Re: rsync -ax and subvolumes

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2019-10-10 17:21, Ulli Horlacher wrote:
> On Thu 2019-10-10 (20:47), Kai Krakow wrote:
> 
>>> I run into the problem that "rsync -ax" sees btrfs subvolumes as "other
>>> filesystems" and ignores them.
>>
>> I worked around it by mounting the btrfs-pool at a special directory:
>>
>> mount -o subvolid=0 /dev/disk/by-label/rootfs /mnt/btrfs-pool
> 
> This is only possible by root, but not by regular users.
> Yes, there are true multi-user systems still out there :-)
And that is what `sudo` or capabilities are for.

`sudo` will even let you get as specific as command line arguments, so 
you can specify an exact mount command that can be run (including 
ensuring that it's read-only) and an exact unmount command that can be run.

If you want to go the capabilities route, you'll need CAP_MOUNT.  This 
is a lot riskier than using `sudo` though.

That said, if you really want _everything_, you're going to need to 
either be root anyway, or have the CAP_DAC_READ_SEARCH (or 
CAP_DAC_OVERRIDE) capability, because there are some files you just 
won't be able to read otherwise (at minimum the contents of `/root` and 
any properly secured authentication logs in `/var/log`, as well as 
possibly other things under `/var` and possibly some things under `/etc`).
> 
> 
>> Actually, you could also just bind-mount into /mnt/btrfs, bind-mounts
>> won't inherit other mounts but will still see pure subvolumes.
> 
> Again, only possible by root.
>