Re: Removing Mandatory Locks

[Amir Goldstein]

On Thu, Aug 19, 2021 at 11:32 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Aug 19, 2021 at 1:18 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > Now that I think about it a little more, I actually did get one
> > complaint a few years ago:
> >
> > Someone had upgraded from an earlier distro that supported the -o mand
> > mount option to a later one that had disabled it, and they had an (old)
> > fstab entry that specified it.
>
> Hmm. We might be able to turn the "return -EINVAL" into just a warning.
>
> Yes, yes, currently if you turn off CONFIG_MANDATORY_FILE_LOCKING, we
> already do that
>
>         VFS: "mand" mount option not supported
>
> warning print, but then we fail the mount.
>
> If CONFIG_MANDATORY_FILE_LOCKING goes away entirely, it might make
> sense to turn that warning into something bigger, but then let the
> mount continue - since now that "mand" flag would be purely a legacy
> thing.
>
> And yes, if we do that, we'd want the warning to be a big ugly thing,
> just to make people very aware of it happening. Right now it's a
> one-liner that is easy to miss, and the "oh, the mount failed" is the
> thing that hopefully informs people about the fact that they need to
> enable CONFIG_MANDATORY_FILE_LOCKING.
>
> The logic being that if you can no longer enable mandatory locking in
> the kernel, the current hard failure seems overly aggressive (and
> might cause boot failures and inability to fix/report things when it
> possibly keeps you from using the system at all).
>

Allow me to play the devil's advocate here - if fstab has '-o mand' we have
no way of knowing if any application is relying on '-o mand' and adding
more !!!!! to the warning is mostly good for clearing our conscious ;-)

Not saying we cannot resort to that and not saying there is an easy
solution, but there is one more solution to consider - force rdonly mount.
Yes, it could break some systems and possibly fail boot, but then again
an ext4 fs can already become rdonly due to errors, so it wouldn't
be the first time that sysadmins/users run into this behavior.

Thanks,
Amir.