[Question] sg_persist failed when using qemu-pr-helper

[Zihao Chang]

On 2020/11/16 23:10, Michal Privoznik wrote:
> On 11/16/20 3:22 PM, Zihao Chang wrote:
>> Hi all,
>>
>> When using qemu-pr-helper，"sg_persist" in guest return Aborted Command after iscsiadm logout & login.
>>
>> My confusion:
>> Is this problem caused by bugs of libvirt/qemu-pr-helper?
>> Or iscsiadm logout & login should be forbidden when using qemu-pr-helper?
>> Or There is something wrong with my config.
>>
>> Steps to Reproduce:
>> 1. setup multipath env
>>    1) set /etc/iscsi/initiatorname.iscsi
>>    2) iscsiadm discovery & login
>>    3) start multipathd
>>
>> 2. add the following xml to guest
>>      <disk type='block' device='lun'>
>>              <driver name='qemu' type='raw' cache='none' io='native'/>
>>              <source dev='/dev/mapper/XXXXXXXXXXXXXXXXX'>
>>                  **<reservations managed='yes'/>**
>>              </source>
>>              <target dev='sdc' bus='scsi'/>
>>      </disk>
>>
>> 3. start the guest
>>     1) "sg_persist -v -i -n -k -d  /dev/sdc" works well in the guest
>>     2) here /dev/sdc(guest)  -> /dev/sdht(host)
>>
>> 4. iscsiadm logout & login on host
>>     1) iscsiadm  -m node -u  (host)
>>     2) iscsiadm -m node -l   (host)
>>     3) here /dev/sdc(guest)  -(change)-> /dev/sdfo(host)
>>
>> 5. exec "sg_persist -v -i -n -k -d  /dev/sdc" in the guest
>>     1) command return:
>>           Persistent Reservation In cmd: 5e 00 00 00 00 00 00 20 00 00
>>           persistent reservation in:
>>           Fixed format, current; Sense key: Aborted Command
>>           Additional sense: Logical unit communication failure
>>           PR in (Read keys): Aborted command sense key, other than protection related (asc=0x10)
>>     2) strace qemu-pr-helper show:
>>           openat(AT_FDCWD, "/dev/sdht", O_RDONLY) = -1 ENOENT (No such file or directory)
>>
>> Additional info:
>> 1. strace qemu-pr-helper show:
>>         openat(AT_FDCWD, "/dev/sdht", O_RDONLY) = -1 ENOENT (No such file or directory)
>> 2. This problem may be related to mnt(namespace). I try to set mnt of qemu-pr-helper as pid 1,  sg_persist works well after logout & login.
>>
>>
> 
> Yeah, this smells like a libvirt bug. But I'm not sure how to solve it. Because from security POV we want to run qemu-pr-helper inside the same namespace as qemu is running (so that it doesn't see more devices than needed). But at the same time, when a /dev node changes from outside libvirt does not reflect that in the namespace.
> 
> BTW: is /dev/mapper/XXX updated on re-login? Becuase if not, then you'd get the same error in qemu, if it wasn't for pr-helper hitting the issue first.
> 
> A possible work around might be to use unmanaged pr-helper (reservations managed='no') and just start it by hand.
> 
> Michal
> 
> .

Actually, if use unmanaged pr-helper (reservations managed='no'), "sg_persist" work well without starting pr-helper by
hand in this case. After logout & login qemu does not meet openat() ENOENT (No such file or directory), and qemu even
does not use openat() according to the result of "strace" command. But the source code of libvirt shows that it start
pr-helper inside the same namespace as qemu is running. It really confused me.

Regards,
Zihao