Re: [PATCH 2/5] x86: Fix early output messages in case of EFI

[Jan Beulich <jbeulich@xxxxxxxx>]

(re-adding xen-devel@)

On 08.08.2024 10:33, Frediano Ziglio wrote:
> On Thu, Aug 8, 2024 at 8:49 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 07.08.2024 15:48, Alejandro Vallejo wrote:
>>> If code is loaded by EFI the loader will relocate the image
>>> under 4GB.
>>
>> This is the MB2 EFI path you're talking about? Since there are two paths,
>> I think this needs clearly separating in all descriptions.
>>
>> If it is the MB2 path, then "relocate" isn't quite correct, I think:
>> Relocations aren't applied in that case, as none are present in xen.gz.
>> I'd rather call this "put at an address below 4G". However, that isn't
>> any different from the non-EFI MB1/2 paths, is it? I feel like I'm
>> missing something here.
>>
> 
> Yes, xen.gz has no relocation, but xen.efi has them,

Of course, I know. I was the one to actually add them, after all. But:
Are you (silently) adding a 4th way of booting Xen, using xen.efi yet
not its PE-header-specified entry point? So far we had three ways only:
Non-EFI (xen.gz:start), native EFI (xen.efi:efi_start), and GrUB2+EFI
(xen.gz:__efi64_mb2_start). You effectively suggest GrUB2+EFI
(xen.efi:__efi64_mb2_start), if I'm not mistaken. Where is such a mode
even specified? When using the MB2 entry point, it's not obvious at all
whether the boot loader is even supposed to be respecting the .reloc
section of the PE binary (imo that's contrary to the original idea of
multiboot).

> Normally probably you are using xen.gz instead of xen.efi however we
> are adding multiboot with PE support and secure boot so we need to use
> PE format for signing.

Note how here is the first time that you actually mention you're adding
a new boot mode. That's quite relevant as context for the entire series,
I would say.

>>> This cause offsets in x86 code generated by
>>> sym_offs(SYMBOL) to be relocated too (basically they won't be
>>> offsets from image base). In order to get real offset the
>>> formulae "sym_offs(SYMBOL) - sym_offs(__image_base__)" is
>>> used instead.
>>
>> The main calculations of %esi are, if I'm not mistaken,
>>
>>         /* Store Xen image load base address in place accessible for 32-bit 
>> code. */
>>         lea     __image_base__(%rip),%esi
>>
> 
> Which is correct
> 
>> and
>>
>>         /* Calculate the load base address. */
>>         call    1f
>> 1:      pop     %esi
>>         sub     $sym_offs(1b), %esi
>>
>> i.e. both deliberately %rip-relative to be position-independent. What's
>> wrong with this?
>>
> 
> This can be wrong if sym_offs(1b) was relocated and not patched by
> efi_arch_relocate_image.

Of course, if in the course of GrUB's loading of xen.efi base relocations
are applied (unlike when loading an ELF binary, where afaik base relocs
would be ignored, even if there were any), then this calculation is of
course going to be wrong. Can't we correct it though, to properly resemble
PIC code:

        /* Calculate the load base address. */
        call    1f
1:      pop     %esi
        sub     1b - start, %esi

or (because start is in a different section):

        /* Calculate the load base address. */
        call    1f
1:      pop     %esi
        sub     $sym_offs(1b), %esi
        add     $sym_offs(start), %esi

(or something along these lines)?

> Consider .Lnot_multiboot label call, here we didn't set %esi and in my
> case (GRUB using PE) the EFI version was used.

How does %esi matter at .Lnot_multiboot? It starts mattering from
x86_32_switch onwards, and it is being set there.

>> There are many more uses of sym_esi(). Why is it only this single one
>> which poses a problem?
> 
> Because potentially %esi is not set (see above example, but better to
> make sure is always valid) and becase potentially for EFI code is
> relocated.
> IMO stripping relocation and calling C code (like efi_multiboot2)
> supposed to be execute at a different location is a bomb waiting to
> explode.

Yet that has been working fine for xen.gz? There definitely is a risk,
but that risk isn't new afaict.

>>> Also, in some case %esi register (that should point to
>>> __image_base__ addresss) is not set so compute in all cases.
>>
>> Which "some case" is this?
> 
> In __efi64_mb2_start path there are some. Note that we use %rsi for
> different purposes there.

All cases there where %esi would matter branch to x86_32_switch first
afaics. As mentioned, %esi is being set one of the first things after
the x86_32_switch label.

>>> Code tested forcing failures in the code.
>>>
>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
>>
>> No Fixes: tag?
> 
> Not sure here, should I open some ticker and refer to it?

Well, now that I learned that you're trying to add a new boot mode,
there's no need for a Fixes: tag. Yet the patch title then also
shouldn't say "Fix" - that made it look like you're addressing some
problem with one of the existing boot modes we have.

Jan