Rather than setting -fPIC wholesale, only set it for the qt objects.
The first commit restructures to move the fPIE flag out, the second is the actual change.
Note that -fPIC is used for testing qt until we get to the actual fPIE test. That’s to prevent us from bombing out early due to qt’s header yelling about PIE before we actually want to test it.
This allows for fPIE to be used selectively.
But only if qt was built with reduced relocations.
Thanks, Concept ACK.
Can confirm default settings ($ ./autogen.sh ;./configure ;make) work again with 69d0513 on latest fedora. (You may want to put “closes #6248” in the OP)
On Ubuntu 15.10 this gives:
0checking whether -fPIE can be used with this Qt config... no
Which is correct. However on On Ubuntu 14.04 I also get:
0checking whether -fPIE can be used with this Qt config... no
While I know in fact that that does work. Error in config.log is:
0configure:23385: g++ -c -fPIE -g -O2 -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter -Wno-self-assign -I/usr/include/qt5/QtCore -I/usr/include/qt5 -I/usr/include/qt5/QtGui -I/usr/include/qt5/QtNetwork -I/usr/include/qt5/QtWidgets -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS conftest.cpp >&5
1conftest.cpp: In function 'int main()':
2conftest.cpp:67:15: error: 'choke' was not declared in this scope
3 choke;
4 ^
5At global scope:
Strange. QT_REDUCE_RELOCATIONS is indeed enabled:
0./qt5/QtCore/qconfig.h:#define QT_REDUCE_RELOCATIONS
However it just works with fPIE. Gcc version issue?
@laanwj I believe it’s a Qt issue. They tightened the requirements with newer qt versions, -fPIE used to compile fine. But -fPIE + gcc5 + older Qt will still likely cause problems regardless if it does compile.
So the real issue is -fPIE + newer gcc regardless of when qt tightened the error. I think it makes sense to always fall back to -fPIC any time that reduce_relocations was used. The alternative would be a blacklist/whitelist for compilers, which i was trying to avoid.
This is the error I get (one for every executable that uses qt):
0In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qnamespace.h:37:0,
1 from /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs.h:41,
2 from /usr/include/x86_64-linux-gnu/qt5/QtCore/qobject.h:40,
3 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QObject:1,
4 from qt/test/uritests.h:8,
5 from qt/test/test_main.cpp:10:
6/usr/include/x86_64-linux-gnu/qt5/QtCore/qglobal.h:1052:4: error: #error "You must build your code with position independent code if Qt was built with -reduce-relocations. " "Compile your code with -fPIC (-fPIE is not enough)."
7 # error "You must build your code with position independent code if Qt was built with -reduce-relocations. "\
8 ^
@laanwj Sorry for being unclear. To summarize the issue:
Qt build can enable -Bsymbolic-functions via the reduce-relocations config option. The qt devs assumed that -fPIC or -fPIE would disable copy-relocations, but gcc5 changed that behavior and allows copy-relocs with -fPIE. So -fPIC is the only way to guarantee that -Bsymbolic-functions will work as intended.
Source: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65886#c16
When Qt discovered this issue (somewhere around 5.4), they added the check in qconfig.h so that compilation would fail in this QT_REDUCE_RELOCATIONS+-fPIE case. Prior to that, the check looked like this:
0#if !defined(QT_BOOTSTRAPPED) && defined(QT_REDUCE_RELOCATIONS) && defined(__ELF__) && !defined(__PIC__) && !defined(__PIE__)
1# error "You must build your code with position independent code if Qt was built with -reduce-relocations. "\
2 "Compile your code with -fPIC or -fPIE."
3#endif
Just because compiling succeeds in versions with the old check, that does not mean that it’s safe to build/run with gcc5+. So testing to see if something builds against qt+-fPIE is not enough, as older versions weren’t aware of the problem.
