[OE-core] Why is systemd installed to / (was: [yocto] Export bitbake variables between recipes)

[Adrian Freihofer]

On Thursday 04 December 2014 21.46:50 Mark Hatle wrote:
> On 12/4/14, 9:00 PM, ChenQi wrote:
> > On 12/04/2014 09:34 PM, Peter Kjellerstedt wrote:
> >>> -----Original Message-----
> >>> From: openembedded-core-bounces at lists.openembedded.org
> >>> [mailto:openembedded-core-bounces at lists.openembedded.org] On Behalf Of
> >>> Mark Hatle
> >>> Sent: den 3 december 2014 16:47
> >>> To: Peter Kjellerstedt; openembedded-core at lists.openembedded.org
> >>> Subject: Re: [OE-core] Why is systemd installed to / (was: [yocto]
> >>> Export bitbake variables between recipes)
> >>>
> >>> On 12/3/14, 9:36 AM, Peter Kjellerstedt wrote:
> >>>>>> Can anyone please explain why OE-core installs systemd to /
> >>>>>> rather than /usr? Because I have traced the recipe all the
> >>>>>> way back to its introduction in OE classic, and I cannot
> >>>>>> find any rationale for this odd decision. And it is extra
> >>>>>> weird given the systemd authors' agenda that everything
> >>>>>> should be in /usr (and /etc)...
> >>>>> It's only strange compared to Fedora.  We're not Fedora.. and
> >>>>> I've got systems that need to boot from a small '/' before
> >>>>> mounting '/usr'.
> >>>> Speaking of Fedora, would an official image feature, e.g.,
> >>>> "unified-fs", be acceptable for OE-Core that sets up the file
> >>>> system with /bin, /sbin and /lib* as links to their /usr
> >>>> counterparts? That would alleviate our problems with the
> >>>> differences in how systemd is installed.
> >>> The system permits developers to set the paths for the various pieces.
> >>> To get a Fedora like unified filesystem, you could do something like:
> >>>
> >>> * provide your own fs-perms.txt:
> >> I had not noticed fs-perms.txt before. And it seems easy enough
> >> to extend via the FILESYSTEMS_PERMS_TABLES variable. :)
> >>
> >>> /usr/bin	link	${base_bindir}
> >>> /usr/sbin	link	${base_sbindir}
> >>> /usr/lib	link	${base_libdir}
> >>>
> >>> Then in your local.conf:
> >>>
> >>> bindir = "${base_bindir}"
> >>> sbindir = "${base_sbindir}"
> >>> libdir = "${base_libdir}"
> >> I think you mean the opposite, i.e., that /bin should be a
> >> link to ${bindir} and ${base_bindir} should be set to ${bindir},
> >> but I get your point.
> >>
> >>> This will result in /usr/bin, sbin and lib being linked to /bin,
> >>> /sbin, /lib -- and all of the package produced for your
> >>> configuration will only reference '/'.
> >>>
> >>> This isn't that unusual of a configuration from what I've been told.
> >> Which is why I think it warrants an official distro feature (an
> >> image feature, as I originally suggested, will of course not do
> >> if it is done the way you suggest since the packages' contents
> >> are modified). That way people would not have to invent the wheel
> >> over and over again (and somehow find out that there is such a
> >> thing as fs-perms.txt which up until now had eluded me).
> >>
> >>> --Mark
> >> //Peter
> >>
> >
> > Hi Peter,
> >
> > It's easy to link /usr/bin to /bin, /usr/sbin to /sbin and make things
> > work in OE.
> > But if we want the opposite to work, we might need several additional
> > patches to OE.
> 
> Exactly.. and going back to the FHS, the FHS says that '/' is required to boot, 
> /usr is not.  So /usr should be linked to / for those items.
> 
> There is a fairly standard OE configuration some people do where they don't 
> define a /usr at all.  They just set the prefix (normally /usr) to '/'.
> 
> That SHOULD be working still, at least it used to be a well tested configuration...
> 
> > Several days ago, I tried to do /usr merge in OE (like what Fedora and
> > Arch does). It turned out that we need to patch several packages to make
> > things work.
> >
> > (I haven't sent out the patches because it's just some initial
> > investigation and I'm not sure if people want this feature or not.)
> >
> > If you want this feature in OE, please open a bug/enhancement in
> > bugzilla https://bugzilla.yoctoproject.org/ to see if the community
> > wants it or not.
> >
> > Regards,
> > Chen Qi
> >
> >
> 
> 
Hi Mark

Lennard Poettering and others have good arguments to have no /bin, /lib and /sbin folder at all. There is an interesting blog here: http://0pointer.net/blog/revisiting-how-we-put-together-linux-systems.html[1]. In short: The goal is to have one mount point (/usr) where all the read only binaries are in. Having /usr on a read only btrfs partition would allow to use the send receive feature (binary diff on filesystem level) of btrfs to deploy software updates to many devices. This approach could provide a generic, robust and optimized SW update procedure for embedded systems as well.

Regards,
Adrian



--------------------
Adrian Freihofer
Gschwaderweg 29
8610 Uster
043 497 84 01

--------
[1] http://0pointer.net/blog/revisiting-how-we-put-together-linux-systems.html
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