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.

2 files changed, 75 insertions, 3 deletions

diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/overflow.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/overflow.cc
index 1141a41..abbba8b 100644
--- a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/overflow.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/overflow.cc
@@ -15,14 +15,14 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-options "-O2 -std=gnu++98" }
+// { dg-options "-O2" }
 // { dg-do run }
 
 // This test checks that operator>> will avoid a buffer overflow when
 // reading into a buffer with a size that is known at compile time.
 
 // Since C++20 this is guaranteed (see LWG 2499), for previous standards
-// we try to check the buffer size as an extension (which depends on -O2).
+// checking the buffer size is an extension and depends on optimisation.
 
 #include <sstream>
 #include <testsuite_hooks.h>
@@ -30,11 +30,24 @@
 void
 test01()
 {
-  std::istringstream in("foolish child");
+  std::istringstream in("foolishly");
   char pc[5];
   in >> pc;
   VERIFY( in.good() );
   VERIFY( std::string(pc) == "fool" );
+
+#if __cplusplus <= 201703L
+  char* p = pc + 1;
+  in >> p;
+  VERIFY( in.good() );
+  VERIFY( std::string(pc) == "fish" );
+
+  p = pc + 4;
+  *p = '#';
+  in >> p;
+  VERIFY( in.fail() ); // if no characters are extracted, failbit is set
+  VERIFY( *p == '\0' );
+#endif
 }
 
 void
@@ -61,4 +74,6 @@ int
 main()
 {
   test01();
+  test02();
+  test03();
 }
diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/overflow.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/overflow.cc
new file mode 100644
index 0000000..6a23f13
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/overflow.cc
@@ -0,0 +1,57 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-O2" }
+// { dg-do run }
+
+// This test checks that operator>> will avoid a buffer overflow when
+// reading into a buffer with a size that is known at compile time.
+
+// Since C++20 this is guaranteed (see LWG 2499), for previous standards
+// checking the buffer size is an extension and depends on optimisation.
+
+#include <sstream>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::wistringstream in(L"foolishly");
+  wchar_t pc[5];
+  in >> pc;
+  VERIFY( in.good() );
+  VERIFY( std::wstring(pc) == L"fool" );
+
+#if __cplusplus <= 201703L
+  wchar_t* p = pc + 1;
+  in >> p;
+  VERIFY( in.good() );
+  VERIFY( std::wstring(pc) == L"fish" );
+
+  p = pc + 4;
+  *p = L'#';
+  in >> p;
+  VERIFY( in.fail() ); // if no characters are extracted, failbit is set
+  VERIFY( *p == L'\0' );
+#endif
+}
+
+int
+main()
+{
+  test01();
+}