Implement hercules stats #3
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This was not correct, see bug #950173 comment #3. Bug: https://bugs.gentoo.org/950173 Signed-off-by: Ionen Wolkens <ionen@gentoo.org>
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Qt's qcompilerdetection.h currently checks for whether -fPIE is being used
along with QT_USE_PROTECTED_VISIBILITY ("reduce relocations", which Qt
automatically uses if supported). It bails out if -fPIE is used, as -fPIC
is required instead.
If LTO is used, when one does something like:
(1) g++ -c -flto -fPIC qtlto.cc
(2) g++ -pie -fPIE qtlto.o -o qtlto
At point (1), the Qt check in the headers fires, and everything is fine,
because we're indeed using -fPIC, and GCC doesn't automatically add -fPIE
when built with --enable-default-pie if -fPIC is present on the command line.
GCC may apply optimisations at this point given Qt is using -mno-direct-extern-access
and it was built with -fPIC not -fPIE.
Later, at point (2), -fPIE is passed. This happens in Wireshark because
`CMAKE_POSITION_INDEPENDENT_CODE` gets set in CMakeLists.txt. With LTO,
there's no opportunity for the Qt sanity check in headers to fire again,
as everything is already long-preprocessed and GCC will have applied some
optimisations already assuming the -fPIC code model in (1). But as slyfox
says at https://bugs.gentoo.org/754021#c12, GCC merges -fPIC -fPIE to -fPIE
at LTO-time (-fPIC coming from the earlier LTO object in (1), and -fPIE
was just-passed on the command line).
qtlto (or Wireshark) then crashes. For Wireshark, this looks like:
```
#0 0x00007ff40e529cf0 in QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData> >::get (this=<optimized out>)
at /usr/src/debug/dev-qt/qtbase-6.8.3/qtbase-everywhere-src-6.8.3/src/corelib/tools/qscopedpointer.h:112
#1 qGetPtrHelper<QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData> > > (ptr=<optimized out>)
at /usr/src/debug/dev-qt/qtbase-6.8.3/qtbase-everywhere-src-6.8.3/src/corelib/global/qtclasshelpermacros.h:128
#2 QObject::d_func (this=<optimized out>) at /usr/src/debug/dev-qt/qtbase-6.8.3/qtbase-everywhere-src-6.8.3/src/corelib/kernel/qobject.h:108
#3 QObjectPrivate::get (o=<optimized out>) at /usr/src/debug/dev-qt/qtbase-6.8.3/qtbase-everywhere-src-6.8.3/src/corelib/kernel/qobject_p.h:150
#4 doActivate<false> (sender=0x0, signal_index=9, argv=argv@entry=0x7ffe59a73c30) at /usr/src/debug/dev-qt/qtbase-6.8.3/qtbase-everywhere-src-6.8.3/src/corelib/kernel/qobject.cpp:4003
#5 0x00007ff40e4d2809 in QMetaObject::activate
(sender=<optimized out>, m=m@entry=0x7ff40f44f6c0 <QGuiApplication::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7ffe59a73c30)
at /usr/src/debug/dev-qt/qtbase-6.8.3
6619
/qtbase-everywhere-src-6.8.3/src/corelib/kernel/qobject.cpp:4183
#6 0x00007ff40ead5676 in QGuiApplication::screenAdded (this=<optimized out>, _t1=<optimized out>)
[...]
```
We need to drop -fPIE somehow at link-time accordingly. There's a few
ways of doing this but I've gone for not calling `check_pie_supported()`
(see (7) below).
(Analysis on fixing this in other packages may depend on whether any static
libraries *installed* by CMake where -fPIC was no longer passed for those,
we would have a problem. I'd tried to use POSITION_INDEPENDENT_CODE at first
but then -fPIC gets dropped as well everywhere, and setting the target
property to false for just the Wireshark executable also doesn't work
because it'll pass -no-pie which isn't what we want.)
There are some questions:
(3) Why doesn't this happen with Clang, given that Clang has -fno-direct-access-external-data
(equivalent to GCC's -mno-direct-extern-access), even when Qt is built
with bfd (not lld)?
The answer seems to be that Clang doesn't implement the optimisation
yet to avoid copy-relocations where possible. GCC implemented that in
5.x in r5-5573-g77ad54d911dd7c.
(4) Why doesn't this (seem to) happen in other distributions?
nextcloud-client suffers from the same issue analysed here, see
https://bugs.gentoo.org/933110. The upstream bug at nextcloud/desktop#2790
was reported by a Debian developer (cgzones), so it's a reasonable assumption
that it can happen on Debian.
Debian is one of few distributions (we're another) to use --enable-default-pie
in GCC rather than just passing it to all package builds in the package manager:
it's possible that some distros are just disabling -fPIE or adding a workaround
like we did for https://bugs.gentoo.org/552440. Not many distros build
with LTO either.
Debian also stopped building Wireshark with LTO because of a bug in Wireshark
itself (https://bugs.gentoo.org/941890), so I guess they disabled LTO
and didn't notice this crash.
(This is enough for me to be more confident in my analysis, anyway.)
(5) Could Qt communicate this somehow automatically?
I think it might be able to if statically linking Qt and Qt was built
with LTO.
Otherwise, I think the only option would be an ELF .note. pkg-config
could maybe work but you can't assume all Qt consumers use that...
See the discussion around <https://bugreports.qt.io/browse/QTBUG-45755?focusedId=282483&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-282483>:
> Thiago Macieira added a comment - 22 May '15 17:11
> There aren't that many autoconf-based Qt5 builds and we've never exported the flag anyway.
(It might be worth bringing this .note idea up to Thiago and/or H.J. but
I'm not sure yet if it'll work.)
On the Qt side, -fPIC gets passed in to various places before because
Qt's CMake config files have INTERFACE_COMPILE_OPTIONS w/ -fPIC. Maybe
the answer is for Qt packages to never use CMAKE_POSITION_INDEPENDENT_CODE
instead. This came up in https://gitlab.kitware.com/cmake/cmake/-/issues/15570.
(6) Could we just disable "reduce relocations" in Qt itself, given that
the workaround here will need to be applied in various Qt consumers?
This would significantly impact startup times of applications using Qt
and there don't seem to be too many applications doing this (only 2
known so far in Gentoo: Wireshark and nextcloud-client).
(7) Is the mechanism used to fix this brittle?
Yes, we're relying on a CMake bug/feature for now at https://gitlab.kitware.com/cmake/cmake/-/issues/25588
so it doesn't try to enable *or* disable PIE at link-time and we can
just rely on our toolchain defaults.
Thanks to Arusekk for producing a minimal example and reporting it upstream
to Wireshark, thanks to slyfox for analysing the interaction with LTO, thanks
to Holger for the discussion around it and testing, and thanks to Eli for
reviewing the commit message.
Bug: https://bugs.gentoo.org/552440
Bug: https://bugs.gentoo.org/754021
Bug: https://bugs.gentoo.org/933110
Bug: https://bugs.gentoo.org/941890
Bug: https://gitlab.kitware.com/cmake/cmake/-/issues/15570
Bug: https://gitlab.kitware.com/cmake/cmake/-/issues/25588
Bug: https://gitlab.kitware.com/cmake/cmake/-/issues/23980
Bug: https://gitlab.com/wireshark/wireshark/-/issues/17040
Bug: https://bugreports.qt.io/browse/QTBUG-45755
Bug: https://bugreports.qt.io/browse/QTBUG-47942
Bug: https://gcc.gnu.org/PR65248
Bug: https://gcc.gnu.org/PR65886
Thanks-to: Arusekk <arek_koz@o2.pl>
Thanks-to: Sergei Trofimovich <slyfox@gentoo.org>
Thanks-to: Holger Hoffstätte <holger@applied-asynchrony.com>
Thanks-to: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Sam James <sam@gentoo.org>
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