On 24/03/14 19:19 +0000, Jonathan Wakely wrote:
There is a lot of code in libsupc++/eh_* that relies on
__cxa_exception and __cxa_dependent_exception having similar layouts,
so tricks like this work:
static inline void*
__gxx_caught_object(_Unwind_Exception* eo)
{
// Bad as it looks, this actually works for dependent exceptions too.
__cxa_exception* header = __get_exception_header_from_ue (eo);
return header->adjustedPtr;
}
The idea is that although the eo might be a pointer to the
unwindHeader member of either __cxa_exception or
__cxa_dependent_exception, the adjustedPtr member will be always be at
the same location relative to the unwindHeader member, so it Just
Works.
Except it doesn't.
offsetof(__cxa_exception, unwindHeader) == 80
offsetof(__cxa_dependent_exception, unwindHeader) == 80
offsetof(__cxa_exception, adjustedPtr) == 72
offsetof(__cxa_dependent_exception, adjustedPtr) == 64
Here's the output of GDB's pahole on __cxa_exception
(gdb) pahole struct __cxxabiv1::__cxa_exception
struct __cxxabiv1::__cxa_exception {
/* 0 8 */ std::type_info * exceptionType
/* 8 8 */ void (*)(void *) exceptionDestructor
/* 16 8 */ void (*)(void) unexpectedHandler
/* 24 8 */ void (*)(void) terminateHandler
/* 32 8 */ __cxxabiv1::__cxa_exception * nextException
/* 40 4 */ int handlerCount
/* 44 4 */ int handlerSwitchValue
/* 48 8 */ const unsigned char * actionRecord
/* 56 8 */ const unsigned char * languageSpecificData
/* 64 8 */ unsigned long catchTemp
/* 72 8 */ void * adjustedPtr
/* 80 32 */ struct _Unwind_Exception {
/* 0 8 */ unsigned long exception_class
/* 8 8 */ void (*)(_Unwind_Reason_Code, _Unwind_Exception *)
exception_cleanup
/* 16 8 */ unsigned long private_1
/* 24 8 */ unsigned long private_2
} unwindHeader
}
And here's the current definition of __cxa_dependent_exception:
(gdb) pahole struct __cxxabiv1::__cxa_dependent_exception
struct __cxxabiv1::__cxa_dependent_exception {
/* 0 8 */ void * primaryException
/* 8 8 */ void (*)(void) unexpectedHandler
/* 16 8 */ void (*)(void) terminateHandler
/* 24 8 */ __cxxabiv1::__cxa_exception * nextException
/* 32 4 */ int handlerCount
/* 36 4 */ int handlerSwitchValue
/* 40 8 */ const unsigned char * actionRecord
/* 48 8 */ const unsigned char * languageSpecificData
/* 56 8 */ unsigned long catchTemp
/* 64 8 */ void * adjustedPtr
/* XXX 64 bit hole, try to pack */
/* 80 32 */ struct _Unwind_Exception {
/* 0 8 */ unsigned long exception_class
/* 8 8 */ void (*)(_Unwind_Reason_Code, _Unwind_Exception *)
exception_cleanup
/* 16 8 */ unsigned long private_1
/* 24 8 */ unsigned long private_2
} unwindHeader
}
Notice the hole right before the unwindHeader member which is not
there in __cxa_exception. All the member which are supposed to be at
the same offsets as in __cxa_exception are 8 bytes earlier.
The attached patch adds a __padding member near the start so changes
that to:
(gdb) pahole struct __cxxabiv1::__cxa_dependent_exception
struct __cxxabiv1::__cxa_dependent_exception {
/* 0 8 */ void * primaryException
/* 8 8 */ void (*)(void *) __padding
/* 16 8 */ void (*)(void) unexpectedHandler
/* 24 8 */ void (*)(void) terminateHandler
/* 32 8 */ __cxxabiv1::__cxa_exception * nextException
/* 40 4 */ int handlerCount
/* 44 4 */ int handlerSwitchValue
/* 48 8 */ const unsigned char * actionRecord
/* 56 8 */ const unsigned char * languageSpecificData
/* 64 8 */ unsigned long catchTemp
/* 72 8 */ void * adjustedPtr
/* 80 32 */ struct _Unwind_Exception {
/* 0 8 */ unsigned long exception_class
/* 8 8 */ void (*)(_Unwind_Reason_Code, _Unwind_Exception *)
exception_cleanup
/* 16 8 */ unsigned long private_1
/* 24 8 */ unsigned long private_2
} unwindHeader
}
That change allows all the pointer tricks in libsupc++/eh_*.cc to
continue working. It's a change to the layout of a low-level type,
which is not visible to users linking to libsupc++.so, but anyone
linking statically will not be able to mix code using GCC 4.9's
libsupc++.a with the libsupc++.a from previous versions if they use
std::rethrow_exception() anywhere.
Tested x86_64-linux, I plan to commit this to trunk soon.
commit 06a845f80204947afd6866109db58cc85dc87117
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Tue Mar 25 14:42:45 2014 +0000
PR libstdc++/60612
* libsupc++/eh_ptr.cc: Assert __cxa_dependent_exception layout is
compatible with __cxa_exception.
* libsupc++/unwind-cxx.h (__cxa_dependent_exception): Add padding.
Fix typos in comments.
* testsuite/18_support/exception_ptr/60612-terminate.cc: New.
* testsuite/18_support/exception_ptr/60612-unexpected.cc: New.
diff --git a/libstdc++-v3/libsupc++/eh_ptr.cc b/libstdc++-v3/libsupc++/eh_ptr.cc
index 6508749..8c25a81 100644
--- a/libstdc++-v3/libsupc++/eh_ptr.cc
+++ b/libstdc++-v3/libsupc++/eh_ptr.cc
@@ -35,6 +35,32 @@
using namespace __cxxabiv1;
+// Verify assumptions about member layout in exception types
+namespace
+{
+template<typename Ex>
+ constexpr std::size_t unwindhdr()
+ { return offsetof(Ex, unwindHeader); }
+
+template<typename Ex>
+ constexpr std::size_t termHandler()
+ { return unwindhdr<Ex>() - offsetof(Ex, terminateHandler); }
+
+static_assert( termHandler<__cxa_exception>()
+ == termHandler<__cxa_dependent_exception>(),
+ "__cxa_dependent_exception::termHandler layout is correct" );
+
+#ifndef __ARM_EABI_UNWINDER__
+template<typename Ex>
+ constexpr std::ptrdiff_t adjptr()
+ { return unwindhdr<Ex>() - offsetof(Ex, adjustedPtr); }
+
+static_assert( adjptr<__cxa_exception>()
+ == adjptr<__cxa_dependent_exception>(),
+ "__cxa_dependent_exception::adjustedPtr layout is correct" );
+#endif
+}
+
std::__exception_ptr::exception_ptr::exception_ptr() _GLIBCXX_USE_NOEXCEPT
: _M_exception_object(0) { }
diff --git a/libstdc++-v3/libsupc++/unwind-cxx.h
b/libstdc++-v3/libsupc++/unwind-cxx.h
index a7df603..f57c536 100644
--- a/libstdc++-v3/libsupc++/unwind-cxx.h
+++ b/libstdc++-v3/libsupc++/unwind-cxx.h
@@ -81,7 +81,7 @@ struct __cxa_exception
// Stack of exceptions in cleanups.
__cxa_exception* nextPropagatingException;
- // The nuber of active cleanup handlers for this exception.
+ // The number of active cleanup handlers for this exception.
int propagationCount;
#else
// Cache parsed handler data from the personality routine Phase 1
@@ -114,6 +114,11 @@ struct __cxa_dependent_exception
// The primary exception this thing depends on.
void *primaryException;
+ // Unused member to get similar layout to __cxa_exception, otherwise the
+ // alignment requirements of _Unwind_Exception would require padding bytes
+ // before the unwindHeader member.
+ void (_GLIBCXX_CDTOR_CALLABI *__padding)(void *);
+
// The C++ standard has entertaining rules wrt calling set_terminate
// and set_unexpected in the middle of the exception cleanup process.
std::unexpected_handler unexpectedHandler;
@@ -130,7 +135,7 @@ struct __cxa_dependent_exception
// Stack of exceptions in cleanups.
__cxa_exception* nextPropagatingException;
- // The nuber of active cleanup handlers for this exception.
+ // The number of active cleanup handlers for this exception.
int propagationCount;
#else
// Cache parsed handler data from the personality routine Phase 1
diff --git a/libstdc++-v3/testsuite/18_support/exception_ptr/60612-terminate.cc
b/libstdc++-v3/testsuite/18_support/exception_ptr/60612-terminate.cc
new file mode 100644
index 0000000..ec5940d
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/exception_ptr/60612-terminate.cc
@@ -0,0 +1,41 @@
+// { dg-options "-std=gnu++11" }
+// { dg-require-atomic-builtins "" }
+
+// Copyright (C) 2014 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/>.
+
+// PR libstdc++/60612
+
+#include <exception>
+#include <stdlib.h>
+
+void terminate() { _Exit(0); }
+
+void f() noexcept
+{
+ try {
+ throw 1;
+ } catch (...) {
+ std::set_terminate(terminate);
+ std::rethrow_exception(std::current_exception());
+ }
+}
+
+int main()
+{
+ f();
+}
diff --git
a/libstdc++-v3/testsuite/18_support/exception_ptr/60612-unexpected.cc
b/libstdc++-v3/testsuite/18_support/exception_ptr/60612-unexpected.cc
new file mode 100644
index 0000000..3f7e2cf
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/exception_ptr/60612-unexpected.cc
@@ -0,0 +1,41 @@
+// { dg-options "-std=gnu++11" }
+// { dg-require-atomic-builtins "" }
+
+// Copyright (C) 2014 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/>.
+
+// PR libstdc++/60612
+
+#include <exception>
+#include <stdlib.h>
+
+void unexpected() { _Exit(0); }
+
+void f() throw()
+{
+ try {
+ throw 1;
+ } catch (...) {
+ std::set_unexpected(unexpected);
+ std::rethrow_exception(std::current_exception());
+ }
+}
+
+int main()
+{
+ f();
+}
