Comment 4 for bug 1092715

I'm now convinced this is a kernel bug.

If I do
   touch xxx
   setfacl -m g::--- xxx
   strace -f -ooutput chmod g+rw xxx
   getfacl xxx

I see that the group acl for xxx is changed to include rw, and the file 'output' shows that chmod simply calls the fchmodat syscall, which uses notify_change to update the acl.

If I do
   # make sure to be logged into console
   sudo apt-get purge qemu-kvm
   sudo modprobe kvm_intel
   sudo apt-get install qemu-kvm
   getfacl xxx
   # see that group acl is still ---
   sudo chmod g+rw /dev/kvm
   # see that group acl is still ---

So the kernel is meant to be updating the acl in fchmodat, but is not doing it. I will try a xfs-backed VM to see if the bug is in ext4 itself, or in the generic fs code.