Date: Sun, 6 Mar 2022 11:13:58 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: David Chisnall <theraven@FreeBSD.org> Cc: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>, John Baldwin <jhb@freebsd.org>, Ed Maste <emaste@freebsd.org>, dev-commits-src-branches@freebsd.org Subject: Re: git: b2127b6f1ae2 - stable/13 - Install unwind.h into /usr/include Message-ID: <D438E16C-AB7D-4759-AB97-53F8417BCB02@FreeBSD.org> In-Reply-To: <4DF32938-153C-4682-8B54-8B9784DB25F0@FreeBSD.org> References: <20220303224535.a0cca57e1f033e930a7f8f9d@dec.sakura.ne.jp> <3166E99F-1616-40D9-BD8B-D18E8104D6FF@FreeBSD.org> <a9863238-04e5-381d-105d-311a2b0c3f56@FreeBSD.org> <6E5AD771-3140-480A-BF20-95B8E8A27189@FreeBSD.org> <20220305113613.7760bb3845cda1c04707fb34@dec.sakura.ne.jp> <7482C54A-1128-4AFF-A74E-53D88334C1D9@FreeBSD.org> <682D0394-CDC8-44AB-B9FF-2B969AE2D39A@FreeBSD.org> <20220306174644.dca71df7cf1400ea48cafaca@dec.sakura.ne.jp> <4DF32938-153C-4682-8B54-8B9784DB25F0@FreeBSD.org>
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--Apple-Mail=_34223E91-A920-47C2-AA63-F723ED9EE3E7 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 In an ideal world, we would just merge = https://github.com/libcxxrt/libcxxrt/commit/88bdf6b290da2f32fe212564bfad9c= 61481fb878 ("Specify double-word alignment for ARM unwind. This matches = the latest revisions from libunwind.") and the corresponding = https://github.com/libcxxrt/libcxxrt/commit/b96169641f794f7b8e28c1b78b66bb= 780445d0cd ("Updated Itanium unwind. This matches the latest revisions = from libunwind."). Then at least libcxxrt's and libunwind's idea of the = struct layouts and alignments would be the same. However, that would still affect older __cxa_exception and = _Unwind_Exception consumers, at least those that rely explicitly on the = old sizes and alignments of those structs. We could bump libcxxrt.so to = .2, but then we'd still have to maintain the old .1 version for those = old consumers. Btw, now that I think about it, I think we merged the unwind.h changes = for 13.1, and we should really fix them up before 13.1 goes out the = door. -Dimitry > On 6 Mar 2022, at 10:17, David Chisnall <theraven@FreeBSD.org> wrote: >=20 > libcxxrt provides its own unwind.h that includes an abstraction layer = over the differences between the Arm and Itanium ABIs. It provides all = of the standard definitions in addition to this, so you should be able = to build other things against it, but you won=E2=80=99t be able to build = it with a different unwind.h. >=20 > You could potentially split the unwind.h that it provides into two = headers, one with only the standard definitions and one with the others, = I=E2=80=99d be happy to review a PR to do this. >=20 > David >=20 >> On 6 Mar 2022, at 08:46, Tomoaki AOKI <junchoon@dec.sakura.ne.jp> = wrote: >>=20 >> On Sat, 5 Mar 2022 23:33:54 +0100 >> Dimitry Andric <dim@FreeBSD.org> wrote: >>=20 >>> On 5 Mar 2022, at 22:34, Dimitry Andric <dim@FreeBSD.org> wrote: >>>>=20 >>>> On 5 Mar 2022, at 03:36, Tomoaki AOKI <junchoon@dec.sakura.ne.jp> = wrote: >>> ... >>>> So according to the spec, casting the void pointer = 'thrown_exception' to >>>> a __cxa_exception pointer, then subtracting 1, should give you the >>>> original __cxa_exception struct. In this case, it subtracts 8 = bytes, >>>> going from 0x87b5aff00 to 0x87b5afe88. >>>=20 >>> Ugh, actually this was 120 bytes! >>>=20 >>>=20 >>>> Now I do exactly the same in the libreoffice frame one below, where = the >>>> incoming void pointer 'pExc' is the previous 'thrown_exception' = value: >>>>=20 >>>> (gdb) frame 1 >>>> #1 gcc3::deleteException (pExc=3D0x87b5aff00) at = bridges/source/cpp_uno/gcc3_linux_x86-64/except.cxx:139 >>>> 139 OUString unoName( toUNOname( = header->exceptionType->name() ) ); >>>> (gdb) print pExc >>>> $33 =3D (void *) 0x87b5aff00 >>>> (gdb) print static_cast<__cxa_exception*>(pExc)-1 >>>> $34 =3D (__cxa_exception *) 0x87b5afe80 >>>>=20 >>>> So in *this* function, subtracting 1 from a __cxa_exception pointer >>>> subtracts 16 bytes instead, going from 0x87b5aff00 to 0x87b5afe80! >>>=20 >>> And this was 128 bytes instead. I think I now know what's going on, >>> which is that our declaration of __cxa_exception changed its size = from >>> 120 bytes to 128 bytes, due to the new unwind headers. >>>=20 >>> Our libcxxrt cxxabi.h header has: >>>=20 >>> struct __cxa_exception >>> { >>> ... lots of stuff ... >>> /** The language-agnostic part of the exception header. */ >>> _Unwind_Exception unwindHeader; >>> }; >>>=20 >>> so the last field is a struct _Unwind_Exception. Our libcxxrt >>> unwind-itanium.h header has: >>>=20 >>> struct _Unwind_Exception >>> { >>> uint64_t exception_class; >>> _Unwind_Exception_Cleanup_Fn exception_cleanup; >>> unsigned long private_1; >>> unsigned long private_2; >>> } ; >>>=20 >>> while libunwind's version has an __aligned__ attribute at the end: >>>=20 >>> struct _Unwind_Exception { >>> uint64_t exception_class; >>> void (*exception_cleanup)(_Unwind_Reason_Code reason, >>> _Unwind_Exception *exc); >>> #if defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__) >>> uintptr_t private_[6]; >>> #else >>> uintptr_t private_1; // non-zero means forced unwind >>> uintptr_t private_2; // holds sp that phase1 found for phase2 to = use >>> #endif >>> #if __SIZEOF_POINTER__ =3D=3D 4 >>> // The implementation of _Unwind_Exception uses an attribute mode = on the >>> // above fields which has the side effect of causing this whole = struct to >>> // round up to 32 bytes in size (48 with SEH). To be more = explicit, we add >>> // pad fields added for binary compatibility. >>> uint32_t reserved[3]; >>> #endif >>> // The Itanium ABI requires that _Unwind_Exception objects are = "double-word >>> // aligned". GCC has interpreted this to mean "use the maximum = useful >>> // alignment for the target"; so do we. >>> } __attribute__((__aligned__)); >>>=20 >>> (Note that upstream libcxxrt also added the reserved field and = aligned >>> attribute, in https://github.com/libcxxrt/libcxxrt/commit/b9616964 = !) >>>=20 >>> The aligned attribute on _Unwind_Exception causes the enclosing >>> __cxa_exception struct to *also* be aligned maximally, growing it = from >>> 120 to 128 bytes on x86_64. >>>=20 >>> So this is a bit of a fine mess we are in. There are multiple issues >>> here: >>>=20 >>> 1) We broke the ABI by increasing __cxa_exception's size. >>>=20 >>> 2) We compile libcxxrt against its *own* unwind headers, so it = assumes a >>> 120-byte __cxa_exception size. But all other programs use the = libunwind >>> headers, so they assume a 128 byte __cxa_exception size. >>>=20 >>> I guess LibreOffice is just a good example which breaks because it = does >>> this deep poking in exception-handling land, which most programs = never >>> go near. That said, LibreOffice also includes the unwind.h header >>> installed by the libunwind-20201110 port, so that is yet *another* >>> possible incompatibility! >>>=20 >>> But I think we must do something about this. The most backward >>> compatible change would be to *remove* the aligned attribute from = our >>> _Unwind_Exception declaration, so the old __cxa_exception size is >>> restored. The problem with that is that we have to carry a patch for >>> libunwind forever. >>>=20 >>> The other way would be to force libcxxrt to use the libunwind = headers >>> instead of its own, so that at least libcxxrt and libunwind agree on = the >>> size and alignment of all these structures! But that may still lead = to >>> crashes for older consumers. >>>=20 >>> No easy way out, in any case... :-/ >>>=20 >>> -Dimitry >>=20 >> Did a quick stupid test by >>=20 >> *Replacing #include "unwind.h" by #include <unwind.h> in >> contrib/libcxxrt/cxxabi.h, the only file including unwind.h on >> contrib/cxxrt directory to pick it from standard place. >>=20 >> *Rename unsind*.h to something else just to be sure. >>=20 >> and got errors by missing macros. At least = BEGIN_PERSONALITY_FUNCTION. >>=20 >> Insufficient part would be needed to be extracted to additional = header. >>=20 >> Emitted errors are as follows: >>=20 >> /usr/src/lib/libcxxrt# make >> Building /usr/obj/usr/src/amd64.amd64/lib/libcxxrt/auxhelper.o >> Building /usr/obj/usr/src/amd64.amd64/lib/libcxxrt/dynamic_cast.o >> Building /usr/obj/usr/src/amd64.amd64/lib/libcxxrt/exception.o >> /usr/src/contrib/libcxxrt/exception.cc:1084:1: error: unknown type = name >> 'BEGIN_PERSONALITY_FUNCTION' >> BEGIN_PERSONALITY_FUNCTION(__gxx_personality_v0) ^ >> /usr/src/contrib/libcxxrt/exception.cc:1084:49: error: expected ';' >> after top level declarator >> BEGIN_PERSONALITY_FUNCTION(__gxx_personality_v0) ^ >> ; >> /usr/src/contrib/libcxxrt/exception.cc:1098:42: error: use of >> undeclared identifier 'exceptionClass'; did you mean = 'exception_class'? >> bool foreignException =3D !isCXXException(exceptionClass); = ^~~~~~~~~~~~~~ >> exception_class >> /usr/src/contrib/libcxxrt/exception.cc:242:23: note: = 'exception_class' >> declared here static const uint64_t exception_class =3D >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1101:2: error: expected >> unqualified-id if (!foreignException) >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1112:91: error: use of >> undeclared identifier 'context' reinterpret_cast<unsigned >> = char*>(static_cast<uintptr_t>(_Unwind_GetLanguageSpecificData(context))); >> ^ /usr/src/contrib/libcxxrt/exception.cc:1116:2: error: expected >> unqualified-id if (0 =3D=3D lsda_addr) { return >> continueUnwinding(exceptionObject, context); } ^ >> /usr/src/contrib/libcxxrt/exception.cc:1137:2: error: expected >> unqualified-id if (actions & _UA_SEARCH_PHASE) >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1176:2: error: expected >> unqualified-id if (!(actions & _UA_HANDLER_FRAME)) >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1188:2: error: expected >> unqualified-id else if (foreignException) >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1195:2: error: expected >> unqualified-id else if (ex->catchTemp =3D=3D 0) >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1200:2: error: expected >> unqualified-id else >> ^ >> /usr/src/contrib/libcxxrt/exception.cc:1209:2: error: C++ requires a >> type specifier for all declarations _Unwind_SetIP(context, >> reinterpret_cast<unsigned long>(action.landing_pad)); ^ >> /usr/src/contrib/libcxxrt/exception.cc:1209:16: error: use of >> undeclared identifier 'context' _Unwind_SetIP(context, >> reinterpret_cast<unsigned long>(action.landing_pad)); ^ >> /usr/src/contrib/libcxxrt/exception.cc:1210:2: error: C++ requires a >> type specifier for all declarations _Unwind_SetGR(context, >> __builtin_eh_return_data_regno(0), ^ >> /usr/src/contrib/libcxxrt/exception.cc:1210:16: error: use of >> undeclared identifier 'context' _Unwind_SetGR(context, >> __builtin_eh_return_data_regno(0), ^ >> /usr/src/contrib/libcxxrt/exception.cc:1211:48: error: use of >> undeclared identifier 'exceptionObject' reinterpret_cast<unsigned >> long>(exceptionObject)); ^ >> /usr/src/contrib/libcxxrt/exception.cc:1212:2: error: C++ requires a >> type specifier for all declarations _Unwind_SetGR(context, >> __builtin_eh_return_data_regno(1), selector); ^ >> /usr/src/contrib/libcxxrt/exception.cc:1212:16: error: use of >> undeclared identifier 'context' _Unwind_SetGR(context, >> __builtin_eh_return_data_regno(1), selector); ^ >> /usr/src/contrib/libcxxrt/exception.cc:1214:2: error: expected >> unqualified-id return _URC_INSTALL_CONTEXT; >> ^ >> fatal error: too many errors emitted, stopping now [-ferror-limit=3D] >> 20 errors generated. >> *** Error code 1 >>=20 >> Stop. >> make: stopped in /usr/src/lib/libcxxrt >> .ERROR_TARGET=3D'exception.o' >> = .ERROR_META_FILE=3D'/usr/obj/usr/src/amd64.amd64/lib/libcxxrt/exception.o.= meta' >> .MAKE.LEVEL=3D'0' >> MAKEFILE=3D'' >> .MAKE.MODE=3D'meta missing-filemon=3Dyes missing-meta=3Dyes = silent=3Dyes >> verbose' _ERROR_CMD=3D'c++ -O2 -pipe -fno-common >> -isystem /usr/src/contrib/libcxxrt -nostdinc++ -march=3Dhaswell >> -mretpoline -Wno-format-zero-length -fstack-protector-strong >> -Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable >> -Wno-error=3Dunused-but-set-variable -Wno-tautological-compare >> -Wno-unused-value -Wno-parentheses-equality -Wno-unused-function >> -Wno-enum-conversion -Wno-unused-local-typedef >> -Wno-address-of-packed-member -Wno-switch -Wno-switch-enum >> -Wno-knr-promoted-parameter -Wno-parentheses -Qunused-arguments >> -mretpoline -std=3Dc++14 -Wno-c++11-extensions >> -c /usr/src/contrib/libcxxrt/exception.cc -o >> exception.o;' .CURDIR=3D'/usr/src/lib/libcxxrt' .MAKE=3D'make' = .OBJDIR=3D'/usr/obj/usr/src/amd64.amd64/lib/libcxxrt' .TARGETS=3D' >> all' DESTDIR=3D'' LD_LIBRARY_PATH=3D'' MACHINE=3D'amd64' = MACHINE_ARCH=3D'amd64' >> MAKEOBJDIRPREFIX=3D'' MAKESYSPATH=3D'/usr/src/share/mk' >> MAKE_VERSION=3D'20220208' >> = PATH=3D'/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/loca= l/bin:/root/bin' >> SRCTOP=3D'/usr/src' OBJTOP=3D'/usr/obj/usr/src/amd64.amd64' >>=20 >>=20 >> -- >> Tomoaki AOKI <junchoon@dec.sakura.ne.jp> >> <libcxxrt_errors_20220306-001.log> >=20 --Apple-Mail=_34223E91-A920-47C2-AA63-F723ED9EE3E7 Content-Transfer-Encoding: 7bit Content-Disposition: attachment; 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