Re: [PATCH 2/2] ieee1275: set real-base in the PowerPC IEEE1275 Note to 32MB

[Michael Chang]

On Tue, Nov 16, 2021 at 02:42:05PM +1100, Daniel Axtens wrote:
> Background
> ----------
> 
> Figuring out how to lay out memory in powerpc OpenFirmware is a bit
> of a complicate dance.
> 
> Firstly, firmware needs to reserve a little bit of space for things
> like interrupt vectors that live at a fixed address. This is usually
> 0x4000 bytes. PFW calls this "stage 1" (of the firmware, not the
> bootloader!)
> 
> As discussed in the previous patch, the ELF image is loaded at
> 'load-base' before it's unpacked. load-base is, by default, set at the
> end of stage 1. From there, the ELF loader unpacks the binary to
> wherever the program headers say and the program is executed.
> 
> Next in memory, we need space for the rest of firmware. (PFW calls
> this "stage 2" of firmware.) The starting address begins at
> 'real-base'.* We need to not run past real-base when loading our ELF
> binary.
> 
> load-base and real-base have defaults from firmware, and may be
> configurable in various ways.
> 
> On PFW:
> 
>  - real-base defaults to 12MB (0xc00000)
> 
>  - real-base is configurable with the IEEE1275 ELF note.
> 
>  - a proprietary OS bootloader configures real-base to
>    0x2000000 (32MB).
> 
> On SLOF:
> 
>  - real-base is set dynamically towards the end of accessible memory.
> 
>  - real-base is not configurable.
> 
> On 32-bit, I'm not sure quite what the story is. I don't have any
> 32-bit Apple hardware to test with. However, with this change, grub
> still loads and runs, including when I hack up the tests to install
> the note by default.
> 
> * I'm assuming here that the firmware doesn't do virtual mode. This is
> true on Power for both PFW and SLOF; Apple OF implementations may do
> things differently.
> 
> Moving real-base
> ----------------
> 
> To account for growing grub binary sizes, we want to try to expand the
> amount of memory that we have to work with. We have the following
> constraints and inputs:
> 
>  - A good practical limit for the size of the grub binary is 8MB, as
>    that's about the size of the PReP partition on a number of distro
>    setups. (See previous patch.)
> 
>  - We also want things to work if people forget to include the
>    note. Even if distros build it in to signed images, people
>    running grub-install will probably forget to set it and we don't
>    want to break their systems. (Helpfully, if you're running
>    grub-install manually there's a decent chance you're not building
>    in a bunch of modules so things are likely to be smaller anyway.)
> 
>  - We want to avoid doing anything too novel with enterprise software.
> 
> So when the note is included, set real-base to 32MB.
> 
> Results
> -------
> 
> This means that:
> 
>  - The ELF file will be loaded at 0x4000 and have just short of 8MB to
>    fit into. (This is entirely the work of the previous patch - the
>    8MB limitation is because we now load at 8MB instead of 2MB.)
> 
>  - Nothing changes on SLOF, because SLOF ignores the note. We have
>    oodles of space in SLOF.
> 
>  - On PFW when the note is not installed, we have about 4MB of space
>    for ELF loader to put the program into (the gap between 8MB and the
>    default real-base of 12MB). This is well more than any current grub
>    image uses.
> 
>  - On PFW when the note _is_ installed, we will have about 24MB of
>    space to put the program bits into. This is well more than we're
>    likely to use unless people start putting very large arrays in BSS,
>    but it means we're aligned with a proprietary bootloader and
>    therefore we can be assured that some testing has been done.

May I ask why 'proprietary' is used here to describe the usecase ? Does
it possess any meaningful words like supportability to the firmware
vendor that OS vendor have to think over before enabling the note for a
'general' feature like powerpc secure boot ?

Thanks,
Michael

> 
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> ---
>  util/grub-mkimagexx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/util/grub-mkimagexx.c b/util/grub-mkimagexx.c
> index d78fa3e53308..2ac40b82a22d 100644
> --- a/util/grub-mkimagexx.c
> +++ b/util/grub-mkimagexx.c
> @@ -467,7 +467,7 @@ SUFFIX (grub_mkimage_generate_elf) (const struct 
> grub_install_image_target_desc
>        note_ptr->header.n_type = grub_host_to_target32 
> (GRUB_IEEE1275_NOTE_TYPE);
>        strcpy (note_ptr->name, GRUB_IEEE1275_NOTE_NAME);
>        note_ptr->descriptor.real_mode = grub_host_to_target32 (0xffffffff);
> -      note_ptr->descriptor.real_base = grub_host_to_target32 (0x00c00000);
> +      note_ptr->descriptor.real_base = grub_host_to_target32 (0x02000000);
>        note_ptr->descriptor.real_size = grub_host_to_target32 (0xffffffff);
>        note_ptr->descriptor.virt_base = grub_host_to_target32 (0xffffffff);
>        note_ptr->descriptor.virt_size = grub_host_to_target32 (0xffffffff);
> -- 
> 2.32.0
> 
> 
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