Re: grubenv on md, Btrfs, LUKS, etc

[Daniel Kiper]

On Wed, Sep 26, 2018 at 04:45:51PM -0600, Chris Murphy wrote:
> On Fri, Sep 21, 2018 at 12:35 PM, Daniel Kiper <address@hidden> wrote:
> > On Tue, Sep 18, 2018 at 01:40:06PM -0600, Chris Murphy wrote:
> >> On Mon, Sep 17, 2018 at 10:59 PM, Chris Murphy <address@hidden> wrote:
> >> > Hi,
> >> >
> >> > GRUB code can certainly read files that are on Btrfs, md devices,
> >> > LUKS, LVM, and so on. But GRUB code can also write to the physical
> >> > block for grubenv - but are there safe guards that prevent it from
> >> > doing so if grubenv is on something like Btrfs, mdadm raid5, LUKS?
> >> >
> >> > And also what about XFS? This used to be safe, but now with reflink
> >> > support, grubenv could be reflink copied, meaning any overwrite is
> >> > disallowed and must be COW'd. How is that handled?
> >> >
> >> > I'm pretty sure on Btrfs GRUB knows is can't write to grubenv, I'm
> >> > just curious about the other cases.
> >>
> >> OK so it allows me to create a grubenv on Btrfs without any complaint.
> >> Will the bootloader actually try to write to this if grub.cfg contains
> >> save_env?
> >>
> >> $ sudo grub2-editenv --verbose grubenv create
> >> [sudo] password for chris:
> >> address@hidden ~]$ ll
> >> -rw-r--r--. 1 root  root     1024 Sep 18 13:37 grubenv
> >> address@hidden ~]$ stat -f grubenv
> >>   File: "grubenv"
> >>     ID: ac9ba8ecdce5b017 Namelen: 255     Type: btrfs
> >> Block size: 4096       Fundamental block size: 4096
> >> Blocks: Total: 46661632   Free: 37479747   Available: 37422535
> >> Inodes: Total: 0          Free: 0
> >> address@hidden ~]$
> >
> > There two things here. grub2-editenv should create grubenv properly
> > and double check that space on disk is reserved. If it is not then
> > it should complain. GRUB2 during boot should check was grubenv file
> > properly created. If it was not it should not update grubenv and
> > complain.
> >
> > I am not sure is anything like that implemented in GRUB2. If does
> > not I am happy to see and review the patches.
>
> When I create a grubenv on Btrfs is does succeed and it's an inline
> extent, so no mattter what it's checksummed. There is a message on the
> next boot:
>
> error: ../../grub-core/commands/loadenv.c:215:sparse file not allowed.
>
> And the grubenv is not overwritten even though the grub.cfg is using
> save_env and when this same grub.cfg is used on ext4 it does overwrite
> grubenv. So... It appears loadenv.c must have some inhibitor for
> writing to Btrfs, which is a good thing.

Great! That is in line with what I said earlier.

> I'm uncertain whether GRUB avoids writing to any other case (LUKS, md
> RAID, LVM). In particular, XFS, because XFS now supports reflinks, so
> strictly speaking even if overwriting 2 sectors does not cause
> corruption today (no inline extent support yet), it probably should
> refuse to write to XFS as well.

Yep!

> Anyway, I've got a couple of different opinions from XFS devel@ and
> ext4 devel@ about this. My understanding is each file system (ext4,
> XFS, Btrfs at least) have reserve areas that can reliably be written
> to by the bootloader (pre-boot), and it seems like those need to be
> used instead of depending on grubenv.
>
> https://www.spinics.net/lists/linux-ext4/msg62364.html
>
> https://www.spinics.net/lists/linux-xfs/msg21902.html

If something like that exits I am happy to use it. However, I would not
change user interface in any way. Everything should happen auto-magically
from user POV.

Daniel