Re: [RFC PATCH v9 1/2] Btrfs: Add a new ioctl to get the label of a mounted file system

[Jeff Liu <jeff.liu@oracle.com>]

Hi Goffredo,

On 01/08/2013 01:32 AM, Goffredo Baroncelli wrote:
> Hi Jeff,
> 
> On 01/07/2013 07:24 AM, Jeff Liu wrote:
>> Hi,
>> On 01/07/2013 02:44 AM, Goffredo Baroncelli wrote:
>>> Hi Jeff,
>>>
>>> On 01/05/2013 03:48 AM, Jeff Liu wrote:
>>>> Add a new ioctl(2) BTRFS_IOC_GET_FSLABLE, so that we can get the label upon a mounted filesystem.
>>>>
>>>> Signed-off-by: Jie Liu <jeff.liu@oracle.com>
>>>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>>>> Cc: Miao Xie <miaox@cn.fujitsu.com>
>>>> Cc: Goffredo Baroncelli <kreijack@inwind.it>
>>>> Cc: David Sterba <dsterba@suse.cz>
>>>>
>>>> ---
>>>>  fs/btrfs/ioctl.c |   21 +++++++++++++++++++++
>>>>  fs/btrfs/ioctl.h |    2 ++
>>>>  2 files changed, 23 insertions(+)
>>>>
>>>> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
>>>> index 8fcf9a5..ef2f55a 100644
>>>> --- a/fs/btrfs/ioctl.c
>>>> +++ b/fs/btrfs/ioctl.c
>>>> @@ -3699,6 +3699,25 @@ out:
>>>>  	return ret;
>>>>  }
>>>>  
>>>
>>> May be that it was already discussed, and I am missing something, but if
>>> we check the label length we should terminate the string with a zero in
>>> case this is too long...
>>>
>>> However this is a minor bug, please push this patch forward..
>> I don't think so.
>> Why we need to terminate the string with a zero?
>> We have already truncated the label string up to maximum 255 bytes if it
>> was too long.
>>
>> Consider the normal conditions, i.e. the label string is less than 256,
>> we just copy it back to the user without a terminated NUL.
> 
> Sorry I don't understand the reason to terminate the string up to 255
> chars without adding a zero. Or we copy all the buffer (BTRFS_LABEL_SIZE
> characters without furthers checks) or we copy BTRFS_LABEL_SIZE-1
> characters, adding a zero at the end.
For normal cases, there should already has a NUL at the end of the input
label string when performing btrfs_ioc_set_fslabel(), and we have also
added a NUL for unmounted label set in btrfs-progs.  In both cases, put
it again on btrfs_ioc_get_fslabel() is redundant IMO.

If we ran into a too long label, return the first 255 bytes is fine for
both mounted and unmounted get_label() routines in btrfs-progs since I
did memset(label, 0, sizeof(label)) at first for fetching mounted fs label.

Also, I have not found any existing kernel code does similar things by
adding a NUL to end up a char array, could you show me an example?


"""Or we copy all the buffer(BTRFS_LABEL_SIZE) characters without
furthers checks"""

If an array is capable of N chars, we can only full it with N - 1 chars.

Thanks,
-Jeff
>>>
>>>
>>>> +static int btrfs_ioctl_get_fslabel(struct file *file, void __user *arg)
>>>> +{
>>>> +	struct btrfs_root *root = BTRFS_I(fdentry(file)->d_inode)->root;
>>>> +	const char *label = root->fs_info->super_copy->label;
>>>> +	size_t len = strnlen(label, BTRFS_LABEL_SIZE);
>>>> +	int ret;
>>> +       int label_is_too_long = 0;
>> 	  bool label_is_too_long = false;
>>>> +
>>>> +	if (len == BTRFS_LABEL_SIZE) {
>>>> +		pr_warn("btrfs: label is too long, return the first %zu bytes\n",
>>>> +			--len);
>>>
>>> +               label_is_too_long = 1;
>> 		  label_is_too_long = true;
>>>> +	}
>>>> +
>>>> +	mutex_lock(&root->fs_info->volume_mutex);
>>>> +	ret = copy_to_user(arg, label, len);
>>> +	if (!ret && label_is_too_long)
>>> +     		ret = copy_to_user(arg+BTRFS_LABEL_SIZE, "", 1);
>> Hmm? 			ret = copy_to_user(arg + len, "", 1);
>>
> My idea was to put a zero at the end of the string, but I did a mistake
> with the cut&paste.. sorry :-)
> 
>>>> +	mutex_unlock(&root->fs_info->volume_mutex);
>> If you or anyone has objection to this patch, please just post yours, I
>> won't follow it up again.
> 
> Sorry I don't want to bother anybody, I know that reviewing a patch nine
> times is a very havvy work. I (and I think more other peoples) really
> appreciate your efforts.
> 
> BR
> G.Baroncelli
> 
>>>> +
>>>> +	return ret ? -EFAULT : 0;
>>>> +}
>>>> +
>>>>  long btrfs_ioctl(struct file *file, unsigned int
>>>>  		cmd, unsigned long arg)
>>>>  {
>>>> @@ -3797,6 +3816,8 @@ long btrfs_ioctl(struct file *file, unsigned int
>>>>  		return btrfs_ioctl_qgroup_create(root, argp);
>>>>  	case BTRFS_IOC_QGROUP_LIMIT:
>>>>  		return btrfs_ioctl_qgroup_limit(root, argp);
>>>> +	case BTRFS_IOC_GET_FSLABEL:
>>>> +		return btrfs_ioctl_get_fslabel(file, argp);
>>>>  	}
>>>>  
>>>>  	return -ENOTTY;
>>>> diff --git a/fs/btrfs/ioctl.h b/fs/btrfs/ioctl.h
>>>> index 731e287..5b2cbef 100644
>>>> --- a/fs/btrfs/ioctl.h
>>>> +++ b/fs/btrfs/ioctl.h
>>>> @@ -451,6 +451,8 @@ struct btrfs_ioctl_send_args {
>>>>  			       struct btrfs_ioctl_qgroup_create_args)
>>>>  #define BTRFS_IOC_QGROUP_LIMIT _IOR(BTRFS_IOCTL_MAGIC, 43, \
>>>>  			       struct btrfs_ioctl_qgroup_limit_args)
>>>> +#define BTRFS_IOC_GET_FSLABEL _IOR(BTRFS_IOCTL_MAGIC, 49, \
>>>> +				   char[BTRFS_LABEL_SIZE])
>>>>  #define BTRFS_IOC_GET_DEV_STATS _IOWR(BTRFS_IOCTL_MAGIC, 52, \
>>>>  				      struct btrfs_ioctl_get_dev_stats)
>>>>  #endif
>>>
>>>
>>
>>
> 
>