From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 12B0B385AC1F; Tue, 12 Jul 2022 22:40:12 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 12B0B385AC1F MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r13-1651] libstdc++: Check for EOF if extraction avoids buffer overflow [PR106248] X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: 6e9d5dfc2911e3acc6039ebfe3837e7ba4be197f X-Git-Newrev: 5ae74944af1de032d4a27fad4a2287bd3a2163fd Message-Id: <20220712224012.12B0B385AC1F@sourceware.org> Date: Tue, 12 Jul 2022 22:40:12 +0000 (GMT) X-BeenThere: libstdc++-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 12 Jul 2022 22:40:12 -0000 https://gcc.gnu.org/g:5ae74944af1de032d4a27fad4a2287bd3a2163fd commit r13-1651-g5ae74944af1de032d4a27fad4a2287bd3a2163fd Author: Jonathan Wakely Date: Tue Jul 12 11:18:47 2022 +0100 libstdc++: Check for EOF if extraction avoids buffer overflow [PR106248] In r11-2581-g17abcc77341584 (for LWG 2499) I added overflow checks to the pre-C++20 operator>>(istream&, char*) overload. Those checks can cause extraction to stop after filling the buffer, where previously it would have tried to extract another character and stopped at EOF. When that happens we no longer set eofbit in the stream state, which is consistent with the behaviour of the new C++20 overload, but is an observable and unexpected change in the C++17 behaviour. What makes it worse is that the behaviour change is dependent on optimization, because __builtin_object_size is used to detect the buffer size and that only works when optimizing. To avoid the unexpected and optimization-dependent change in behaviour, set eofbit manually if we stopped extracting because of the buffer size check, but had reached EOF anyway. If the stream's rdstate() != goodbit or width() is non-zero and smaller than the buffer, there's nothing to do. Otherwise, we filled the buffer and need to check for EOF, and maybe set eofbit. The new check is guarded by #ifdef __OPTIMIZE__ because otherwise __builtin_object_size is useless. There's no point compiling and emitting dead code that can't be eliminated because we're not optimizing. We could add extra checks that the next character in the buffer is not whitespace, to detect the case where we stopped early and prevented a buffer overflow that would have happened otherwise. That would allow us to assert or set badbit in the stream state when undefined behaviour was prevented. However, those extra checks would increase the size of the function, potentially reducing the likelihood of it being inlined, and so making the buffer size detection less reliable. It seems preferable to prevent UB and silently truncate, rather than miss the UB and allow the overflow to happen. libstdc++-v3/ChangeLog: PR libstdc++/106248 * include/std/istream [C++17] (operator>>(istream&, char*)): Set eofbit if we stopped extracting at EOF. * testsuite/27_io/basic_istream/extractors_character/char/pr106248.cc: New test. * testsuite/27_io/basic_istream/extractors_character/wchar_t/pr106248.cc: New test. Diff: --- libstdc++-v3/include/std/istream | 31 ++++++++++++++--- .../extractors_character/char/pr106248.cc | 40 ++++++++++++++++++++++ .../extractors_character/wchar_t/pr106248.cc | 40 ++++++++++++++++++++++ 3 files changed, 106 insertions(+), 5 deletions(-) diff --git a/libstdc++-v3/include/std/istream b/libstdc++-v3/include/std/istream index b506c4f7504..416ef556fa1 100644 --- a/libstdc++-v3/include/std/istream +++ b/libstdc++-v3/include/std/istream @@ -784,7 +784,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION * - if `width()` is greater than zero, `n` is `min(width(), n)` * - otherwise `n` is the number of elements of the array * - (before C++20 the pointer is assumed to point to an array of - * - the largest possible size for an array of `char_type`). + * the largest possible size for an array of `char_type`). * * Characters are extracted and stored until one of the following happens: * - `n - 1` characters are stored @@ -802,19 +802,40 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION inline basic_istream<_CharT, _Traits>& operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s) { +#ifdef __OPTIMIZE__ + // Function inlining might make the buffer size known, allowing us to + // prevent overflow. size_t __n = __builtin_object_size(__s, 0); - if (__builtin_expect(__n < sizeof(_CharT), false)) + if (__n < sizeof(_CharT)) { // There is not even space for the required null terminator. __glibcxx_assert(__n >= sizeof(_CharT)); + // No point calling __istream_extract, but still need to reset width. __in.width(0); __in.setstate(ios_base::failbit); } + else if (__n != (size_t)-1) + { + __n /= sizeof(_CharT); + streamsize __w = __in.width(); + std::__istream_extract(__in, __s, __n); + if (__in.good() && (__w <= 0 || __n < __w)) + { + // Stopped extracting early to avoid overflowing the buffer, + // but might have stopped anyway (and set eofbit) if at EOF. + const typename _Traits::int_type __c = __in.rdbuf()->sgetc(); + const bool __eof = _Traits::eq_int_type(__c, _Traits::eof()); + if (__builtin_expect(__eof, true)) // Assume EOF, not overflow. + __in.setstate(ios_base::eofbit); + } + } else +#endif // __OPTIMIZE { - if (__n == (size_t)-1) - __n = __gnu_cxx::__numeric_traits::__max; - std::__istream_extract(__in, __s, __n / sizeof(_CharT)); + // Buffer size is unknown, have to assume it's huge. + streamsize __n = __gnu_cxx::__numeric_traits::__max; + __n /= sizeof(_CharT); + std::__istream_extract(__in, __s, __n); } return __in; } diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/pr106248.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/pr106248.cc new file mode 100644 index 00000000000..6d89a0e5fef --- /dev/null +++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/pr106248.cc @@ -0,0 +1,40 @@ +// { dg-do run } + +#include +#include + +void +test_pr106248() +{ + char buf[5] = {'x', 'x', 'x', 'x', 'x'}; + std::string s(" four"); + std::istringstream in(s); + in >> buf; +#if __cplusplus >= 202002L + // Extraction stops because buffer is full. + VERIFY( in.good() ); +#else + // PR libstdc++/106248 + // Extraction stops because all input has been consumed and eofbit is set. + VERIFY( in.eof() ); +#endif + // Extracted string must be null-terminated. + VERIFY( buf[4] == '\0' ); + VERIFY( std::string(buf) == "four" ); + + in.clear(); + in.str(s); + for (int i = 0; i < 5; ++i) + s[i] = 'x'; + + in.width(5); + in >> buf; + // Extraction stops due to field width, eofbit not set. + VERIFY( in.good() ); + VERIFY( std::string(buf) == "four" ); +} + +int main() +{ + test_pr106248(); +} diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/pr106248.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/pr106248.cc new file mode 100644 index 00000000000..7c226600b9e --- /dev/null +++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/pr106248.cc @@ -0,0 +1,40 @@ +// { dg-do run } + +#include +#include + +void +test_pr106248() +{ + wchar_t buf[5] = {L'x', L'x', L'x', L'x', L'x'}; + std::wstring s(L" four"); + std::wistringstream in(s); + in >> buf; +#if __cplusplus >= 202002L + // Extraction stops because buffer is full. + VERIFY( in.good() ); +#else + // PR libstdc++/106248 + // Extraction stops because all input has been consumed and eofbit is set. + VERIFY( in.eof() ); +#endif + // Extracted string must be null-terminated. + VERIFY( buf[4] == L'\0' ); + VERIFY( std::wstring(buf) == L"four" ); + + in.clear(); + in.str(s); + for (int i = 0; i < 5; ++i) + s[i] = L'x'; + + in.width(5); + in >> buf; + // Extraction stops due to field width, eofbit not set. + VERIFY( in.good() ); + VERIFY( std::wstring(buf) == L"four" ); +} + +int main() +{ + test_pr106248(); +}