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

[Michal Privoznik]

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