[PATCH] qemu: domfsinfo should print a target name on s390x, not a complete device path

[Peter Krempa]

On Thu, Nov 12, 2020 at 23:48:02 +0100, Thomas Huth wrote:
> While fixing domfsinfo for non-PCI (i.e. CCW) devices on s390x,
> I accidentally used the whole device path for the devAlias field.
> However, it should only contain the base target name.
> 
> Currently we have the wrong output:
> 
>  $ virsh domfsinfo guestname
>   Mountpoint   Name   Type   Target
>  ---------------------------------------
>   /            sda3   xfs    /dev/sda3
>   /boot        sda1   xfs    /dev/sda1
> 
> It should look like this instead:
> 
>  $ virsh domfsinfo guestname
>   Mountpoint   Name   Type   Target
>  ------------------------------------
>   /            sda3   xfs    sda
>   /boot        sda1   xfs    sda
> 
> Thus we have to strip the "/dev/" prefix and the partition number
> from the string.

This is not the correct approach. While our "target" looks suspiciously
close to the guest /dev/name it can't be guaranteed to be the same.

Stripping prefix and suffix may work in many cases, but if you want to
uphold what's written in the API docs this won't be always correct.

I think we need to just relax the documentation of 'virsh domfsinfo' and
report the path as we do now if the s390x platform is unable to report
back any information which would allow us to match the device otherwise.

> 
> Fixes: f8333b3b0a ("qemu: Fix domfsinfo for non-PCI device information ...")
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1858771
> Reported-by: Sebastian Mitterle <smitterl at redhat.com>
> ---
>  src/qemu/qemu_driver.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index 05f8eb2cb7..d92bee1d35 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -18874,6 +18874,30 @@ qemuDomainGetFSInfoAgent(virQEMUDriverPtr driver,
>      return ret;
>  }
>  
> +/* Turn device node string like "/dev/vda1" into a target name like "vda" */
> +static char *
> +qemuAgentDevNodeToTarget(const char *devnode)
> +{
> +    char *str = g_strdup(devnode);
> +    size_t len = strlen(str);
> +
> +    /* Remove the "/dev/" prefix from the string */
> +    if (g_str_has_prefix(str, "/dev/")) {
> +        len -= 5;
> +        memmove(str, str + 5, len + 1);
> +    }
> +
> +    /* Remove the partition number from the end of the string */
> +    while (len > 0) {
> +        len--;
> +        if (!g_ascii_isdigit(str[len]))
> +            break;
> +        str[len] = 0;

Note that the guest-reported partition name does not necessarily match
what's configured in the domain XML, so this algorithm is not correct.


> +    }
> +
> +    return str;
> +}
> +
>  static virDomainFSInfoPtr
>  qemuAgentFSInfoToPublic(qemuAgentFSInfoPtr agent,
>                          virDomainDefPtr vmdef)
> @@ -18903,7 +18927,7 @@ qemuAgentFSInfoToPublic(qemuAgentFSInfoPtr agent,
>          if (diskDef != NULL)
>              ret->devAlias[i] = g_strdup(diskDef->dst);
>          else if (agentdisk->devnode != NULL)
> -            ret->devAlias[i] = g_strdup(agentdisk->devnode);
> +            ret->devAlias[i] = qemuAgentDevNodeToTarget(agentdisk->devnode);
>          else
>              VIR_DEBUG("Missing devnode name for '%s'.", ret->mountpoint);
>      }
> -- 
> 2.18.4
>