libstdc++: Adjust overflow prevention to operator>>

This adjusts the overflow prevention added to operator>> so that we can
distinguish "unknown size" from "zero size", and avoid writing anything
at all in to zero sized buffers.

This also removes the incorrect comment saying extraction stops at a
null byte.

libstdc++-v3/ChangeLog:

	* include/std/istream (operator>>(istream&, char*)): Add
	attributes to get warnings for pointers that are null or known
	to point to the end of a buffer. Request upper bound from
	__builtin_object_size check and handle zero-sized buffer case.
	(operator>>(istream&, signed char))
	(operator>>(istream&, unsigned char*)): Add attributes.
	* testsuite/27_io/basic_istream/extractors_character/char/overflow.cc:
	Check extracting into the middle of a buffer.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/overflow.cc: New test.

1 files changed, 19 insertions, 7 deletions

diff --git a/libstdc++-v3/include/std/istream b/libstdc++-v3/include/std/istream
index cb8e9f8..20a455a 100644
--- a/libstdc++-v3/include/std/istream
+++ b/libstdc++-v3/include/std/istream
@@ -790,7 +790,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  - `n - 1` characters are stored
    *  - EOF is reached
    *  - the next character is whitespace according to the current locale
-   *  - the next character is a null byte (i.e., `charT()`)
    *
    *  `width(0)` is then called for the input stream.
    *
@@ -799,25 +798,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus <= 201703L
   template<typename _CharT, typename _Traits>
+    __attribute__((__nonnull__(2), __access__(__write_only__, 2)))
     inline basic_istream<_CharT, _Traits>&
     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
     {
-      streamsize __n = __builtin_object_size(__s, 2) / sizeof(_CharT);
-      if (__n == 0)
-	__n = __gnu_cxx::__numeric_traits<streamsize>::__max / sizeof(_CharT);
-      std::__istream_extract(__in, __s, __n);
+      size_t __n = __builtin_object_size(__s, 0);
+      if (__builtin_expect(__n < sizeof(_CharT), false))
+	{
+	  // There is not even space for the required null terminator.
+	  __glibcxx_assert(__n >= sizeof(_CharT));
+	  __in.width(0);
+	  __in.setstate(ios_base::failbit);
+	}
+      else
+	{
+	  if (__n == (size_t)-1)
+	    __n = __gnu_cxx::__numeric_traits<streamsize>::__max;
+	  std::__istream_extract(__in, __s, __n / sizeof(_CharT));
+	}
       return __in;
     }
 
   template<class _Traits>
+    __attribute__((__nonnull__(2), __access__(__write_only__, 2)))
     inline basic_istream<char, _Traits>&
     operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
-    { return (__in >> reinterpret_cast<char*>(__s)); }
+    { return __in >> reinterpret_cast<char*>(__s); }
 
   template<class _Traits>
+    __attribute__((__nonnull__(2), __access__(__write_only__, 2)))
     inline basic_istream<char, _Traits>&
     operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
-    { return (__in >> reinterpret_cast<char*>(__s)); }
+    { return __in >> reinterpret_cast<char*>(__s); }
 #else
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // 2499. operator>>(istream&, char*) makes it hard to avoid buffer overflows