[libc++][format] adds a basic fuzzer test.

[mordante]

This adds an initial fuzzer. Different formatting arguments will execute different code paths. This will be tested by different fuzzer tests.

The code is based on a sample provided by Louis.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Mark de Wever (mordante)

Changes

This adds an initial fuzzer. Different formatting arguments will execute different code paths. This will be tested by different fuzzer tests.

The code is based on a sample provided by Louis.

---

Full diff: https://github.com/llvm/llvm-project/pull/87883.diff

1 Files Affected:

• (added) libcxx/test/libcxx/fuzzing/format_no_args.pass.cpp (+26)

diff --git a/libcxx/test/libcxx/fuzzing/format_no_args.pass.cpp b/libcxx/test/libcxx/fuzzing/format_no_args.pass.cpp
new file mode 100644
index 00000000000000..32e468eb54fc6c
--- /dev/null
+++ b/libcxx/test/libcxx/fuzzing/format_no_args.pass.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-exceptions
+
+#include <cstdint>
+#include <format>
+#include <string_view>
+
+#include "fuzz.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const std::uint8_t* data, std::size_t size) {
+  try {
+    [[maybe_unused]] auto result = std::vformat(std::string_view{(const char*)(data), size}, std::make_format_args());
+  } catch (std::format_error const&) {
+    // If the fuzzing input isn't a valid thing we can format and we detect it, it's okay. We are looking for crashes.
+    return 0;
+  }
+  return 0;
+}