Create a poor-developer's msan for libc wide read functions.

[Sterling-Augustine]

Most libcs optimize functions like strlen by reading in chunks larger than a single character. As part of "the implementation", they can legally do this as long as they are careful not to read invalid memory.

However, such tricks prevents those functions from being tested under the various sanitizers.

This PR creates a test framework that can report when one of these functions read or write in an invalid way without using the sanitizers.

[llvmbot]

@llvm/pr-subscribers-libc

Author: None (Sterling-Augustine)

Changes

Most libcs optimize functions like strlen by reading in chunks larger than a single character. As part of "the implementation", they can legally do this as long as they are careful not to read invalid memory.

However, such tricks prevents those functions from being tested under the various sanitizers.

This PR creates a test framework that can report when one of these functions read or write in an invalid way without using the sanitizers.

---

Full diff: https://github.com/llvm/llvm-project/pull/170586.diff

1 Files Affected:

• (modified) libc/test/src/strings/CMakeLists.txt (+10)

diff --git a/libc/test/src/strings/CMakeLists.txt b/libc/test/src/strings/CMakeLists.txt
index 5f70dc024f6ce..0ccd0dc302943 100644
--- a/libc/test/src/strings/CMakeLists.txt
+++ b/libc/test/src/strings/CMakeLists.txt
@@ -110,5 +110,15 @@ add_libc_test(
     libc.src.strings.strncasecmp_l
 )
 
+add_libc_test(
+  wide_read_memory_test
+  SUITE
+    libc-strings-tests
+  SRCS
+    wide_read_memory_test.cpp
+  DEPENDS
+    libc.src.string.strlen
+)
+
 add_libc_multi_impl_test(bcmp libc-strings-tests SRCS bcmp_test.cpp)
 add_libc_multi_impl_test(bzero libc-strings-tests SRCS bzero_test.cpp)

[github-actions]

🐧 Linux x64 Test Results

✅ The build succeeded and no tests ran. This is expected in some build configurations.

[c8ef]

Is this file supposed to be committed? It's missing.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions cpp -- libc/test/src/strings/wide_read_memory_test.cpp --diff_from_common_commit

⚠️
The reproduction instructions above might return results for more than one PR
in a stack if you are using a stacked PR workflow. You can limit the results by
changing origin/main to the base branch/commit you want to compare against.
⚠️

View the diff from clang-format here.

diff --git a/libc/test/src/strings/wide_read_memory_test.cpp b/libc/test/src/strings/wide_read_memory_test.cpp
index e353d49dd..5e2e02bb9 100644
--- a/libc/test/src/strings/wide_read_memory_test.cpp
+++ b/libc/test/src/strings/wide_read_memory_test.cpp
@@ -27,14 +27,13 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-
 class LlvmLibcWideAccessMemoryTest : public testing::Test {
   char *page0_;
   char *page1_;
   char *page2_;
   size_t page_size;
 
- public:
+public:
   void SetUp() override {
     page_size = getpagesize();
     page0_ =
@@ -65,7 +64,7 @@ class LlvmLibcWideAccessMemoryTest : public testing::Test {
   // 512-byte arrays. The termination condition, eg, end-of string or character
   // being searched for, should be near the end of the data.
   template <typename TestFunc>
-  void TestMemoryAccess(const std::vector<char>& buf, TestFunc func) {
+  void TestMemoryAccess(const std::vector<char> &buf, TestFunc func) {
     // Run func on data near the start boundary of valid memory.
     for (unsigned long offset = 0;
          offset < std::alignment_of<std::max_align_t>::value; ++offset) {
@@ -76,7 +75,7 @@ class LlvmLibcWideAccessMemoryTest : public testing::Test {
     // Run func on data near the end boundary of valid memory.
     for (unsigned long offset = 0;
          offset < std::alignment_of<std::max_align_t>::value; ++offset) {
-      char *test_addr = page2_  - buf.size() - offset - 1;
+      char *test_addr = page2_ - buf.size() - offset - 1;
       assert(test_addr + buf.size() < page2_);
       BasicMemCopy(test_addr, buf.data(), buf.size());
       func(test_addr);
@@ -89,10 +88,10 @@ TEST_F(LlvmLibcWideAccessMemoryTest, StringLength) {
   std::vector<char> buf(1536, 'a');
   // Make sure it is null terminated.
   buf.push_back('\0');
-  this->TestMemoryAccess(buf, [this, buf](const char* test_data) {
+  this->TestMemoryAccess(buf, [this, buf](const char *test_data) {
     // -1 for the null character.
     ASSERT_EQ(internal::string_length(test_data), size_t(buf.size() - 1));
   });
 }
 
-}
+} // namespace LIBC_NAMESPACE_DECL

[lntue]

You'll need to replace these with our internal implementations / headers. I see we already have mprotect, mmap, munmap. You might need to add getpagesize entrypoint (in a separate PR). Also you can use our inline_memcpy instead of BasicMemCopy.

One good thing with using those is that with correct dependency, this test will be skipped automatically for any build configurations where those syscalls are not available / implemented.