Re: cannot read default subvolume id?

[Qu Wenruo <wqu@suse.com>]

在 2025/7/28 19:38, Ulli Horlacher 写道:
> On Mon 2025-07-28 (08:06), Qu Wenruo wrote:
>>
>>
>> 圚 2025/7/28 03:16, Ulli Horlacher 写é":
>>
>>>
>>> Why is there a "cannot read default subvolume id"?
>>>
>>> tux@quak:~: btrfs sub create xx
>>> Create subvolume './xx'
>>>
>>> tux@quak:~: btrfs sub del xx
>>> WARNING: cannot read default subvolume id: Operation not permitted
>>
>> It's a warning, not an error. And you can see on the next line, the
>> deletion happened without any problem.
> 
> But this warning confuses a user:
> Is there a problem?
> What is going wrong?
> Have I made a mistake?

Yes, that's indeed confusing, and it should be hidden by commit 
0e66228959c4 ("btrfs-progs: subvol delete: hide a warning on an 
unprivileged delete"), which is included in v5.16.

Unfortunately that fix has a bug that the uid check is completely 
opposite...
> 
>> You're running as a non-root user, meanwhile default subvolume id search
>> is done using tree search ioctl, which require root privilege.
> 
> Why is this subvolume id search necessary for subvolume deletion and why
> is it just a warning and apparently no problem at all?
> If it does not have any consequences, it should not be displayed.
> 

It is an extra check to output a more meaningful error message if the 
end user is trying to delete the default subvolume.

Thus we need to search if the current subvolume is a default one.

Thanks,
Qu