Re: [Qemu-devel] VirtIO 9p mount_tag (bogus?) limit of 32 bytes

[Harsh Bora <harsh@linux.vnet.ibm.com>]

On 09/22/2011 11:12 PM, Daniel P. Berrange wrote:
> I've noticed that if you use a virtio 9p filesystem with a mount_tag
> property value that is longer than 32 bytes, it gets silently truncated.
>
> In virtio-9p-device.c
>
>      len = strlen(conf->tag);
>      if (len>  MAX_TAG_LEN) {
>          len = MAX_TAG_LEN;

I think its better to return here with a failure message saying 
mount_tag too long. IIUC, The 32 byte limit has been kept because of 
understanding that mount_tag is a device name in guest (and not a path 
location).

Aneesh, any inputs ?

- Harsh

>      }
>
>
> The header  virtio-9p.h contains
>
>
>    /* from Linux's linux/virtio_9p.h */
>
>    /* The ID for virtio console */
>    #define VIRTIO_ID_9P    9
>    #define MAX_REQ         128
>    #define MAX_TAG_LEN     32
>
>
> The Linux kernel's  virtio_9p.h, however, does not have any MAX_TAG_LEN
> constant and AFAICT the code in Linux's net/9p/trans_virtio.c is not
> placing any 32 byte length restriction on the mount tag.
>
> So is this QEMU length limit legacy code that can be removed ?
>
> If using the mount_tag to specify the desired guest mount location path,
> then 32 bytes is really quite limiting - a good 255 bytes is much more
> desirable.
>
> Finally, regardless of what limit is imposed, it would be better to
> return an error if the user attempts to specify an excessively long
> mount tag, rather than truncate it breaking the guest app relying on
> the full tag.
>
> Regards,
> Daniel