Re: unshare -m and mount propagation

["Yuriy M. Kaminskiy" <yumkam@gmail.com>]

On 18.04.2016 15:22, Karel Zak wrote:
> On Mon, Apr 18, 2016 at 02:51:37PM +0300, Yuriy M. Kaminskiy wrote:
>> Karel Zak <kzak@redhat.com> writes:
>>
>>> On Fri, Mar 18, 2016 at 05:26:25AM +0300, Yuriy M. Kaminskiy wrote:
>>>> I think this issue should be at least documented. And, maybe, default
>>>> `--propagation` should be changed to `slave`.
>>>
>>> The reason why we use 'private' is that it's the kernel default for
>>> years and it's what has been expected by users for long time before we
>>> introduced --propagation and any unshare(1) default.
>>>
>>> The current --propagation default unifies things and makes unshare(1)
>>> portable to distributions where root fs is mounted as 'shared' (e.g.
>>> systemd distros) and all this in backwardly compatible way for users

Opposite. It does not change anything for older systems, but breaks 
things for new systems.

>>> who have no clue about --propagation.

And it is *especially* harmful for users that are not aware about 
--propagation. As private (new 2.27+ default) break umount propagation, 
and results in nasty surprises (up to data loss).

>>> So, I don't think we want to change any default to corrupt scripts where
>>> is no explicitly specified --propagation.
>>
>> By you already broke scripts that expected old a-la '--propagation
>> unchanged' behavior.
>
> Only if your system uses something else that kernel default 'private'
> and you depend on this non-default setting. (IMHO relatively small
> groups of users)

All systemd systems use shared. Which is not "small group of users".

> The old "--propagation unchanged" makes unshare useless on some
> mainstream distros where default is 'shared'.

No, it is not. --propagation does not do *anything* that cannot be done 
without it. On pre-2.27 util-linux,

   unshare -m sh -c 'mount --make-rprivate /; ...'

does exactly same as `unshare -m [--propagation=private] ...` in 2.27+.

Reverse is not true! With 2.27 you *must* use new 
[backward-incompatible] options to revert to sane behavior [which is 
*slave*, not *private*].

> Anyway, we will not change any default now.