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libstdc++: Fix std::to_chars buffer overflow (PR 95851)

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The __detail::__to_chars_2 function assumes it won't be called with zero
values. However, when the output buffer is empty the caller doesn't
handle zero values correctly, and calls __to_chars_2 with a zero value,
resulting in an overflow of the empty buffer.

The __detail::__to_chars_i function should just return immediately for
an empty buffer, and otherwise ensure zero values are handled properly.

libstdc++-v3/ChangeLog:

	PR libstdc++/95851
	* include/std/charconv (__to_chars_i): Check for zero-sized
	buffer unconditionally.
	* testsuite/20_util/to_chars/95851.cc: New test.
This commit is contained in:
Jonathan Wakely 2020-06-23 22:47:58 +01:00
parent 3fb2c2f4d0
commit be50843754
2 changed files with 40 additions and 1 deletions
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5
libstdc++-v3/include/std/charconv
View File

@ -327,7 +327,10 @@ namespace __detail
using _Up = __detail::__unsigned_least_t<_Tp>;
_Up __unsigned_val = __value;
if (__value == 0 && __first != __last)
if (__first == __last) [[__unlikely__]]
return { __last, errc::value_too_large };
if (__value == 0)
{
*__first = '0';
return { __first + 1, errc{} };

36
libstdc++-v3/testsuite/20_util/to_chars/95851.cc Normal file
View File

@ -0,0 +1,36 @@
// 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-do run { target c++14 } }
#include <charconv>
#include <testsuite_hooks.h>
void
test01()
{
char* p = nullptr;
auto res = std::to_chars(p, p, 0, 2); // PR libstdc++/95851
VERIFY( res.ptr == p );
VERIFY( res.ec == std::errc::value_too_large );
}
int
main()
{
test01();
}