[PATCH] D20334: [libcxx] Fix a bug in strstreambuf::overflow

Akira Hatanaka via cfe-commits cfe-commits at lists.llvm.org
Tue May 17 12:20:16 PDT 2016

ahatanak created this revision.
ahatanak added reviewers: mclow.lists, EricWF, howard.hinnant.
ahatanak added a subscriber: cfe-commits.

The end pointer should point to one past the end of the newly allocated buffer.

Without this fix, asan reports an error when the following code is compiled and executed:

$ cat test.cpp

std::string stringOfLength(const size_t length) {
  std::string s("");

  std::string characters("abcdefghijklmnopqrstuvwxyz0123456789+-*/");
  for (size_t i = 0; i < length; ++i) {
    s += characters[i % characters.size()];

  return s;

int main(int, char const **argv) {
  std::ostrstream oss;

  oss << stringOfLength(atoi(argv[1])) << std::ends;
  std::cout << oss.str();

  return 0;

$ clang++  -fsanitize=address test.cpp && ./a.out 4096
==18970==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62100001dd00 at pc 0x00010277c45d bp 0x7fff5d4ce6e0 sp 0x7fff5d4cdea0
READ of size 4097 at 0x62100001dd00 thread T0
    #0 0x10277c45c in wrap_strlen (libclang_rt.asan_osx_dynamic.dylib+0x4345c)
    #1 0x102733954 in std::__1::char_traits<char>::length(char const*) (a.out+0x100002954)
    #2 0x10273390b in std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<<<std::__1::char_traits<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, char const*) (a.out+0x10000290b)
    #3 0x102733346 in main (a.out+0x100002346)
    #4 0x7fff901905ac in start (libdyld.dylib+0x35ac)
    #5 0x1  (<unknown module>)

0x62100001dd00 is located 0 bytes to the right of 4096-byte region [0x62100001cd00,0x62100001dd00)
allocated by thread T0 here:
    #0 0x10278d42b in wrap__Znam (libclang_rt.asan_osx_dynamic.dylib+0x5442b)
    #1 0x7fff9bdc9fa1 in std::__1::strstreambuf::overflow(int) (libc++.1.dylib+0x44fa1)
    #2 0x7fff9bd901cc in std::__1::basic_streambuf<char, std::__1::char_traits<char> >::xsputn(char const*, long) (libc++.1.dylib+0xb1cc)
    #3 0x10273547c in std::__1::ostreambuf_iterator<char, std::__1::char_traits<char> > std::__1::__pad_and_output<char, std::__1::char_traits<char> >(std::__1::ostreambuf_iterator<char, std::__1::char_traits<char> >, char const*, char const*, char const*, std::__1::ios_base&, char) (a.out+0x10000447c)
    #4 0x102734312 in std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::__put_character_sequence<char, std::__1::char_traits<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, char const*, unsigned long) (a.out+0x100003312)
    #5 0x10273389d in std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<<<char, std::__1::char_traits<char>, std::__1::allocator<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) (a.out+0x10000289d)
    #6 0x1027332c4 in main (a.out+0x1000022c4)
    #7 0x7fff901905ac in start (libdyld.dylib+0x35ac)
    #8 0x1  (<unknown module>)



Index: src/strstream.cpp
--- src/strstream.cpp
+++ src/strstream.cpp
@@ -180,7 +180,7 @@
                 delete [] eback();
         setg(buf, buf + ninp, buf + einp);
-        setp(buf + einp, buf + einp + eout);
+        setp(buf + einp, buf + new_size);
         __strmode_ |= __allocated;

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