fix(elf): preserve compiler metadata after an empty kernel builder

Squashed from #9950 and rebased onto main.

Fixes #9942
Supersedes #9950

Co-Authored-By: gensokyo-dev <342580529+gensokyo-dev@users.noreply.github.com>

docs/guides/native/kernel-build-config.md

@@ -31,8 +31,9 @@ otherwise.
 `banner` is the first printable string that starts with `Linux version `
 followed by a version token beginning with a digit and ends at a NUL or
 newline within 512 bytes. `version` is that token. `builder` is the first
-parenthesized group (`user@host`). `compiler` is the second group up to the
-last `) #`, so nested parentheses in compiler strings are kept. Since Linux
+parenthesized group (`user@host`) and is null when that group is empty.
+`compiler` is the second group up to the last `) #`, so nested parentheses in
+compiler strings are kept. Since Linux
 6.1, kbuild also leaves the weak banner of `init/version.o`. It is compiled
 before the build number and timestamp exist, so unless `KBUILD_BUILD_VERSION`
 and `KBUILD_BUILD_TIMESTAMP` are both set, it has a space right after `#` or at

src/native/elf/KernelBuildConfig.ts

@@ -530,8 +530,8 @@ const parseBannerText = (
   let compiler: string | null = null;
   if (rest.startsWith(" (")) {
     const close = rest.indexOf(")");
-    if (close > 2) {
-      builder = rest.slice(2, close);
+    if (close >= 2) {
+      builder = rest.slice(2, close) || null;
       rest = rest.slice(close + 1);
     }
   }

tests/acceptance/kernelBuildConfigCli.test.ts

@@ -40,6 +40,29 @@ cliTest(
   },
 );
 
+cliTest(
+  "keeps the compiler group separate from an empty builder group",
+  async ({ cli }) => {
+    const root = await createTestTempDirectory("rea-kernel-banner-cli-");
+    const path = join(root, "banner.bin");
+    const text = "Linux version 6.1.0 () (gcc 12.2.0) #1 SMP";
+    await writeFile(path, `${text}\0`);
+    const result = await cli.run({
+      arguments: ["inspect-kernel-build-config", path, "--json"],
+    });
+    expect(result.exitCode, result.stderr).toBe(0);
+    const evidence = contract.outputSchema.parse(result.json);
+    expect(evidence.normalized_result.banner).toMatchObject({
+      status: "present",
+      text,
+      version: "6.1.0",
+      builder: null,
+      compiler: "gcc 12.2.0",
+      location: { space: "file", offset: 0, bytes: text.length },
+    });
+  },
+);
+
 cliTest(
   "resolves a relative bzImage path and pages its inflated config",
   async ({ cli }) => {