On 11/5/18, Anna.Lyons(a)data61.csiro.au <Anna.Lyons(a)data61.csiro.au> wrote:
The boot code of seL4 is currently unverified, but we
do have ambitions for
verification in the future. As such, we'd want to keep a minimal, static
boot environment. That said, we'd be more than happy to take a look at any
patches you have in more detail.
My patches make relatively minor changes and probably wouldn't break
any future boot code verification efforts.
I'm curious as to why you need to place your
desired functionality into the
boot loader, rather than at user level in a root task or some kind of chain
loader? How much trust is placed into your boot loader?
UX/RT will be a full self-hosting general-purpose OS that runs on
workstations/servers as well as embedded systems, and the components
required at boot will be split across several deb packages (except for
minimal embedded systems without dpkg), so the boot image will be
rebuilt any time any included package is installed/upgraded. I want to
keep the image building process as simple as possible (think Linux
initramfs) and don't want to have to include a linker on every system
just to link the boot modules into the root server. I want to be able
to treat the kernel and root server as normal files that are placed at
fixed physical addresses within the filesystem image (much like QNX
does), since that is simpler than having to link the modules into the
root server and then possibly update a bootloader configuration file.
Also, as I said before, it is more convenient to manually boot an
image for recovery or testing if you just have to specify one file
rather than two (and basically guarantees that you're not accidentally
booting a mismatched kernel and root server).
The changes I made (only on x86/PC, which is the initial development
platform; once I port UX/RT to ARM and RISC-V I will port the
Multiboot2 code to those platforms):
- Perform a few bounds checks on the boot FS image (a module tag for
the whole image with a different type code is created by the
- Disable boot module relocation if an FS image is present (the image
specifies a starting physical address in its headers and is already
placed there by the bootloader, and the kernel and root server are
already placed at the proper addresses within the FS image)
- Accept a root server module that's already been ELF-loaded by the
bootloader, much like what the elfloader does on non-PC platforms
(with an extended module tag specifying the section placement)
- Create a new extra boot info structure specifying the address of the
MBI, which is required to follow the boot FS image
- Replaced the magic numbers in the Multiboot2 header assembly with
proper symbolic constants, just like for the Multiboot1 header
No parsing of the boot image FS is added to the kernel and any format
will work as long as files are contiguous and page-aligned (the UX/RT
root server is also completely independent of the boot FS format as
well, and will create an initial filesystem for the init process out
of the Multiboot modules, and a normal user program will mount the
image filesystem to get access to the full image FS contents and full
Unix FS features). The existing boot code paths are not disabled
(although I may add CMake configuration options to disable them).
When you say "implement a full disk-based
bootloader" -- this would be at
user-level, right? We definitely don't want to have to trust disk drivers to
boot seL4 + the root task.
My disk-based bootloader will be comparable to GRUB or
syslinux/isolinux (except more Unix-like than either). The kernel
won't have to trust it any more than those bootloaders.
Our current image format (cpio) is one picked for
we'd be more than happy to consider proposals for a different format. MBI
passthrough is a desirable feature.
I am not using CAmkES or CapDL at all (my root server is written in
Rust and is based on feL4 and some code forked from Robigalia), so I
am not using your cpio code. Since the existing boot code paths are
still functional, non-UX/RT OSes don't have to change their boot code
even with my patches in place. If other OSes want to change their boot
code for commonality with UX/RT they can, but there's no reason why
they have to (except maybe to make it easier to implement some form of
limited UX/RT-compatible environment based on CAmkES, for systems
where UX/RT would be overkill). UX/RT is a rather different type of OS
than any other seL4-based system (it will be the only QNX-like
L4-based OS ever written AFAIK; I've always wondered about why nobody
else has written one, since the L3/L4 microkernel family and
especially seL4 has several similarities with the QNX microkernel) and
has its own requirements for the boot process that may not be
priorities for other systems.
Here are my patches for the seL4 kernel (the first is the main one and
the second just cleans up a couple things I missed in the previous
Here are the repositories for my bootloader (it is currently in-memory only):
And here is the top-level repository for UX/RT (currently all the root
server does is print hello world; I'm currently working on importing
some of the bootinfo code from Robigalia):