[PATCH] libstdc++: avoid uninitialized read in basic_string constructor

[Ben Sherman]

Tested on x86_64-pc-linux-gnu, please let me know if there's anything
else needed. I haven't contributed before and don't have write access, so
apologies if I've missed anything.

-- >8 --

The basic_string input iterator constructor incrementally reads data and
allocates the internal buffer as-needed. When _M_dispose() is called, there
is a check for whether the local buffer is being used - if it is, there is
an additional check guarding __builtin_unreachable() for the value of
_M_string_length. The constructor does not initialize _M_string_length
until all data has been read, so the first re-allocation out of the local
buffer will have an uninitialized read.

This updates the basic_string input iterator constructor to properly set
_M_string_length as data is being read.  It additionally introduces a new
_M_assign_terminator() function to assign the null-terminator based on the
currently-stored _M_string_length.

libstdc++-v3/ChangeLog:

        * include/bits/basic_string.h (_M_assign_terminator()): New
          function.
          (_M_set_length()): Use _M_assign_terminator().
        * include/bits/basic_string.tcc (_M_construct(InIter, InIter,
          input_iterator_tag)): Set length incrementally, use
          _M_assign_terminator().

diff --git a/libstdc++-v3/include/bits/basic_string.h 
b/libstdc++-v3/include/bits/basic_string.h
index 0fa32afeb..ba02d8f0f 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -258,12 +258,17 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _M_capacity(size_type __capacity)
       { _M_allocated_capacity = __capacity; }

+      _GLIBCXX20_CONSTEXPR
+      void
+      _M_assign_terminator()
+      { traits_type::assign(_M_data()[_M_string_length], _CharT()); }
+
       _GLIBCXX20_CONSTEXPR
       void
       _M_set_length(size_type __n)
       {
        _M_length(__n);
-       traits_type::assign(_M_data()[__n], _CharT());
+       _M_assign_terminator();
       }

       _GLIBCXX20_CONSTEXPR
diff --git a/libstdc++-v3/include/bits/basic_string.tcc 
b/libstdc++-v3/include/bits/basic_string.tcc
index f0a44e5e8..84366a44a 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -182,6 +182,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
            ++__beg;
          }

+       _M_length(__len);
+
        struct _Guard
        {
          _GLIBCXX20_CONSTEXPR
@@ -206,12 +208,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                _M_capacity(__capacity);
              }
            traits_type::assign(_M_data()[__len++], *__beg);
+           _M_length(__len);
            ++__beg;
          }

        __guard._M_guarded = 0;

-       _M_set_length(__len);
+       _M_assign_terminator();
       }

   template<typename _CharT, typename _Traits, typename _Alloc>
--
2.21.0







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