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Re: Remounting tmpfs

From: Ludovic Courtès
Subject: Re: Remounting tmpfs
Date: Wed, 17 Apr 2019 22:31:22 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux)


address@hidden skribis:

> On 4/9/19 11:58 PM, Ludovic Courtès wrote:
>> Hello,
>> address@hidden skribis:
>>> On a system I'm porting to guix I have 2GB tmpfs with subdirectories
>>> like /tmpfs/etc that I remount to /etc with an overlay filesystem.
>>> The current way I do this in systemd is making a service dependency
>>> between the /tmpfs and /etc mounts that mkdirs /tmpfs/etc and
>>> /tmpfs/etc_work, but AFAICT filesystem definitions in guix can only have
>>> filesystem dependencies.
>>> Are there any other ways I can do this without copying/pasting/modifying
>>> gobs of core guix code into my system definition?  Like somehow
>>> appending (mkdir /tmpfs/etc) onto the tmpfs filesystem service start
>>> procedure or something.
>> In Guix /etc is mostly populated by “activation programs”, which are
>> generated from your config.  So I’m not sure what you describe would
>> make much sense.
> So if /etc can be read-only and boot I'm probably fine... my experience
> with other distros was that some other processes needed to write to it. 
> Ex: modifying resolv.conf.

/etc is writable because of things like ‘resolv.conf’.

/etc consists mostly of immutable files derived directly from your OS
config (/etc/passwd, /etc/hosts, /etc/polkit-1, /etc/pam.d, etc.), along
with files that contains bits of state (/etc/shadow, /etc/resolv.conf.)

The former are directly managed by Guix, while the latter are either
left as is or touched with care by Guix (/etc/shadow in particular.)

>> Now, you could try to add a file system declaration that mounts /etc,
>> with (needed-for-boot? #t).
> My goal is to have a read-only / mount with the ability for programs to
> make temporary modifications for operational purposes when necessary, in
> limited scopes (like /etc).  Can you elaborate on what you're suggesting
> here?  Mounting something other than the overlayfs on /etc would hide
> the system config files.  I might be able to use another mount to create
> a pseudo- /tmpfs/etc_work subdirectory but it sounds kind of wormy and
> overlayfs requires the upper dir and workdir to be the same filesystem
> which I think precludes doing any mounting for those subdirectories.

The overlay makes a lot of sense.  This is what ‘guix system vm’ does:
see the #:volatile-root? parameter of ‘raw-initrd’.

Perhaps you could simply set #:volatile-root? #t in your initrd to
obtain what you want?


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