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Re: grubenv on md, Btrfs, LUKS, etc

From: Daniel Kiper
Subject: Re: grubenv on md, Btrfs, LUKS, etc
Date: Fri, 21 Sep 2018 20:35:03 +0200
User-agent: Mutt/1.3.28i

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

I am not sure is anything like that implemented in GRUB2. If does
not I am happy to see and review the patches.


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