linux-kernel - Re: [dm-devel] LVM snapshot broke between 4.14 and 4.16

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Date:   Fri, 3 Aug 2018 11:20:34 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>, dm-devel@...hat.com,
        Ilya Dryomov <idryomov@...il.com>, wgh@...lan.ru,
        Zdenek Kabelac <zkabelac@...hat.com>
Subject: Re: [dm-devel] LVM snapshot broke between 4.14 and 4.16

On Fri, Aug 03, 2018 at 09:31:03AM -0400, Mike Snitzer wrote:
> 
> Debian is notorious for having a stale and/or custom lvm2.
> Generally speaking, it is recommended that lvm2 not be older than the
> kernel (but the opposite is fine).

On Fri, Aug 03, 2018 at 03:31:18PM +0200, Zdenek Kabelac wrote:
> IMHO (as the author of fixing lvm2 patch) user should not be upgrading
> kernels and keep running older lvm2 user-land tool (and there are very good
> reasons for this).

I'm going to have to strenuously disagree.

In *general* it's quite common for users to update their kernel
without updating their userspace.  For example, I as a *developer*, I
am often running bleeding kernels (e.g., at the moment I am running
something based on 4.18-rc6 on a Debian testing system; and it's not
at all uncommon for users to run a newer kernel on older
distribution).

This is the *first* I've heard that I should be continuously updating
lvm because I'm running bleeding edge kernels --- and I would claim
that this is entirely unreasonable.

I'll also note that very often users will update kernels while running
distribution userspace.  And if you are using Linode, very often
*Linode* will offer a newer kernel to better take advantage of the
Linode VM, and this is done without needing to install the Linode
kernel into the userspace.

It *used* to be the case that users running RHEL 2 or RHEL 3 could try
updating to the latest upstream kernel, and everything would break and
fall apart.  This was universally considered to be a failure, and a
Bad Thing.  So if LVM2 is not backwards compatible, and breaks in the
face of newer kernels running older distributions, that is a bug.

If there is a fundamental bug in the userspace API, and it can't be
fixed without a serious security bug, sometimes we need to have an
exception to the "you can't mandate newer userspace" rule.  But I
don't think this falls into this category; how would a user "exploit"
what people are calling a "security bug" to break root?

    	      	      	      	    		   	- Ted