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

[Nicholas Piggin <npiggin@gmail.com>]

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>

> ---
>  usr/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/usr/Kconfig b/usr/Kconfig
> index c0c48507e44e..ad0543e21760 100644
> --- a/usr/Kconfig
> +++ b/usr/Kconfig
> @@ -220,6 +220,7 @@ config INITRAMFS_COMPRESSION_LZ4
>  endchoice
>  
>  config INITRAMFS_COMPRESSION
> +	depends on INITRAMFS_SOURCE!=""
>  	string
>  	default ""      if INITRAMFS_COMPRESSION_NONE
>  	default ".gz"   if INITRAMFS_COMPRESSION_GZIP