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

[john cooper]

Ryan Harper wrote:
> * 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.

So we return a nul string in the case the qemu user
didn't specify an id string and also in the case a
legacy qemu doesn't support retrieval of an id string.
Not too much difference and if needed going forward the
error return can be elaborated.

>>>     /* 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. 

Which we're getting from serial_sysfs() without
having to accommodate an unused nul.  I'd hazard the
primary reason the sysfs calling code keys off a
return of byte count vs. traversing the string itself
is due to the called function almost always having the
byte count available.

-john

-- 
address@hidden