libbpf: Add bpf_link pinning/unpinning

With bpf_link abstraction supported by kernel explicitly, add
pinning/unpinning API for links. Also allow to create (open) bpf_link from BPF
FS file.

This API allows to have an "ephemeral" FD-based BPF links (like raw tracepoint
or fexit/freplace attachments) surviving user process exit, by pinning them in
a BPF FS, which is an important use case for long-running BPF programs.

As part of this, expose underlying FD for bpf_link. While legacy bpf_link's
might not have a FD associated with them (which will be expressed as
a bpf_link with fd=-1), kernel's abstraction is based around FD-based usage,
so match it closely. This, subsequently, allows to have a generic
pinning/unpinning API for generalized bpf_link. For some types of bpf_links
kernel might not support pinning, in which case bpf_link__pin() will return
error.

With FD being part of generic bpf_link, also get rid of bpf_link_fd in favor
of using vanialla bpf_link.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/libbpf.c

···
 struct bpf_link {
 	int (*detach)(struct bpf_link *link);
 	int (*destroy)(struct bpf_link *link);
+	char *pin_path;		/* NULL, if not pinned */
+	int fd;			/* hook FD, -1 if not applicable */
 	bool disconnected;
 };
 
···
 		err = link->detach(link);
 	if (link->destroy)
 		link->destroy(link);
+	if (link->pin_path)
+		free(link->pin_path);
 	free(link);
 
 	return err;
 }
 
-struct bpf_link_fd {
-	struct bpf_link link; /* has to be at the top of struct */
-	int fd; /* hook FD */
-};
+int bpf_link__fd(const struct bpf_link *link)
+{
+	return link->fd;
+}
+
+const char *bpf_link__pin_path(const struct bpf_link *link)
+{
+	return link->pin_path;
+}
+
+static int bpf_link__detach_fd(struct bpf_link *link)
+{
+	return close(link->fd);
+}
+
+struct bpf_link *bpf_link__open(const char *path)
+{
+	struct bpf_link *link;
+	int fd;
+
+	fd = bpf_obj_get(path);
+	if (fd < 0) {
+		fd = -errno;
+		pr_warn("failed to open link at %s: %d\n", path, fd);
+		return ERR_PTR(fd);
+	}
+
+	link = calloc(1, sizeof(*link));
+	if (!link) {
+		close(fd);
+		return ERR_PTR(-ENOMEM);
+	}
+	link->detach = &bpf_link__detach_fd;
+	link->fd = fd;
+
+	link->pin_path = strdup(path);
+	if (!link->pin_path) {
+		bpf_link__destroy(link);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	return link;
+}
+
+int bpf_link__pin(struct bpf_link *link, const char *path)
+{
+	int err;
+
+	if (link->pin_path)
+		return -EBUSY;
+	err = make_parent_dir(path);
+	if (err)
+		return err;
+	err = check_path(path);
+	if (err)
+		return err;
+
+	link->pin_path = strdup(path);
+	if (!link->pin_path)
+		return -ENOMEM;
+
+	if (bpf_obj_pin(link->fd, link->pin_path)) {
+		err = -errno;
+		zfree(&link->pin_path);
+		return err;
+	}
+
+	pr_debug("link fd=%d: pinned at %s\n", link->fd, link->pin_path);
+	return 0;
+}
+
+int bpf_link__unpin(struct bpf_link *link)
+{
+	int err;
+
+	if (!link->pin_path)
+		return -EINVAL;
+
+	err = unlink(link->pin_path);
+	if (err != 0)
+		return -errno;
+
+	pr_debug("link fd=%d: unpinned from %s\n", link->fd, link->pin_path);
+	zfree(&link->pin_path);
+	return 0;
+}
 
 static int bpf_link__detach_perf_event(struct bpf_link *link)
 {
-	struct bpf_link_fd *l = (void *)link;
 	int err;
 
-	err = ioctl(l->fd, PERF_EVENT_IOC_DISABLE, 0);
+	err = ioctl(link->fd, PERF_EVENT_IOC_DISABLE, 0);
 	if (err)
 		err = -errno;
 
-	close(l->fd);
+	close(link->fd);
 	return err;
 }
 
···
 						int pfd)
 {
 	char errmsg[STRERR_BUFSIZE];
-	struct bpf_link_fd *link;
+	struct bpf_link *link;
 	int prog_fd, err;
 
 	if (pfd < 0) {
···
 	link = calloc(1, sizeof(*link));
 	if (!link)
 		return ERR_PTR(-ENOMEM);
-	link->link.detach = &bpf_link__detach_perf_event;
+	link->detach = &bpf_link__detach_perf_event;
 	link->fd = pfd;
 
 	if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
···
 			   libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
 		return ERR_PTR(err);
 	}
-	return (struct bpf_link *)link;
+	return link;
 }
 
 /*
···
 	return link;
 }
 
-static int bpf_link__detach_fd(struct bpf_link *link)
-{
-	struct bpf_link_fd *l = (void *)link;
-
-	return close(l->fd);
-}
-
 struct bpf_link *bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
 						    const char *tp_name)
 {
 	char errmsg[STRERR_BUFSIZE];
-	struct bpf_link_fd *link;
+	struct bpf_link *link;
 	int prog_fd, pfd;
 
 	prog_fd = bpf_program__fd(prog);
···
 	link = calloc(1, sizeof(*link));
 	if (!link)
 		return ERR_PTR(-ENOMEM);
-	link->link.detach = &bpf_link__detach_fd;
+	link->detach = &bpf_link__detach_fd;
 
 	pfd = bpf_raw_tracepoint_open(tp_name, prog_fd);
 	if (pfd < 0) {
···
 		return ERR_PTR(pfd);
 	}
 	link->fd = pfd;
-	return (struct bpf_link *)link;
+	return link;
 }
 
 static struct bpf_link *attach_raw_tp(const struct bpf_sec_def *sec,
···
 struct bpf_link *bpf_program__attach_trace(struct bpf_program *prog)
 {
 	char errmsg[STRERR_BUFSIZE];
-	struct bpf_link_fd *link;
+	struct bpf_link *link;
 	int prog_fd, pfd;
 
 	prog_fd = bpf_program__fd(prog);
···
 	link = calloc(1, sizeof(*link));
 	if (!link)
 		return ERR_PTR(-ENOMEM);
-	link->link.detach = &bpf_link__detach_fd;
+	link->detach = &bpf_link__detach_fd;
 
 	pfd = bpf_raw_tracepoint_open(NULL, prog_fd);
 	if (pfd < 0) {
···
 
 static int bpf_link__detach_struct_ops(struct bpf_link *link)
 {
-	struct bpf_link_fd *l = (void *)link;
 	__u32 zero = 0;
 
-	if (bpf_map_delete_elem(l->fd, &zero))
+	if (bpf_map_delete_elem(link->fd, &zero))
 		return -errno;
 
 	return 0;
···
 struct bpf_link *bpf_map__attach_struct_ops(struct bpf_map *map)
 {
 	struct bpf_struct_ops *st_ops;
-	struct bpf_link_fd *link;
+	struct bpf_link *link;
 	__u32 i, zero = 0;
 	int err;
 
···
 		return ERR_PTR(err);
 	}
 
-	link->link.detach = bpf_link__detach_struct_ops;
+	link->detach = bpf_link__detach_struct_ops;
 	link->fd = map->fd;
 
-	return (struct bpf_link *)link;
+	return link;
 }
 
 enum bpf_perf_event_ret

tools/lib/bpf/libbpf.h

···
 
 struct bpf_link;
 
+LIBBPF_API struct bpf_link *bpf_link__open(const char *path);
+LIBBPF_API int bpf_link__fd(const struct bpf_link *link);
+LIBBPF_API const char *bpf_link__pin_path(const struct bpf_link *link);
+LIBBPF_API int bpf_link__pin(struct bpf_link *link, const char *path);
+LIBBPF_API int bpf_link__unpin(struct bpf_link *link);
 LIBBPF_API void bpf_link__disconnect(struct bpf_link *link);
 LIBBPF_API int bpf_link__destroy(struct bpf_link *link);
 

tools/lib/bpf/libbpf.map

···
 
 LIBBPF_0.0.8 {
 	global:
+		bpf_link__fd;
+		bpf_link__open;
+		bpf_link__pin;
+		bpf_link__pin_path;
+		bpf_link__unpin;
 		bpf_program__set_attach_target;
 } LIBBPF_0.0.7;