 @@ LIBBPF_API int bpf_link__unpin(struct bpf_link *link);
 LIBBPF_API int bpf_link__update_program(struct bpf_link *link,
 					struct bpf_program *prog);
 LIBBPF_API void bpf_link__disconnect(struct bpf_link *link);
+LIBBPF_API int bpf_link__detach(struct bpf_link *link);
 LIBBPF_API int bpf_link__destroy(struct bpf_link *link);
 
 LIBBPF_API struct bpf_link *

tools/lib/bpf/libbpf.map

@@ -273,6 +273,8 @@ LIBBPF_0.0.9 {
 
 LIBBPF_0.1.0 {
 	global:
+		bpf_link__detach;
+		bpf_link_detach;
 		bpf_map__ifindex;
 		bpf_map__key_size;
 		bpf_map__map_flags;