Introduce bpf_link abstraction, representing an attachment of BPF program to
a BPF hook point (e.g., tracepoint, perf event, etc). bpf_link encapsulates
ownership of attached BPF program, reference counting of a link itself, when
reference from multiple anonymous inodes, as well as ensures that release
callback will be called from a process context, so that users can safely take
mutex locks and sleep.

Additionally, with a new abstraction it's now possible to generalize pinning
of a link object in BPF FS, allowing to explicitly prevent BPF program
detachment on process exit by pinning it in a BPF FS and let it open from
independent other process to keep working with it.

Convert two existing bpf_link-like objects (raw tracepoint and tracing BPF
program attachments) into utilizing bpf_link framework, making them pinnable
in BPF FS. More FD-based bpf_links will be added in follow up patches.

Signed-off-by: Andrii Nakryiko <redacted>
---
 include/linux/bpf.h  |  13 +++
 kernel/bpf/inode.c   |  42 ++++++++-
 kernel/bpf/syscall.c | 209 ++++++++++++++++++++++++++++++++++++-------
 3 files changed, 226 insertions(+), 38 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 6015a4daf118..f13c78c6f29d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1056,6 +1056,19 @@ extern int sysctl_unprivileged_bpf_disabled;
 int bpf_map_new_fd(struct bpf_map *map, int flags);
 int bpf_prog_new_fd(struct bpf_prog *prog);
 
+struct bpf_link;
+
+struct bpf_link_ops {
+	void (*release)(struct bpf_link *link);
+};
+
+void bpf_link_init(struct bpf_link *link, const struct bpf_link_ops *ops,
+		   struct bpf_prog *prog);
+void bpf_link_inc(struct bpf_link *link);
+void bpf_link_put(struct bpf_link *link);
+int bpf_link_new_fd(struct bpf_link *link);
+struct bpf_link *bpf_link_get_from_fd(u32 ufd);
+
 int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
 int bpf_obj_get_user(const char __user *pathname, int flags);
 

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 5e40e7fccc21..95087d9f4ed3 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c

@@ -25,6 +25,7 @@ enum bpf_type {
 	BPF_TYPE_UNSPEC	= 0,
 	BPF_TYPE_PROG,
 	BPF_TYPE_MAP,
+	BPF_TYPE_LINK,
 };
 
 static void *bpf_any_get(void *raw, enum bpf_type type)

@@ -36,6 +37,9 @@ static void *bpf_any_get(void *raw, enum bpf_type type)
 	case BPF_TYPE_MAP:
 		bpf_map_inc_with_uref(raw);
 		break;
+	case BPF_TYPE_LINK:
+		bpf_link_inc(raw);
+		break;
 	default:
 		WARN_ON_ONCE(1);
 		break;

@@ -53,6 +57,9 @@ static void bpf_any_put(void *raw, enum bpf_type type)
 	case BPF_TYPE_MAP:
 		bpf_map_put_with_uref(raw);
 		break;
+	case BPF_TYPE_LINK:
+		bpf_link_put(raw);
+		break;
 	default:
 		WARN_ON_ONCE(1);
 		break;

@@ -63,20 +70,32 @@ static void *bpf_fd_probe_obj(u32 ufd, enum bpf_type *type)
 {
 	void *raw;
 
-	*type = BPF_TYPE_MAP;
 	raw = bpf_map_get_with_uref(ufd);
-	if (IS_ERR(raw)) {
+	if (!IS_ERR(raw)) {
+		*type = BPF_TYPE_MAP;
+		return raw;
+	}
+
+	raw = bpf_prog_get(ufd);
+	if (!IS_ERR(raw)) {
 		*type = BPF_TYPE_PROG;
-		raw = bpf_prog_get(ufd);
+		return raw;
 	}
 
-	return raw;
+	raw = bpf_link_get_from_fd(ufd);
+	if (!IS_ERR(raw)) {
+		*type = BPF_TYPE_LINK;
+		return raw;
+	}
+
+	return ERR_PTR(-EINVAL);
 }
 
 static const struct inode_operations bpf_dir_iops;
 
 static const struct inode_operations bpf_prog_iops = { };
 static const struct inode_operations bpf_map_iops  = { };
+static const struct inode_operations bpf_link_iops  = { };
 
 static struct inode *bpf_get_inode(struct super_block *sb,
 				   const struct inode *dir,

@@ -114,6 +133,8 @@ static int bpf_inode_type(const struct inode *inode, enum bpf_type *type)
 		*type = BPF_TYPE_PROG;
 	else if (inode->i_op == &bpf_map_iops)
 		*type = BPF_TYPE_MAP;
+	else if (inode->i_op == &bpf_link_iops)
+		*type = BPF_TYPE_LINK;
 	else
 		return -EACCES;
 

@@ -335,6 +356,12 @@ static int bpf_mkmap(struct dentry *dentry, umode_t mode, void *arg)
 			     &bpffs_map_fops : &bpffs_obj_fops);
 }
 
+static int bpf_mklink(struct dentry *dentry, umode_t mode, void *arg)
+{
+	return bpf_mkobj_ops(dentry, mode, arg, &bpf_link_iops,
+			     &bpffs_obj_fops);
+}
+
 static struct dentry *
 bpf_lookup(struct inode *dir, struct dentry *dentry, unsigned flags)
 {

@@ -411,6 +438,9 @@ static int bpf_obj_do_pin(const char __user *pathname, void *raw,
 	case BPF_TYPE_MAP:
 		ret = vfs_mkobj(dentry, mode, bpf_mkmap, raw);
 		break;
+	case BPF_TYPE_LINK:
+		ret = vfs_mkobj(dentry, mode, bpf_mklink, raw);
+		break;
 	default:
 		ret = -EPERM;
 	}

@@ -487,6 +517,8 @@ int bpf_obj_get_user(const char __user *pathname, int flags)
 		ret = bpf_prog_new_fd(raw);
 	else if (type == BPF_TYPE_MAP)
 		ret = bpf_map_new_fd(raw, f_flags);
+	else if (type == BPF_TYPE_LINK)
+		ret = bpf_link_new_fd(raw);
 	else
 		return -ENOENT;
 

@@ -504,6 +536,8 @@ static struct bpf_prog *__get_prog_inode(struct inode *inode, enum bpf_prog_type
 
 	if (inode->i_op == &bpf_map_iops)
 		return ERR_PTR(-EINVAL);
+	if (inode->i_op == &bpf_link_iops)
+		return ERR_PTR(-EINVAL);
 	if (inode->i_op != &bpf_prog_iops)
 		return ERR_PTR(-EACCES);
 

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c536c65256ad..fca8de7e7872 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -2173,23 +2173,153 @@ static int bpf_obj_get(const union bpf_attr *attr)
 				attr->file_flags);
 }
 
-static int bpf_tracing_prog_release(struct inode *inode, struct file *filp)
+struct bpf_link {
+	atomic64_t refcnt;