Fuzzer for base::nix::GetFileMimeType()

Change-Id: I0160bba4f9a5e9feb99b02a9de052f946fd98066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4982470
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1216720}

base/BUILD.gn

@@ -4987,6 +4987,13 @@ fuzzer_test("file_path_fuzzer") {
   deps = [ "//base" ]
 }
 
+if (is_linux || is_chromeos) {
+  fuzzer_test("mime_util_xdg_fuzzer") {
+    sources = [ "nix/mime_util_xdg_fuzzer.cc" ]
+    deps = [ "//base" ]
+  }
+}
+
 fuzzer_test("string_number_conversions_fuzzer") {
   sources = [ "strings/string_number_conversions_fuzzer.cc" ]
   deps = [ "//base" ]

base/nix/mime_util_xdg_fuzzer.cc

@@ -0,0 +1,38 @@
+// Copyright 2023 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <stdlib.h>
+#include <string>
+
+#include "base/containers/span.h"
+#include "base/files/file_path.h"
+#include "base/files/file_util.h"
+#include "base/files/scoped_temp_dir.h"
+#include "base/logging.h"
+#include "base/nix/mime_util_xdg.h"
+
+// Entry point for LibFuzzer.
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  base::ScopedTempDir temp_dir;
+  if (!temp_dir.CreateUniqueTempDir()) {
+    // Not a fuzzer error, so we return 0.
+    LOG(ERROR) << "Failed to create temp dir";
+    return 0;
+  }
+
+  // The parser reads file $XDG_DATA_DIRS/mime/mime.cache.
+  setenv("XDG_DATA_DIRS", temp_dir.GetPath().value().c_str(), 1);
+  base::FilePath mime_dir = temp_dir.GetPath().Append("mime");
+  base::FilePath mime_cache = mime_dir.Append("mime.cache");
+  if (!base::CreateDirectory(mime_dir) ||
+      !base::WriteFile(mime_cache, base::make_span(data, size))) {
+    LOG(ERROR) << "Failed to create " << mime_cache;
+    // Not a fuzzer error, so we return 0.
+    return 0;
+  }
+
+  base::FilePath dummy_path("foo.txt");
+  std::string type = base::nix::GetFileMimeType(dummy_path);
+  return 0;
+}