Re: [PATCH 2/2] kernel-initramfs.bbclass: Handle built-in /dev/console dependence

[Nathan Rossi <nathan@nathanrossi.com>]

On Fri, 9 Nov 2018 at 00:20, Andrea Adami <andrea.adami@gmail.com> wrote:
>
> On Thu, Nov 8, 2018 at 2:05 PM Nathan Rossi <nathan@nathanrossi.com> wrote:
> >
> > On Wed, 7 Nov 2018 at 23:32, Andrea Adami <andrea.adami@gmail.com> wrote:
> > >
> > > On Wed, Nov 7, 2018 at 1:16 PM Nathan Rossi <nathan@nathanrossi.com> wrote:
> > > >
> > > > The kernel has two differing code paths when handling a built-in cpio
> > > > versus a cpio passed to the kernel from a boot loader. When the kernel
> > > > is booted with the built-in cpio it expects the /dev/console node to
> > > > exist and will panic if it does not exist.
> > > >
> > > >   Kernel panic - not syncing: /dev/console is missing or not a character device!
> > > >
> > > > When the cpio is passed to the kernel via a bootloader, this behaviour
> > > > is not observed.
> > > >
> > >
> > > Well, of course once booted you have devtmpfs or you have provided
> > > your devices-list.
> > > We fight since long with this early issues and I think this approach
> > > is wrong: there are more things needed, not only /dev/consoleI'd say
> > > the responsability to check for devtmpfs or recreate the devices is up
> > > to init manager of the image.
> >
> > Ok, so I do not think my commit message was clear enough in this regard.
> >
> > Firstly the kernel requires the /dev/console node before the init
> > process is started, but after it has unpacked the built-in ramfs and
> > after it has mounted the initrd (rd or ramfs). You can see the exact
> > call that attempts to open /dev/console here:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/init/main.c?h=v4.20-rc1#n1154
> >
> > The exact problem this change is solving is the fact that by embedding
> > a user source cpio into the kernel, the default kernel initramfs is
> > replaced. The default kernel ramfs contains /dev, /dev/console and
> > /root.
> >
> > Also for reference when the kernel is built without
> > initramfs/BLK_DEV_INITRD support it will create this default rootfs
> > with an alternate initcall:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/init/noinitramfs.c?h=v4.20-rc1
> >
> > So essentially the kernel is always expecting the rootfs to contain
> > this /dev/console node.
> >
> > >
> > > We don't use busybox, just a single binary running as init: kexecboot
> > > will exactly do that, check and recreate devices if missing:
> > > http://cgit.openembedded.org/meta-openembedded/tree/meta-initramfs/recipes-bsp/kexecboot/kexecboot_git.bb
> >
> > This only solves the user space requirements for device nodes. Since
> > kexecboot or busybox will not be executed before the kernel tries to
> > open the /dev/console file.
> >
> Rightm and nothing bad will happen, just the
> pr_err("Warning: unable to open an initial console.\n");
>
> then init is run.
>
>
> > >
> > > Please don't inject anything in the cpio, let the image-engineer decide.
> > >
> > > This poor image is regularly attacked by new changes filling the cpio
> > > with undesidered stuff:
> > > http://cgit.openembedded.org/meta-openembedded/tree/meta-initramfs/recipes-bsp/images/initramfs-kexecboot-image.bb
> >
> > How does this image handle this /dev/console dependency? Is there
> > something I am missing with how it is built into the kernel, or with
> > how the kernel you are using is configured?
> >
> > It appears the initramfs-framework images of oe-core solve this by
> > creating the node in the image itself.
> > http://git.openembedded.org/openembedded-core/tree/meta/recipes-core/initrdscripts/initramfs-framework_1.0.bb#n51
> > http://git.openembedded.org/openembedded-core/tree/meta/recipes-core/initrdscripts/initramfs-live-boot_1.0.bb#n15
> > http://git.openembedded.org/openembedded-core/tree/meta/recipes-core/initrdscripts/initramfs-live-boot-tiny_1.0.bb
> >
> > Looking at the log of those files, it appears this might have been a
> > kernel version and/or architecture specific thing back in 2013...
> > http://git.openembedded.org/openembedded-core/commit/?id=0352841cd92f6316bcac092e2fff9d28c352b36b
> >
> > >
>
> And I had to revert it...
> http://cgit.openembedded.org/meta-openembedded/commit/meta-initramfs/recipes-kernel?id=7183bacd5c866d433467a750e485e6c01af4aafc

Oh now I feel silly :|. I confused myself thinking is was testing with
a mainline kernel, and all this time it was still using linux-yocto.
You are right this only affects linux-yocto kernels.

If this appending was only done for linux-yocto kernels you would have
no issue with it correct?

Thanks,
Nathan

>
> > > Maybe we are on the corner-case, size-contraints limit us to be within
> > > 1MiB, neverthless I think it is a good exercise to keep things small
> > > and clean ;)
> >
> > I would also like to avoid this change if there is an alternative that
> > works better.
> >
>
> Well, at the beginning we used IMAGE_DEVICE_TABLES to populate it.
> Then whit the introduction of devtmpfs (3.2 iirc) we switched using
> it, to save the few kb of the /dev:the trick here is that devtmpfs
> automount does not work in the initramfs so you have to mount devtmpfs
> by hand in your init, like here
> https://github.com/kexecboot/kexecboot/blob/master/src/kexecboot.c#L734
>
> Cheers
> Andrea
>
> > Thanks,
> > Nathan
> >
> > >
> > > As for the previous patch touching initramfs, I hold my comments for
> > > the moment: let me first test it.
> > > Cheers
> > >
> > > Andrea
> > >
> > > > To resolve this issue, ensure any cpio that is built into the kernel is
> > > > prepared with a /dev/console node. This is done by appending to the
> > > > copied cpio of the INITRAMFS_IMAGE. In order to create the node to
> > > > append, the task must be executed with root permission (to run mknod).
> > > > As such this change creates and intermediate _prepare task between the
> > > > existing _bundle and _copy tasks.
> > > >
> > > > Note: The default/minimal initramfs that the kernel sources
> > > > gen_initramfs_list.sh generates contains this device node.
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/usr/gen_initramfs_list.sh?h=master#n55
> > > >
> > > > Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
> > > > ---
> > > >  meta/classes/kernel-initramfs.bbclass | 15 ++++++++++++++-
> > > >  1 file changed, 14 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/meta/classes/kernel-initramfs.bbclass b/meta/classes/kernel-initramfs.bbclass
> > > > index b23fb51495..135f4c8949 100644
> > > > --- a/meta/classes/kernel-initramfs.bbclass
> > > > +++ b/meta/classes/kernel-initramfs.bbclass
> > > > @@ -16,7 +16,8 @@ python __anonymous () {
> > > >      if image and bundle:
> > > >          # add all the tasks
> > > >          bb.build.addtask('do_initramfs_copy', 'do_initramfs_bundle', 'do_install', d)
> > > > -        bb.build.addtask('do_initramfs_bundle', 'do_deploy', 'do_initramfs_copy', d)
> > > > +        bb.build.addtask('do_initramfs_prepare', 'do_initramfs_bundle', 'do_initramfs_copy', d)
> > > > +        bb.build.addtask('do_initramfs_bundle', 'do_deploy', 'do_initramfs_prepare', d)
> > > >
> > > >          # make the do_initramfs_copy task depend on the image do_image_complete task
> > > >          d.appendVarFlag('do_initramfs_copy', 'depends', ' ${INITRAMFS_IMAGE}:do_image_complete')
> > > > @@ -67,6 +68,18 @@ do_initramfs_copy () {
> > > >      echo "Finished copy of initramfs into ./usr"
> > > >  }
> > > >
> > > > +do_initramfs_prepare[dirs] = "${B}"
> > > > +fakeroot do_initramfs_prepare () {
> > > > +    echo "Preparing initramfs by creating required /dev/ nodes"
> > > > +    # append a /dev/console node, this node must be created in the rootfs
> > > > +    # for built-in initramfs cpios otherwise it will error with:
> > > > +    #   Kernel panic - not syncing: /dev/console is missing or not a character device!
> > > > +    rm -rf ${B}/usr/append
> > > > +    mkdir -p ${B}/usr/append/dev
> > > > +    mknod -m 600 ${B}/usr/append/dev/console c 5 1
> > > > +    (cd  ${B}/usr/append && echo "./dev/console" | cpio -oA -H newc -F ${B}/usr/${INITRAMFS_IMAGE_NAME}.cpio)
> > > > +}
> > > > +
> > > >  do_initramfs_bundle[dirs] = "${B}"
> > > >  do_initramfs_bundle () {
> > > >      echo "Creating a kernel image with a bundled initramfs..."
> > > > ---
> > > > 2.19.1
> > > > --
> > > > _______________________________________________
> > > > Openembedded-core mailing list
> > > > Openembedded-core@lists.openembedded.org
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