Hi,
On 8/3/20 5:27 PM, Daniel P. Berrangé wrote:
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.
Maybe it was not bound at link time, but inlined at compile time?
I think it might be worthwhile to try and mark the qemuProcessStartManagedPRDaemon
implementation which is used normally (no LD_PRELOAD) with some function
attribute that it may be never inlined. I'm sure Florian and/or Jakub
can help with what that function attribute should actually look like...
Regards,
Hans
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