[libc] Add a differential fuzzer for inet_aton (#200341)

libc/fuzzing/CMakeLists.txt

@@ -2,6 +2,7 @@ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=fuzzer")
 add_custom_target(libc-fuzzer)
 
 add_subdirectory(__support)
+add_subdirectory(arpa)
 # TODO(#85680): Re-enable math fuzzing after headers are sorted out
 add_subdirectory(math)
 add_subdirectory(stdlib)

libc/fuzzing/arpa/CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory(inet)

libc/fuzzing/arpa/inet/CMakeLists.txt

@@ -0,0 +1,9 @@
+add_libc_fuzzer(
+  inet_aton_differential_fuzz
+  SRCS
+    inet_aton_differential_fuzz.cpp
+  DEPENDS
+    libc.src.__support.CPP.scope
+    libc.src.arpa.inet.inet_aton
+    libc.src.string.memcpy
+)

libc/fuzzing/arpa/inet/inet_aton_differential_fuzz.cpp

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Differential fuzz test for llvm-libc inet_aton implementation.
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/CPP/scope.h"
+#include "src/arpa/inet/inet_aton.h"
+#include "src/string/memcpy.h"
+#include <arpa/inet.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // Create a null-terminated copy of the data
+  char *str = new char[size + 1];
+  LIBC_NAMESPACE::cpp::scope_exit delete_str([&] { delete[] str; });
+  LIBC_NAMESPACE::memcpy(str, data, size);
+  str[size] = '\0';
+
+  struct in_addr ref_addr = {};
+  struct in_addr impl_addr = {};
+  int ref = ::inet_aton(str, &ref_addr);
+  int impl = LIBC_NAMESPACE::inet_aton(str, &impl_addr);
+
+  if (ref != impl) {
+    fprintf(stderr,
+            "Different result (reference: %d, implementation: %d) for \"%s\"\n",
+            ref, impl, str);
+    __builtin_trap();
+  }
+
+  if (ref == 1 && ref_addr.s_addr != impl_addr.s_addr) {
+    fprintf(
+        stderr,
+        "Different address (reference: %x, implementation: %x) for \"%s\"\n",
+        ref_addr.s_addr, impl_addr.s_addr, str);
+    __builtin_trap();
+  }
+
+  return 0;
+}