Q: tests/ioctl_kvm_run.c segfaults on rawhide

[Masatake YAMATO]

On Wed, 4 Jul 2018 05:52:16 +0300, "Dmitry V. Levin" <ldv at altlinux.org> wrote:
> On Tue, Jul 03, 2018 at 11:32:06PM +0300, Dmitry V. Levin wrote:
>> Hi,
>> 
>> Our ioctl_kvm_run test started to fail on rawhide with the following symptoms:
>> 
>> strace/tests$ $ ./ioctl_kvm_run >/dev/null; echo \$?=$?
>> $?=0
>> strace/tests-m32$ ./ioctl_kvm_run >/dev/null; echo \$?=$?
>> Segmentation fault (core dumped)
>> $?=139
>> strace/tests-m32$ ../strace -enone ./ioctl_kvm_run >/dev/null 
>> ../strace: [ Process PID=12345 runs in 32 bit mode. ]
>> +++ exited with 0 +++
>> 
>> The segfault happens inside glibc right after the first ioctl(KVM_RUN)
>> invocation.
>> 
>> All this oddness is happening in the following system:
>> Linux rawhide-test.fedorainfracloud.org 4.18.0-0.rc2.git4.1.fc29.x86_64 #1 SMP Fri Jun 29 15:03:54 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
> 
> The same symptoms could be seen in the following system:
> Linux f27-test.fedorainfracloud.org 4.17.3-100.fc27.x86_64 #1 SMP Tue Jun 26 14:19:03 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
> 
> This essentially leaves the kernel as the only plausible cause of this bug.
> 
>> There are no such problems in the following system:
>> Linux f28-test.fedorainfracloud.org 4.16.3-301.fc28.x86_64 #1 SMP Mon Apr 23 21:59:58 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
>> 
>> I suspect it's a bug of kvm nesting in rawhide, most likely in the kernel.
>> Any ideas where to report this?

I think the report should go to bugzilla.redhat.com because the kernel is built at Fedora project.
However, if the issue is reproachable with a kernel binary built locally, the report should go to
one of upstream mailing list.

    KERNEL VIRTUAL MACHINE (KVM)
    M:	Paolo Bonzini <pbonzini at redhat.com>
    M:	Radim Krčmář <rkrcmar at redhat.com>
    L:	kvm at vger.kernel.org

I guess we can get more interests from more experts if we can report it to kvm at vger.kernel.org.


We have a good reproducer, so it is worth to verify the issue on the
upstream kernel.

Though I don't read but I found an interesting article for tracking down
this issue.

     http://blog.oddbit.com/2014/07/21/tracking-down-a-kernel-bug-wit/

I think it will be nice that strace community (including I) has knowledges
for trakcing down the change of kernel behavior with GIT BISECT.

I will study this area more.

Masatake YAMATO
> 
> -- 
> ldv