verifier: parse BTF type tags for function arguments

The BTF parsing logic for function arguments goes through
the arguments' decl tags, but does not go into their type
tags. Add type tag parsing for function arguments.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260602004120.17087-3-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: etsal

kernel/bpf/btf.c

@@ -7802,6 +7802,84 @@ enum btf_arg_tag {
 	ARG_TAG_ARENA	  = BIT_ULL(5),
 };
 
+static int btf_scan_decl_tags(struct bpf_verifier_env *env,
+			      const struct btf *btf,
+			      const struct btf_type *fn_t,
+			      u32 arg_idx, bool is_global, u32 *tags)
+{
+	int id = btf_named_start_id(btf, false) - 1;
+
+	/*
+	 * The 'arg:<tag>' decl_tag takes precedence over the derivation
+	 * of the register type from the BTF type itself.
+	 */
+	while ((id = btf_find_next_decl_tag(btf, fn_t, arg_idx, "arg:", id)) > 0) {
+		const struct btf_type *tag_t = btf_type_by_id(btf, id);
+		const char *tag = __btf_name_by_offset(btf, tag_t->name_off) + 4;
+
+		/* disallow arg tags in static subprogs */
+		if (!is_global) {
+			bpf_log(&env->log,
+				"arg#%d type tag is not supported in static functions\n",
+				arg_idx);
+			return -EOPNOTSUPP;
+		}
+
+		if (strcmp(tag, "ctx") == 0) {
+			*tags |= ARG_TAG_CTX;
+		} else if (strcmp(tag, "trusted") == 0) {
+			*tags |= ARG_TAG_TRUSTED;
+		} else if (strcmp(tag, "untrusted") == 0) {
+			*tags |= ARG_TAG_UNTRUSTED;
+		} else if (strcmp(tag, "nonnull") == 0) {
+			*tags |= ARG_TAG_NONNULL;
+		} else if (strcmp(tag, "nullable") == 0) {
+			*tags |= ARG_TAG_NULLABLE;
+		} else if (strcmp(tag, "arena") == 0) {
+			*tags |= ARG_TAG_ARENA;
+		} else {
+			bpf_log(&env->log, "arg#%d has unsupported set of tags\n", arg_idx);
+			return -EOPNOTSUPP;
+		}
+	}
+	if (id != -ENOENT) {
+		bpf_log(&env->log, "arg#%d type tag fetching failure: %d\n", arg_idx, id);
+		return id;
+	}
+
+	return 0;
+}
+
+static int btf_scan_type_tags(struct bpf_verifier_env *env,
+			      const struct btf *btf, u32 type_id,
+			      u32 *tags)
+{
+	const struct btf_type *t;
+
+	/* Find the first pointer type in the chain. */
+	t = btf_type_skip_modifiers(btf, type_id, NULL);
+	if (!t || !btf_type_is_ptr(t))
+		return 0;
+
+	/* We got a pointer, get all associated type tags. */
+	t = btf_type_by_id(btf, t->type);
+	while (t && btf_type_is_type_tag(t)) {
+		const char *tag = __btf_name_by_offset(btf, t->name_off);
+
+		if (strcmp(tag, "arena") == 0) {
+			*tags |= ARG_TAG_ARENA;
+		} else {
+			bpf_log(&env->log, "function signature member has unsupported type tag '%s'\n",
+				tag);
+			return -EOPNOTSUPP;
+		}
+
+		t = btf_type_by_id(btf, t->type);
+	}
+
+	return 0;
+}
+
 /* Process BTF of a function to produce high-level expectation of function
  * arguments (like ARG_PTR_TO_CTX, or ARG_PTR_TO_MEM, etc). This information
  * is cached in subprog info for reuse.
@@ -7820,6 +7898,7 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 	struct btf *btf = prog->aux->btf;
 	const struct btf_param *args;
 	const struct btf_type *t, *ref_t, *fn_t;
+	int err;
 	u32 i, nargs, btf_id;
 	const char *tname;
 
@@ -7903,42 +7982,13 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 	 */
 	for (i = 0; i < nargs; i++) {
 		u32 tags = 0;
-		int id = btf_named_start_id(btf, false) - 1;
-
-		/* 'arg:<tag>' decl_tag takes precedence over derivation of
-		 * register type from BTF type itself
-		 */
-		while ((id = btf_find_next_decl_tag(btf, fn_t, i, "arg:", id)) > 0) {
-			const struct btf_type *tag_t = btf_type_by_id(btf, id);
-			const char *tag = __btf_name_by_offset(btf, tag_t->name_off) + 4;
-
-			/* disallow arg tags in static subprogs */
-			if (!is_global) {
-				bpf_log(log, "arg#%d type tag is not supported in static functions\n", i);
-				return -EOPNOTSUPP;
-			}
+		err = btf_scan_decl_tags(env, btf, fn_t, i, is_global, &tags);
+		if (err)
+			return err;
 
-			if (strcmp(tag, "ctx") == 0) {
-				tags |= ARG_TAG_CTX;
-			} else if (strcmp(tag, "trusted") == 0) {
-				tags |= ARG_TAG_TRUSTED;
-			} else if (strcmp(tag, "untrusted") == 0) {
-				tags |= ARG_TAG_UNTRUSTED;
-			} else if (strcmp(tag, "nonnull") == 0) {
-				tags |= ARG_TAG_NONNULL;
-			} else if (strcmp(tag, "nullable") == 0) {
-				tags |= ARG_TAG_NULLABLE;
-			} else if (strcmp(tag, "arena") == 0) {
-				tags |= ARG_TAG_ARENA;
-			} else {
-				bpf_log(log, "arg#%d has unsupported set of tags\n", i);
-				return -EOPNOTSUPP;
-			}
-		}
-		if (id != -ENOENT) {
-			bpf_log(log, "arg#%d type tag fetching failure: %d\n", i, id);
-			return id;
-		}
+		err = btf_scan_type_tags(env, btf, args[i].type, &tags);
+		if (err)
+			return err;
 
 		t = btf_type_by_id(btf, args[i].type);
 		while (btf_type_is_modifier(t))