[LEDE-DEV] Supporting crashdumps w/ kexec

[Luiz Angelo Daros de Luca]

1st: Is where does /boot get unmounted, and is there an easy way to
keep it around a bit longer, at least until the init.d scripts have
finished running?
A: /boot (aka sda1) is never mounted on x86/x86_64. It could but it is
simply unnecessary. grub is the only one that reads it (for loading
grub confs and the kernel)

2nd: Or what if I want to configure an extra menuentry in /boot/grub/grub.cfg?
A: If you need to add extra kernel parameters, you simply need to
mount sda1 somewhere (like /boot) and do as you would in a normal
linux. You can edit grub conf there.

3rd: how does a regular package force these kernel options?
A: you can depend on a kernel config just like you depend on any other
kernel config. CONFIG_KERNEL_XXX becomes CONFIG_XXX in kernel config.
See procd makefile for reference. However, note that any selecting the
package is enough to enable the config for any kernel compiled, even
those that do not install your package.

4th:  Is there an easy way to have the image include a 3rd partition
for collecting crash dumps?
A: your kexec package could configure the mounting. I don't know where
is the best place for mounting, if /etc/fstab, /etc/config/fstab or
mounting with a custom /etc/init.d/kexec service

5th: what would be involved in mashing /dev/sda1 into the root unionfs?
A: kernel could live inside rootfs. There is even an existing menu
option (TARGET_ROOTFS_INCLUDE_KERNEL) for it (but not for x86). I
guess grub2 can read files both from squashfs and ext4. Grub just need
to look for the kernel there (you need to change the target partition
from sda1 to sda2) and you must put the kernel inside sda2 and not
sda1 during image build. The only drawback is that even if you change
the kernel (writing to overlayfs), grub would still read from pristine
squashfs. I don't know if this is really a problem because in
LEDE/OpenWRT, you normally change kernel reflashing your system and
not updating kernel.

I did some code (CC era) in order to put the kernel inside rootfs. It
was used for a A/B upgrade strategy. grub conf was changed to boot
either sda2 or sda3 rootfs. sysupgade could replace only the other
rootfs partition or the whole system (boot+rootfs). It still misses
some kind of hardware watchdog (I used a human watchdog). If there is
interest for this particular A/B image solution (only for x86/x86_64
and no watchdog), I can spend some time to bring it to LEDE trunk,

Regards,

---
     Luiz Angelo Daros de Luca, Me.
            luizluca at gmail.com