libstdc++: Another attempt to ensure g++ 13+ compiled programs enforce gcc...

libstdc++: Another attempt to ensure g++ 13+ compiled programs enforce gcc 13.2+ libstdc++.so.6 [PR108969]

GCC used to emit an instance of an empty ios_base::Init class in
every TU which included <iostream> to ensure it is std::cout etc.
is initialized, but thanks to Patrick work on some targets (which have
init_priority attribute support) it is now initialized only inside of
libstdc++.so.6/libstdc++.a.

This causes a problem if people do something that has never been supported,
try to run GCC 13 compiled C++ code against GCC 12 or earlier
libstdc++.so.6 - std::cout etc. are then never initialized because code
including <iostream> expects the library to initialize it and the library
expects code including <iostream> to do that.

The following patch is second attempt to make this work cheaply as the
earlier attempt of aliasing the std::cout etc. symbols with another symbol
version didn't work out due to copy relocation breaking the aliases appart.

The patch forces just a _ZSt21ios_base_library_initv undefined symbol
into all *.o files which include <iostream> and while there is no runtime
relocation against that, it seems to enforce the right version of
libstdc++.so.6.  /home/jakub/src/gcc/obj08i/usr/local/ is the install
directory of trunk patched with this patch, /home/jakub/src/gcc/obj06/
is builddir of trunk without this patch, system g++ is GCC 12.1.1.
$ cat /tmp/hw.C
 #include <iostream>

int
main ()
{
  std::cout << "Hello, world!" << std::endl;
}
$ cd /home/jakub/src/gcc/obj08i/usr/local/bin
$ ./g++ -o /tmp/hw /tmp/hw.C
$ readelf -Wa /tmp/hw 2>/dev/null | grep initv
     4: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND _ZSt21ios_base_library_initv@GLIBCXX_3.4.32 (4)
    71: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND _ZSt21ios_base_library_initv@GLIBCXX_3.4.32
$ /tmp/hw
/tmp/hw: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.32' not found (required by /tmp/hw)
$ LD_LIBRARY_PATH=/home/jakub/src/gcc/obj08i/usr/local/lib64/ /tmp/hw
Hello, world!
$ LD_LIBRARY_PATH=/home/jakub/src/gcc/obj06/x86_64-pc-linux-gnu/libstdc++-v3/src/.libs/ /tmp/hw
/tmp/hw: /home/jakub/src/gcc/obj06/x86_64-pc-linux-gnu/libstdc++-v3/src/.libs/libstdc++.so.6: version `GLIBCXX_3.4.32' not found (required by /tmp/hw)
$ g++ -o /tmp/hw /tmp/hw.C
$ /tmp/hw
Hello, world!
$ LD_LIBRARY_PATH=/home/jakub/src/gcc/obj06/x86_64-pc-linux-gnu/libstdc++-v3/src/.libs/ /tmp/hw
Hello, world!
$ LD_LIBRARY_PATH=/home/jakub/src/gcc/obj08i/usr/local/lib64/ /tmp/hw
Hello, world!

On sparc-sun-solaris2.11 one I've actually checked a version which had
defined(_GLIBCXX_SYMVER_SUN) next to defined(_GLIBCXX_SYMVER_GNU), but
init_priority attribute doesn't seem to be supported there and so I couldn't
actually test how this works there.  Using gas and Sun ld, Rainer, does one
need to use gas + gld for init_priority or something else?

2023-04-28  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/108969
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.32): Export
	_ZSt21ios_base_library_initv.
	* testsuite/util/testsuite_abi.cc (check_version): Add GLIBCXX_3.4.32
	symver and make it the latestp.
	* src/c++98/ios_init.cc (ios_base_library_init): New alias.
	* acinclude.m4 (libtool_VERSION): Change to 6:32:0.
	* include/std/iostream: If init_priority attribute is supported
	and _GLIBCXX_SYMVER_GNU, force undefined _ZSt21ios_base_library_initv
	symbol into the object.
	* configure: Regenerated.