[dm-devel] [PATCH v2 17/20] multipath.rules: find_multipaths "smart" logic

[Martin Wilck]

On Tue, 2018-03-27 at 18:07 -0500, Benjamin Marzinski wrote:
> On Tue, Mar 27, 2018 at 11:34:00PM +0200, Martin Wilck wrote:
> > 
> > The following happens: multipath -u temporarily claims the device.
> > When
> > multipathd starts, it fails to set up the map, sets the "failed"
> > marker, and retriggers udev. The second time, multipath -u unclaims
> > the
> > device because it recognizes it as failed.
> 
> But if that device is already in use because multipath didn't claim
> it
> in the initramfs, and you suddenly mark it as ENV{SYSTEMD_READY}="0",
> this can cause systemd to automatically unmount any filesystem on it.
> This isn't just a problem with Red Hat's setup.  If it's not a
> configured device type, there will only be a short timeout, but
> that's
> still enough to mess with devices that are already in use.  I'm
> pretty
> sure that the multipath temporary claiming is only safe the very
> first
> time a device appears. Otherwise, it's possible that something else
> will
> claim it first, and then multipath will claim it and mess with that
> other user.
> 

Arrgh. Thanks for pointing that out. It's even worse because even if we
could figure out whether we're being called for the first or second
time, it wouldn't be sufficient. multipath.rules may not even be
present in the initrd, so the device may be present in the system, and
used, without multipath.rules having ever been called.

The only way I see to avoid this is to try calling open(O_EXCL) on the
path device in "multipath -u". So far we've avoided that because it's
not completely race-free. But we're not talking about a race here, but
a situation where some entity grabbed a device before pivot-root.
So we could attempt open(O_EXCL) only in the "is maybe a valid path"
case, and only return "maybe" if that open succeeds. Otherwise we'd
return "no", as we already checked that the device isn't currently part
of a multipath map.

Agreed?

Martin

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)