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c++libstdc++std-filesystem

Uninitialized Usage in libstdc++ filesystem?

Consider the following code:

#include <filesystem>

int main() {
    std::filesystem::path p{"/"};
}

When compiling with clang10 with the flags -std=c++17 -fsanitize=memory -g -O1 -stdlib=libstdc++ it compiles totally fine, but when running, the memory sanitizer finds the following error:

==166467==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x560e38eafc41 in std::basic_ostream<char, std::char_traits<char> >& std::__detail::operator<<<char, std::char_traits<char>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&>(std::basic_ostream<char, std::char_traits<char> >&, std::__detail::_Quoted_string<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char> const&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/quoted_string.h:124:32
    #1 0x560e38eaf03c in std::basic_ostream<char, std::char_traits<char> >& std::filesystem::__cxx11::operator<<<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::filesystem::__cxx11::path const&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/fs_path.h:441:7
    #2 0x560e38eaecdb in main ~/test/test.cpp:15:15
    #3 0x7f5d0fa58001 in __libc_start_main (/usr/lib/libc.so.6+0x27001)
    #4 0x560e38e3033d in _start (~/test/a.out+0x2133d)

SUMMARY: MemorySanitizer: use-of-uninitialized-value /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/quoted_string.h:124:32 in std::basic_ostream<char, std::char_traits<char> >& std::__detail::operator<<<char, std::char_traits<char>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&>(std::basic_ostream<char, std::char_traits<char> >&, std::__detail::_Quoted_string<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char> const&)
Exiting

When compiling with -stdlib=libc++ it works completely fine. See also godbolt (sadly, the msan-output is a bit non-descriptive there).

Is the memory sanitizer a overzealous or is there something wrong? I looked briefly into the offending file and the code

    for (auto __c : __str._M_string)
      {
        if (__c == __str._M_delim || __c == __str._M_escape) // <- line 124
          __ostr << __str._M_escape;
        __ostr << __c;
      }

seems ok to me (__str is of type std::__detail::_Quoted_string<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const& that only has one user defined constructor, that value initializes all three members from parameters).

Output of clang++ -v:

clang version 10.0.0 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-pc-linux-gnu/10.1.0
Found candidate GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0
Found candidate GCC installation: /usr/lib64/gcc/x86_64-pc-linux-gnu/10.1.0
Selected GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
Solution

  • This is a false positive.

    You should report it as a Sanitizer bug; it appears to be similar to issue 1238.