Re: [PATCH] initramfs: Fix disabling of initramfs (and its compression)

[Nicholas Piggin <npiggin@gmail.com>]

On Mon, 28 Aug 2017 13:03:31 -0700
Florian Fainelli <f.fainelli@gmail.com> wrote:

> On 05/21/2017 07:46 PM, Nicholas Piggin wrote:
> > On Sat, 20 May 2017 20:33:35 -0700
> > Florian Fainelli <f.fainelli@gmail.com> wrote:
> >   
> >> Commit db2aa7fd15e8 ("initramfs: allow again choice of the embedded
> >> initram compression algorithm") introduced the possibility to select the
> >> initramfs compression algorithm from Kconfig and while this is a nice
> >> feature it broke the use case described below.
> >>
> >> Here is what my build system does:
> >>
> >> - kernel is initially configured not to have an initramfs included
> >> - build the user space root file system
> >> - re-configure the kernel to have an initramfs included
> >> (CONFIG_INITRAMFS_SOURCE="/path/to/romfs") and set relevant
> >> CONFIG_INITRAMFS options, in my case, no compression option
> >> (CONFIG_INITRAMFS_COMPRESSION_NONE)
> >> - kernel is re-built with these options -> kernel+initramfs image is
> >>   copied
> >> - kernel is re-built again without these options -> kernel image is
> >>   copied
> >>
> >> Building a kernel without an initramfs means setting this option:
> >>
> >> CONFIG_INITRAMFS_SOURCE="" (and this one only)
> >>
> >> whereas building a kernel with an initramfs means setting these options:
> >>
> >> CONFIG_INITRAMFS_SOURCE="/home/fainelli/work/uclinux-rootfs/romfs
> >> /home/fainelli/work/uclinux-rootfs/misc/initramfs.dev"
> >> CONFIG_INITRAMFS_ROOT_UID=1000
> >> CONFIG_INITRAMFS_ROOT_GID=1000
> >> CONFIG_INITRAMFS_COMPRESSION_NONE=y
> >> CONFIG_INITRAMFS_COMPRESSION=""
> >>
> >> Commit db2aa7fd15e857891cefbada8348c8d938c7a2bc ("initramfs: allow again
> >> choice of the embedded initram compression algorithm") is problematic
> >> because CONFIG_INITRAMFS_COMPRESSION which is used to determine the
> >> initramfs_data.cpio extension/compression is a string, and due to how
> >> Kconfig works it will evaluate in order, how to assign it.
> >>
> >> Setting CONFIG_INITRAMFS_COMPRESSION_NONE with
> >> CONFIG_INITRAMFS_SOURCE="" cannot possibly work (because of the depends
> >> on INITRAMFS_SOURCE!="" imposed on CONFIG_INITRAMFS_COMPRESSION ) yet we
> >> still get CONFIG_INITRAMFS_COMPRESSION assigned to ".gz" because
> >> CONFIG_RD_GZIP=y is set in my kernel, even when there is no initramfs
> >> being built.
> >>
> >> So we basically end-up generating two initramfs_data.cpio* files, one
> >> without extension, and one with .gz. This causes usr/Makefile to track
> >> usr/initramfs_data.cpio.gz, and not usr/initramfs_data.cpio anymore,
> >> that is also largely problematic after
> >> 9e3596b0c6539e28546ff7c72a06576627068353 ("kbuild: initramfs cleanup,
> >> set target from Kconfig") because we used to track all possible
> >> initramfs_data files in the $(targets) variable before that commit.
> >>
> >> The end result is that the kernel with an initramfs clearly does not
> >> contain what we expect it to, it has a stale initramfs_data.cpio file
> >> built into it, and we keep re-generating an initramfs_data.cpio.gz file
> >> which is not the one that we want to include in the kernel image proper.
> >>
> >> The fix consists in hiding CONFIG_INITRAMFS_COMPRESSION when
> >> CONFIG_INITRAMFS_SOURCE="". This puts us back in a state to the pre-4.10
> >> behavior where we can properly disable and re-enable initramfs within
> >> the same kernel .config file, and be in control of what
> >> CONFIG_INITRAMFS_COMPRESSION is set to.
> >>
> >> Fixes: db2aa7fd15e8 ("initramfs: allow again choice of the embedded initram compression algorithm")
> >> Fixes: 9e3596b0c653 ("kbuild: initramfs cleanup, set target from Kconfig")
> >> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>  
> > 
> > This is very thorough, thank you for tracking it down and fixing it.
> > 
> > I can't say I've worked through the problem in the code, but your
> > changelog and the proposed fix seem reasonable to me. So for what
> > it's worth:
> > 
> > Acked-by: Nicholas Piggin <npiggin@gmail.com>  
> 
> Well, I am looking at this again, and it's still broken, the same test
> case is involved, except this time, I am switching beween no-initramfs
> and initramfs with gzip compression (the key thing is using a
> compression of some sort). The end result is the following:
> 
> - change stuff in the rootfs
> - build the kernel with initramfs, CONFIG_INITRAMFS_COMPRESSION_GZIP=y,
> usr/initramfs_data.cpio.gz gets generated correctly the first time
> - build the kernel without initramfs,
> CONFIG_INITRAMFS_COMPRESSION_NONE=y, usr/initramfs_data.cpio gets generated
> 
> Now back to step 1 add some files, and we can see that
> usr/initramfs_data.cpio.gz is now stale from before...
> 
> So while my earlier fix switched the initramfs w/o compression to no
> initramfs rebuild, now this does not work because we still have two
> files left to be tracked:
> 
> usr/initramfs_data.cpio (no compression, or when initramfs is disabled)
> and usr/initramfs_data.cpio.$(suffix_y)
> 
> How would you go about solving this?

I don't see the problem. When I change back to .gz, modify the source
directory, and rebuild, it rebuilds a new initramfs_data.cpio.gz here.

Thanks,
Nick