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

[Ryan Harper]

* john cooper <address@hidden> [2010-06-21 01:11]:
> Rusty Russell wrote:
> > 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.
> 
> Propagating a prospective error from virtblk_get_id() should
> be possible.  Unsure if doing so is more useful from the
> user's perspective compared to just a nul id string.

I'm not sure we can do any thing else here; maybe printk a warning?

Documentation/filesystems/sysfs.txt says that showing attributes should
always return the number of chars put into the buffer; so when there is
an error; zero is the right value to return since we're not filling the
buffer.
> 
> >> +  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);
> 
> Agreed, that's a better wrench in the gearworks.
> Note padding buf[] by 1 isn't necessary as indicated
> below.

Yep; that's a good one to take.

> 
> >     /* id_str is not necessarily nul-terminated! */
> >     buf[VIRTIO_BLK_ID_BYTES] = '\0';
> >     return virtblk_get_id(disk, buf);
> 
> The /sys file is rendered according to the length
> returned from this function and the trailing nul
> is not interpreted in this context.  In fact if a
> nul is added and included in the byte count of the
> string it will appear in the /sys file.

Yeah; I like the simplicity; but we do need to know how long the string
is so we can return that value. 

> 
> Thanks,
> 
> -john
> 
> 
> -- 
> address@hidden

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
address@hidden