bpf: Refactor bpf_link update handling

Make bpf_link update support more generic by making it into another
bpf_link_ops methods. This allows generic syscall handling code to be agnostic
to various conditionally compiled features (e.g., the case of
CONFIG_CGROUP_BPF). This also allows to keep link type-specific code to remain
static within respective code base. Refactor existing bpf_cgroup_link code and
take advantage of this.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429001614.1544-2-andriin@fb.com

include/linux/bpf-cgroup.h

@@ -100,8 +100,6 @@ int __cgroup_bpf_attach(struct cgroup *cgrp,
 int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 			struct bpf_cgroup_link *link,
 			enum bpf_attach_type type);
-int __cgroup_bpf_replace(struct cgroup *cgrp, struct bpf_cgroup_link *link,
-			 struct bpf_prog *new_prog);
 int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 		       union bpf_attr __user *uattr);
 
@@ -112,8 +110,6 @@ int cgroup_bpf_attach(struct cgroup *cgrp,
 		      u32 flags);
 int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 		      enum bpf_attach_type type);
-int cgroup_bpf_replace(struct bpf_link *link, struct bpf_prog *old_prog,
-		       struct bpf_prog *new_prog);
 int cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 		     union bpf_attr __user *uattr);
 
@@ -353,7 +349,6 @@ int cgroup_bpf_prog_query(const union bpf_attr *attr,
 #else
 
 struct bpf_prog;
-struct bpf_link;
 struct cgroup_bpf {};
 static inline int cgroup_bpf_inherit(struct cgroup *cgrp) { return 0; }
 static inline void cgroup_bpf_offline(struct cgroup *cgrp) {}
@@ -377,13 +372,6 @@ static inline int cgroup_bpf_link_attach(const union bpf_attr *attr,
 	return -EINVAL;
 }
 
-static inline int cgroup_bpf_replace(struct bpf_link *link,
-				     struct bpf_prog *old_prog,
-				     struct bpf_prog *new_prog)
-{
-	return -EINVAL;
-}
-
 static inline int cgroup_bpf_prog_query(const union bpf_attr *attr,
 					union bpf_attr __user *uattr)
 {

include/linux/bpf.h

@@ -1093,7 +1093,8 @@ struct bpf_link {
 struct bpf_link_ops {
 	void (*release)(struct bpf_link *link);
 	void (*dealloc)(struct bpf_link *link);
-
+	int (*update_prog)(struct bpf_link *link, struct bpf_prog *new_prog,
+			   struct bpf_prog *old_prog);
 };
 
 void bpf_link_init(struct bpf_link *link, const struct bpf_link_ops *ops,

kernel/bpf/cgroup.c

@@ -557,8 +557,9 @@ static void replace_effective_prog(struct cgroup *cgrp,
  *
  * Must be called with cgroup_mutex held.
  */
-int __cgroup_bpf_replace(struct cgroup *cgrp, struct bpf_cgroup_link *link,
+static int __cgroup_bpf_replace(struct cgroup *cgrp,
-			 struct bpf_prog *new_prog)
+				struct bpf_cgroup_link *link,
+				struct bpf_prog *new_prog)
 {
 	struct list_head *progs = &cgrp->bpf.progs[link->type];
 	struct bpf_prog *old_prog;
@@ -583,6 +584,30 @@ int __cgroup_bpf_replace(struct cgroup *cgrp, struct bpf_cgroup_link *link,
 	return 0;
 }
 
+static int cgroup_bpf_replace(struct bpf_link *link, struct bpf_prog *new_prog,
+			      struct bpf_prog *old_prog)
+{
+	struct bpf_cgroup_link *cg_link;
+	int ret;
+
+	cg_link = container_of(link, struct bpf_cgroup_link, link);
+
+	mutex_lock(&cgroup_mutex);
+	/* link might have been auto-released by dying cgroup, so fail */
+	if (!cg_link->cgroup) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
+	if (old_prog && link->prog != old_prog) {
+		ret = -EPERM;
+		goto out_unlock;
+	}
+	ret = __cgroup_bpf_replace(cg_link->cgroup, cg_link, new_prog);
+out_unlock:
+	mutex_unlock(&cgroup_mutex);
+	return ret;
+}
+
 static struct bpf_prog_list *find_detach_entry(struct list_head *progs,
 					       struct bpf_prog *prog,
 					       struct bpf_cgroup_link *link,
@@ -811,6 +836,7 @@ static void bpf_cgroup_link_dealloc(struct bpf_link *link)
 const struct bpf_link_ops bpf_cgroup_link_lops = {
 	.release = bpf_cgroup_link_release,
 	.dealloc = bpf_cgroup_link_dealloc,
+	.update_prog = cgroup_bpf_replace,
 };
 
 int cgroup_bpf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)

kernel/bpf/syscall.c

@@ -3645,13 +3645,10 @@ static int link_update(union bpf_attr *attr)
 		goto out_put_progs;
 	}
 
-#ifdef CONFIG_CGROUP_BPF
+	if (link->ops->update_prog)
-	if (link->ops == &bpf_cgroup_link_lops) {
+		ret = link->ops->update_prog(link, new_prog, old_prog);
-		ret = cgroup_bpf_replace(link, old_prog, new_prog);
+	else
-		goto out_put_progs;
+		ret = EINVAL;
-	}
-#endif
-	ret = -EINVAL;
 
 out_put_progs:
 	if (old_prog)

kernel/cgroup/cgroup.c

@@ -6508,33 +6508,6 @@ int cgroup_bpf_attach(struct cgroup *cgrp,
 	return ret;
 }
 
-int cgroup_bpf_replace(struct bpf_link *link, struct bpf_prog *old_prog,
-		       struct bpf_prog *new_prog)
-{
-	struct bpf_cgroup_link *cg_link;
-	int ret;
-
-	if (link->ops != &bpf_cgroup_link_lops)
-		return -EINVAL;
-
-	cg_link = container_of(link, struct bpf_cgroup_link, link);
-
-	mutex_lock(&cgroup_mutex);
-	/* link might have been auto-released by dying cgroup, so fail */
-	if (!cg_link->cgroup) {
-		ret = -EINVAL;
-		goto out_unlock;
-	}
-	if (old_prog && link->prog != old_prog) {
-		ret = -EPERM;
-		goto out_unlock;
-	}
-	ret = __cgroup_bpf_replace(cg_link->cgroup, cg_link, new_prog);
-out_unlock:
-	mutex_unlock(&cgroup_mutex);
-	return ret;
-}
-
 int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 		      enum bpf_attach_type type)
 {