On Mon, Aug 03, 2020 at 05:01:18PM +0200, Florian Weimer wrote:
> * Daniel P. Berrangé:
>
> > Disabling LTO in the RPM spec confirms this and makes things pass
> > again. Hacking the makefiles to remove the -fno-lto option when
> > building the test suite binaries also fixes things.
> >
> > I don't see any mention of LD_PRELOAD being impacted by LTO in the
> > Fedora feature change page, but I can imagine how it would be.
>
> LTO should still export the same functions as before, and should not
> imply -fno-semantic-interposition by default. The linker plugin
> provides the necessary information to GCC. What you are observing could
> be the result of a toolchain bug.
Libvirt has a test program "qemuhotplugtest".
This test links to a shared library libqemutestdriver.so which contains
a function "qemuProcessStartManagedPRDaemon".
qemuhotplugtest test does not call "qemuProcessStartManagedPRDaemon"
directly. It invokes "qemuDomainAttachDeviceDiskLive" which eventually
ends up calling "qemuProcessStartManagedPRDaemon" some way further
down the stack.
Then there is a shared library libqemuhotplugmock.so which contains a
replacement "qemuProcessStartManagedPRDaemon" to avoid us spawning
external programs.
When it starts "qemuhotplugtest" will set LD_PRELOAD=libqemuhotplugmock.so
and then execve() itself.
So when the test runs, the "qemuProcessStartManagedPRDaemon" impl from
the mock library is supposed to be used.
If I run with LD_DEBUG=all on a build /without/ LTO, I can see this lookup
and override happening:
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/lt-qemuhotplugtest [0]
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libqemutestdriver.so [0]
381018: binding file /home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libqemutestdriver.so [0] to /home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libqemutestdriver.so [0]: normal symbol `qemuProcessStartManagedPRDaemon'
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/lt-qemuhotplugtest [0]
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libvirhostdevmock.so [0]
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libvirpcimock.so [0]
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libdomaincapsmock.so [0]
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libvirprocessmock.so [0]
381018: symbol=qemuProcessStartManagedPRDaemon; lookup in file=/home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libqemuhotplugmock.so [0]
381018: binding file /home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libqemutestdriver.so [0] to /home/berrange/src/fedora/libvirt/libvirt-6.5.0/x86_64-redhat-linux-gnu/tests/.libs/libqemuhotplugmock.so [0]: normal symbol `qemuProcessStartManagedPRDaemon'
If I run LD_DEBUG=all on a build /with/ LTO, there are no symbol lookups
at all for qemuProcessStartManagedPRDaemon. It looks very much like the
call was resolved and bound at link time when built with LTO.
Regards,
Daniel
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