On 18.04.2016 20:48, Karel Zak wrote:
On Mon, Apr 18, 2016 at 04:05:29PM +0300, Yuriy M. Kaminskiy wrote:
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@xxxxxxxxxx> 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).
Well, you see only umount propagation...
Because *breaking* it causes real problems?
The problem is that the original implementation (try emulate by
"--propagation unchanged") makes "unshare --mount" useless at all on
systems with shared root fs.
The very old (since year 2009) and very common use-case is:
# unshare --mount
mount --make-rslave / # or --make-rprivate or whatever
# mount /dev/foo /mnt
And? I've posted how this could be solved *without* changing anything in
unshare.
and user expects that /mnt will be visible *only* in the session
(namespace). This is the way how many users use unshare for years.
Unfortunately, after systemd installation it does not work anymore
and /mnt is visible everywhere. For users it's regression and it has
been reported many many times.
You can blame systemd, but the problem is that unshare(1) was not
robust enough. So we have forced unshare to use "private" by default
to keep the *original behavior* independently on root fs propagation
flag.
You can have same effect with *slave* as default, with exception that
host->guest mount/unmount propagation still works.
When kernel have to get rid of shared propagation (in userns), it
downgrades shared mounts to *slave*, not to *private*.
When systemd itself downgrades shared propagation (for running service
in new mount namespace; Protect*/Private*/etc), it also downgrades
*shared* propagation to *slave*, not to *private*.
P.S. For sure, *any* of propagation variants have some or other
drawbacks and corner cases; however, without special care about
removable media (and alike), *private* has higher chance to beat you in
a nasty way.
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