From f6121bb258b8a477956fedd0cb03c0fa90d82a0e Mon Sep 17 00:00:00 2001 From: Nick Desaulniers Date: Thu, 14 Dec 2023 13:20:53 -0800 Subject: [PATCH] [libc] fix msan failure in mempcpy_test Internal builds of the unittests with msan flagged mempcpy_test. ==6862==WARNING: MemorySanitizer: use-of-uninitialized-value #0 0x55e34d7d734a in length llvm-project/libc/src/__support/CPP/string_view.h:41:11 #1 0x55e34d7d734a in string_view llvm-project/libc/src/__support/CPP/string_view.h:71:24 #2 0x55e34d7d734a in __llvm_libc_9999_0_0_git::testing::Test::testStrEq(char const*, char const*, char const*, char const*, __llvm_libc_9999_0_0_git::testing::internal::Location) llvm-project/libc/test/UnitTest/LibcTest.cpp:284:13 #3 0x55e34d7d4e09 in LlvmLibcMempcpyTest_Simple::Run() llvm-project/libc/test/src/string/mempcpy_test.cpp:20:3 #4 0x55e34d7d6dff in __llvm_libc_9999_0_0_git::testing::Test::runTests(char const*) llvm-project/libc/test/UnitTest/LibcTest.cpp:133:8 #5 0x55e34d7d86e0 in main llvm-project/libc/test/UnitTest/LibcTestMain.cpp:21:10 SUMMARY: MemorySanitizer: use-of-uninitialized-value llvm-project/libc/src/__support/CPP/string_view.h:41:11 in length What's going on here is that mempcpy_test.cpp's Simple test is using ASSERT_STREQ with a partially initialized char array. ASSERT_STREQ calls Test::testStrEq which constructs a cpp:string_view. That constructor calls the private method cpp::string_view::length. When built with msan, the loop is transformed into multi-byte access, which then fails upon access. I took a look at libc++'s __constexpr_strlen which just calls __builtin_strlen(). Replacing the implementation of cpp::string_view::length with a call to __builtin_strlen() may still result in out of bounds access when the test is built with msan. It's not safe to use ASSERT_STREQ with a partially initialized array. Initialize the whole array so that the test passes. --- libc/test/src/string/mempcpy_test.cpp | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libc/test/src/string/mempcpy_test.cpp b/libc/test/src/string/mempcpy_test.cpp index 43ad5a0f3e1b7e..877ee8104880eb 100644 --- a/libc/test/src/string/mempcpy_test.cpp +++ b/libc/test/src/string/mempcpy_test.cpp @@ -14,7 +14,7 @@ // mempcpy behavior (returning the end of what was copied). TEST(LlvmLibcMempcpyTest, Simple) { const char *src = "12345"; - char dest[10]; + char dest[10] = {}; void *result = LIBC_NAMESPACE::mempcpy(dest, src, 6); ASSERT_EQ(static_cast(result), dest + 6); ASSERT_STREQ(src, dest);