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Re: [PATCH 0/2] Support plain encryption mode.

From: Maxim Fomin
Subject: Re: [PATCH 0/2] Support plain encryption mode.
Date: Tue, 01 Feb 2022 15:48:01 +0000

------- Original Message -------

On Monday, January 31st, 2022 at 23:40, Glenn Washburn 
<> wrote:

> On Sun, 30 Jan 2022 19:40:37 +0000
> Maxim Fomin wrote:
> > This patch adds support for plain encryption mode (plain dm-crypt) via new
> >
> > module/command named 'plainmount'. This is an extension of previous patch
> >
> > (member of crypto enhancement patch series) sent to grub-devel by John Lane.
> >
> > The plain mode patch was fully reworked, the most important changes are:
> >
> > 1.  The code is rewritten as a separate 'plainmount' module and command. 
> > This
> >
> >     change addresses several issues raised in previous patch discussion: 
> > complex
> >
> >     implementation and expanding too much cryptomount parameter list. 
> > Plainmount
> >
> >     module still depends on several functions in cryptodisk module just 
> > like LUKS,
> >
> >     LUKS2 and geli modules. I tried to minimize link to cryptodisk, but 
> > some link
> >
> >     is unavoidable.
> >
> > 2.  Plainmount now uses GPT partition unique UUID to point to exact 
> > partition.
> >
> >     Plain mode does not have special header (as LUKS/LUKS2 does), so it 
> > does not
> >
> >     have any UUID. Absence of UUID was key issue in discussions of original 
> > plain
> >
> >     mode patch - without UUID any link like 'hdX,gptY' in grub.cfg can be 
> > broken
> >
> >     in multiboot setup because hdX can map to different disks each boot.
> >
> So plain mount can only be used for volumes located on GPT partitions,
> leaving out MBR or any other partitioning format, full-disk encryption,
> and loopback file encryption. I think this is an undesirable and
> unnecessary limitation.
> Instead, I suggest assuming the user knows what they are doing in the
> GRUB shell/script and letting them do it. This should not be allowed
> when automatically generating the grub.cfg using GRUB user-space tools.
> GRUB should fail to automagically install to a device hosted on a plain
> encrypted container (unless perhaps an override is given). I suspect
> that this is the default, but worth mentioning.
> Glenn

Plainmount can work with '(hdX,gptY)' syntax in config or shell (actually, this
is the base syntax) and thus it is not limited to GPT paritions, what is limited
is the ability to use UUID - currently only on GPT. If partition scheme does not
have UUID then UUID as a convenience feature cannot be supported - inconvenient,
but technically fair. I will take a look at MBR UUID and see whether they can be
supported. Possible situations (under current implementaion) are follows:

a) GPT disk, multi-disk environment, disks map unpredictably: can name 
   by GPT UUID in config file/shell, no problem, ability to name by UUID has 
b) GPT disk, single disk environment: no problem, UUID feature has less value
b) not GPT, single disk environment: no problem, config like 'plainmount 
   always works
d) not GPT, multi-disk environment, disks map unpredictably: worst case, link
   '(hd0,gpt2)' can randomly fail, but user can type manually in shell
e) cannot mount plain mode device in grub shell <- this is impossible, 
   can always be called in shell.

Best regards,
Maxim Fomin

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