[Qemu-devel] Re: [PATCH 1/2] Add 'serial' attribute to virtio-blk devices

[Rusty Russell]

On Sat, 19 Jun 2010 04:08:02 am Ryan Harper wrote:
> Create a new attribute for virtio-blk devices that will fetch the serial 
> number
> of the block device.  This attribute can be used by udev to create disk/by-id
> symlinks for devices that don't have a UUID (filesystem) associated with them.
> 
> ATA_IDENTIFY strings are special in that they can be up to 20 chars long
> and aren't required to be NULL-terminated.  The buffer is also zero-padded
> meaning that if the serial is 19 chars or less that we get a NULL terminated
> string.  When copying this value into a string buffer, we must be careful to
> copy up to the NULL (if it present) and only 20 if it is longer and not to
> attempt to NULL terminate; this isn't needed.
> 
> Signed-off-by: Ryan Harper <address@hidden>
> Signed-off-by: john cooper <address@hidden>
> ---
>  drivers/block/virtio_blk.c |   32 ++++++++++++++++++++++++++++++++
>  1 files changed, 32 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 258bc2a..f1ef26f 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -281,6 +281,31 @@ static int index_to_minor(int index)
>       return index << PART_BITS;
>  }
>  
> +/* Copy serial number from *s to *d.  Copy operation terminates on either
> + * encountering a nul in *s or after n bytes have been copied, whichever
> + * occurs first.  *d is not forcibly nul terminated.  Return # of bytes 
> copied.
> + */
> +static inline int serial_sysfs(char *d, char *s, int n)
> +{
> +     char *di = d;
> +
> +     while (*s && n--)
> +             *d++ = *s++;
> +     return d - di;
> +}
> +
> +static ssize_t virtblk_serial_show(struct device *dev,
> +                             struct device_attribute *attr, char *buf)
> +{
> +     struct gendisk *disk = dev_to_disk(dev);
> +     char id_str[VIRTIO_BLK_ID_BYTES];
> +
> +     if (IS_ERR(virtblk_get_id(disk, id_str)))
> +             return 0;

0?  Really?  That doesn't seem very informative.

> +     return serial_sysfs(buf, id_str, min(VIRTIO_BLK_ID_BYTES, PAGE_SIZE));

How about something like this:

        BUILD_BUG_ON(PAGE_SIZE < VIRTIO_BLK_ID_BYTES + 1);

        /* id_str is not necessarily nul-terminated! */
        buf[VIRTIO_BLK_ID_BYTES] = '\0';
        return virtblk_get_id(disk, buf);

Thanks,
Rusty.