[PATCH bpf-next] bpf: clean up btf_scan_decl_tags()

[Matt Bobrowski <mattbobrowski@google.com>]

Refactor the newly introduced btf_scan_decl_tags() to improve
readability and maintainability. The current implementation uses a
manual if-else chain and a magic number offset to strip the "arg:"
prefix from declaration tags.

Replace the if-else logic with a table-driven approach using a static
const array. This separates the tag data from the scanning logic, making
the helper more extensible for future tags. Additionally, replace the
magic number '4' with a sizeof-based calculation on the prefix string to
ensure the offset remains synchronized with the search key.

Finally, optimize the loop by moving the is_global check to the top of
the block. This allows the verifier to fail-fast on static subprograms
without performing unnecessary BTF string and type lookups.

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 kernel/bpf/btf.c | 46 ++++++++++++++++++++++++++++++----------------
 1 file changed, 30 insertions(+), 16 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 68921d9172b5..55aa3ba1b1e0 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -7808,14 +7808,28 @@ static int btf_scan_decl_tags(struct bpf_verifier_env *env,
 			      u32 arg_idx, bool is_global, u32 *tags)
 {
 	int id = btf_named_start_id(btf, false) - 1;
+	const char tag_key[] = "arg:";
+	static const struct {
+		const char *tag_value;
+		enum btf_arg_tag arg_tag;
+	} tag_values[] = {
+		{ "ctx", ARG_TAG_CTX },
+		{ "trusted", ARG_TAG_TRUSTED },
+		{ "untrusted", ARG_TAG_UNTRUSTED },
+		{ "nonnull", ARG_TAG_NONNULL },
+		{ "nullable", ARG_TAG_NULLABLE },
+		{ "arena", ARG_TAG_ARENA },
+	};
 
 	/*
 	 * 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;
+	while ((id = btf_find_next_decl_tag(btf, fn_t, arg_idx, tag_key, id)) > 0) {
+		const struct btf_type *tag_t;
+		const char *tag;
+		int i;
+		bool found;
 
 		/* disallow arg tags in static subprogs */
 		if (!is_global) {
@@ -7825,19 +7839,19 @@ static int btf_scan_decl_tags(struct bpf_verifier_env *env,
 			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 {
+		tag_t = btf_type_by_id(btf, id);
+		tag = __btf_name_by_offset(btf, tag_t->name_off) + (sizeof(tag_key) - 1);
+
+		found = false;
+		for (i = 0; i < ARRAY_SIZE(tag_values); ++i) {
+			if (!strcmp(tag, tag_values[i].tag_value)) {
+				*tags |= tag_values[i].arg_tag;
+				found = true;
+				break;
+			}
+		}
+
+		if (!found) {
 			bpf_log(&env->log, "arg#%d has unsupported set of tags\n", arg_idx);
 			return -EOPNOTSUPP;
 		}
-- 
2.54.0.1032.g2f8565e1d1-goog