bpf: Add bpf_link iterator

Implement bpf_link iterator to traverse links via bpf_seq_file
operations. The changeset is mostly shamelessly copied from
commit a228a64fc1 ("bpf: Add bpf_prog iterator")

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220510155233.9815-2-9erthalion6@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -1544,6 +1544,7 @@ void bpf_link_put(struct bpf_link *link);
 int bpf_link_new_fd(struct bpf_link *link);
 struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
 struct bpf_link *bpf_link_get_from_fd(u32 ufd);
+struct bpf_link *bpf_link_get_curr_or_next(u32 *id);
 
 int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
 int bpf_obj_get_user(const char __user *pathname, int flags);

kernel/bpf/Makefile

@@ -6,7 +6,7 @@ cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
 endif
 CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
 
-obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
+obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
 obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
 obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o

kernel/bpf/link_iter.c

@@ -0,0 +1,107 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2022 Red Hat, Inc. */
+#include <linux/bpf.h>
+#include <linux/fs.h>
+#include <linux/filter.h>
+#include <linux/kernel.h>
+#include <linux/btf_ids.h>
+
+struct bpf_iter_seq_link_info {
+	u32 link_id;
+};
+
+static void *bpf_link_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	struct bpf_iter_seq_link_info *info = seq->private;
+	struct bpf_link *link;
+
+	link = bpf_link_get_curr_or_next(&info->link_id);
+	if (!link)
+		return NULL;
+
+	if (*pos == 0)
+		++*pos;
+	return link;
+}
+
+static void *bpf_link_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct bpf_iter_seq_link_info *info = seq->private;
+
+	++*pos;
+	++info->link_id;
+	bpf_link_put((struct bpf_link *)v);
+	return bpf_link_get_curr_or_next(&info->link_id);
+}
+
+struct bpf_iter__bpf_link {
+	__bpf_md_ptr(struct bpf_iter_meta *, meta);
+	__bpf_md_ptr(struct bpf_link *, link);
+};
+
+DEFINE_BPF_ITER_FUNC(bpf_link, struct bpf_iter_meta *meta, struct bpf_link *link)
+
+static int __bpf_link_seq_show(struct seq_file *seq, void *v, bool in_stop)
+{
+	struct bpf_iter__bpf_link ctx;
+	struct bpf_iter_meta meta;
+	struct bpf_prog *prog;
+	int ret = 0;
+
+	ctx.meta = &meta;
+	ctx.link = v;
+	meta.seq = seq;
+	prog = bpf_iter_get_info(&meta, in_stop);
+	if (prog)
+		ret = bpf_iter_run_prog(prog, &ctx);
+
+	return ret;
+}
+
+static int bpf_link_seq_show(struct seq_file *seq, void *v)
+{
+	return __bpf_link_seq_show(seq, v, false);
+}
+
+static void bpf_link_seq_stop(struct seq_file *seq, void *v)
+{
+	if (!v)
+		(void)__bpf_link_seq_show(seq, v, true);
+	else
+		bpf_link_put((struct bpf_link *)v);
+}
+
+static const struct seq_operations bpf_link_seq_ops = {
+	.start	= bpf_link_seq_start,
+	.next	= bpf_link_seq_next,
+	.stop	= bpf_link_seq_stop,
+	.show	= bpf_link_seq_show,
+};
+
+BTF_ID_LIST(btf_bpf_link_id)
+BTF_ID(struct, bpf_link)
+
+static const struct bpf_iter_seq_info bpf_link_seq_info = {
+	.seq_ops		= &bpf_link_seq_ops,
+	.init_seq_private	= NULL,
+	.fini_seq_private	= NULL,
+	.seq_priv_size		= sizeof(struct bpf_iter_seq_link_info),
+};
+
+static struct bpf_iter_reg bpf_link_reg_info = {
+	.target			= "bpf_link",
+	.ctx_arg_info_size	= 1,
+	.ctx_arg_info		= {
+		{ offsetof(struct bpf_iter__bpf_link, link),
+		  PTR_TO_BTF_ID_OR_NULL },
+	},
+	.seq_info		= &bpf_link_seq_info,
+};
+
+static int __init bpf_link_iter_init(void)
+{
+	bpf_link_reg_info.ctx_arg_info[0].btf_id = *btf_bpf_link_id;
+	return bpf_iter_reg_target(&bpf_link_reg_info);
+}
+
+late_initcall(bpf_link_iter_init);

kernel/bpf/syscall.c

@@ -4680,6 +4680,25 @@ struct bpf_link *bpf_link_by_id(u32 id)
 	return link;
 }
 
+struct bpf_link *bpf_link_get_curr_or_next(u32 *id)
+{
+	struct bpf_link *link;
+
+	spin_lock_bh(&link_idr_lock);
+again:
+	link = idr_get_next(&link_idr, id);
+	if (link) {
+		link = bpf_link_inc_not_zero(link);
+		if (IS_ERR(link)) {
+			(*id)++;
+			goto again;
+		}
+	}
+	spin_unlock_bh(&link_idr_lock);
+
+	return link;
+}
+
 #define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id
 
 static int bpf_link_get_fd_by_id(const union bpf_attr *attr)