bpf: Remove anonymous union in bpf_kfunc_call_arg_meta

For kfuncs like bpf_obj_drop and bpf_refcount_acquire - which take
user-defined types as input - the verifier needs to track the specific
type passed in when checking a particular kfunc call. This requires
tracking (btf, btf_id) tuple. In commit 7c50b1cb
("bpf: Add bpf_refcount_acquire kfunc") I added an anonymous union with
inner structs named after the specific kfuncs tracking this information,
with the goal of making it more obvious which kfunc this data was being
tracked / expected to be tracked on behalf of.

In a recent series adding a new user of this tuple, Alexei mentioned
that he didn't like this union usage as it doesn't really help with
readability or bug-proofing ([0]). In an offline convo we agreed to
have the tuple be fields (arg_btf, arg_btf_id), with comments in
bpf_kfunc_call_arg_meta definition enumerating the uses of the fields by
kfunc-specific handling logic. Such a pattern is used by struct
bpf_reg_state without trouble.

Accordingly, this patch removes the anonymous union in favor of arg_btf
and arg_btf_id fields and comment enumerating their current uses. The
patch also removes struct btf_and_id, which was only being used by the
removed union's inner structs.

This is a mechanical change, existing linked_list and rbtree tests will
validate that correct (btf, btf_id) are being passed.

  [0]: https://lore.kernel.org/bpf/20230505021707.vlyiwy57vwxglbka@dhcp-172-26-102-232.dhcp.thefacebook.comSigned-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230510213047.1633612-1-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -279,11 +279,6 @@ struct bpf_call_arg_meta {
 	struct btf_field *kptr_field;
 };
 
-struct btf_and_id {
-	struct btf *btf;
-	u32 btf_id;
-};
-
 struct bpf_kfunc_call_arg_meta {
 	/* In parameters */
 	struct btf *btf;
@@ -302,10 +297,18 @@ struct bpf_kfunc_call_arg_meta {
 		u64 value;
 		bool found;
 	} arg_constant;
-	union {
-		struct btf_and_id arg_obj_drop;
-		struct btf_and_id arg_refcount_acquire;
-	};
+
+	/* arg_btf and arg_btf_id are used by kfunc-specific handling,
+	 * generally to pass info about user-defined local kptr types to later
+	 * verification logic
+	 *   bpf_obj_drop
+	 *     Record the local kptr type to be drop'd
+	 *   bpf_refcount_acquire (via KF_ARG_PTR_TO_REFCOUNTED_KPTR arg type)
+	 *     Record the local kptr type to be refcount_incr'd
+	 */
+	struct btf *arg_btf;
+	u32 arg_btf_id;
+
 	struct {
 		struct btf_field *field;
 	} arg_list_head;
@@ -10680,8 +10683,8 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			}
 			if (meta->btf == btf_vmlinux &&
 			    meta->func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
-				meta->arg_obj_drop.btf = reg->btf;
-				meta->arg_obj_drop.btf_id = reg->btf_id;
+				meta->arg_btf = reg->btf;
+				meta->arg_btf_id = reg->btf_id;
 			}
 			break;
 		case KF_ARG_PTR_TO_DYNPTR:
@@ -10892,8 +10895,8 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 				verbose(env, "bpf_refcount_acquire calls are disabled for now\n");
 				return -EINVAL;
 			}
-			meta->arg_refcount_acquire.btf = reg->btf;
-			meta->arg_refcount_acquire.btf_id = reg->btf_id;
+			meta->arg_btf = reg->btf;
+			meta->arg_btf_id = reg->btf_id;
 			break;
 		}
 	}
@@ -11125,12 +11128,12 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			} else if (meta.func_id == special_kfunc_list[KF_bpf_refcount_acquire_impl]) {
 				mark_reg_known_zero(env, regs, BPF_REG_0);
 				regs[BPF_REG_0].type = PTR_TO_BTF_ID | MEM_ALLOC;
-				regs[BPF_REG_0].btf = meta.arg_refcount_acquire.btf;
-				regs[BPF_REG_0].btf_id = meta.arg_refcount_acquire.btf_id;
+				regs[BPF_REG_0].btf = meta.arg_btf;
+				regs[BPF_REG_0].btf_id = meta.arg_btf_id;
 
 				insn_aux->kptr_struct_meta =
-					btf_find_struct_meta(meta.arg_refcount_acquire.btf,
-							     meta.arg_refcount_acquire.btf_id);
+					btf_find_struct_meta(meta.arg_btf,
+							     meta.arg_btf_id);
 			} else if (meta.func_id == special_kfunc_list[KF_bpf_list_pop_front] ||
 				   meta.func_id == special_kfunc_list[KF_bpf_list_pop_back]) {
 				struct btf_field *field = meta.arg_list_head.field;
@@ -11260,8 +11263,8 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		if (meta.btf == btf_vmlinux && btf_id_set_contains(&special_kfunc_set, meta.func_id)) {
 			if (meta.func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
 				insn_aux->kptr_struct_meta =
-					btf_find_struct_meta(meta.arg_obj_drop.btf,
-							     meta.arg_obj_drop.btf_id);
+					btf_find_struct_meta(meta.arg_btf,
+							     meta.arg_btf_id);
 			}
 		}
 	}