[libc] Add fuzz test for strlen, memchr.

[cgyurgyik]

Summary

Rationale: While the implementations for these are trivial, this may change at some point. Having a fuzz test is just another sanity check for the optimizer.

libc/fuzzing/string/CMakeLists.txt

 add_libc_fuzzer(
+  memchr_fuzz
+  SRCS
+    memchr_fuzz.cpp
+  DEPENDS
+    libc.src.string.memchr
+)
+
+add_libc_fuzzer(
   strcmp_fuzz
   SRCS
     strcmp_fuzz.cpp
   DEPENDS
     libc.src.string.strcmp
 )
 
 add_libc_fuzzer(
   strcpy_fuzz
   SRCS
     strcpy_fuzz.cpp
   DEPENDS
     libc.src.string.memcpy
     libc.src.string.strcpy
     libc.src.string.strlen
 )
 
 add_libc_fuzzer(
+  strlen_fuzz
+  SRCS
+    strlen_fuzz.cpp
+  DEPENDS
+    libc.src.string.strlen
+)
+
+add_libc_fuzzer(
   strstr_fuzz
   SRCS
     strstr_fuzz.cpp
   DEPENDS
     libc.src.string.strstr
     libc.src.string.strlen
 )

libc/fuzzing/string/memchr_fuzz.cpp

+//===-- memchr_fuzz.cpp ---------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// Fuzzing test for llvm-libc memchr implementation.
+///
+//===----------------------------------------------------------------------===//
+#include "src/string/memchr.h"
+#include <stddef.h>
+#include <stdint.h>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // Verify the size is at least 1, since we're using the final index as the
+  // value to search for.
+  if (!size)
+    return 0;
+
+  const unsigned char *src = reinterpret_cast<const unsigned char *>(data);
+  const unsigned char c = data[size - 1]; // final index.
+
+  // Searches all indices except the last one. This ensures
+  // the value being searched for isn't always found.
+  const size_t new_size = size - 1;
+
+  size_t i;
+  for (i = 0; i < new_size; ++i) {
+    if (src[i] == c)
+      break;
+  }
+  const unsigned char *expected = i < new_size ? src + i : nullptr;
+
+  if (__llvm_libc::memchr(data, c, new_size) != expected)
+    __builtin_trap();
+
+  return 0;
+}

libc/fuzzing/string/strlen_fuzz.cpp

+//===-- strlen_fuzz.cpp ---------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// Fuzzing test for llvm-libc strlen implementation.
+///
+//===----------------------------------------------------------------------===//
+#include "src/string/strlen.h"
+#include <stddef.h>
+#include <stdint.h>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // Verify the size is at least 1 and the data is null terminated.
+  if (!size || data[size - 1] != '\0')
+    return 0;
+
+  size_t count;
+  for (count = 0; data[count]; ++count)
+    ;
+
+  if (__llvm_libc::strlen(reinterpret_cast<const char *>(data)) != count)
+    __builtin_trap();
+
+  return 0;
+}