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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.08.2024 14:50, Frediano Ziglio wrote:
> On Thu, Aug 8, 2024 at 10:29 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> (re-adding xen-devel@)

Did you notice this in my earlier reply? You dropped the list again.

>> On 08.08.2024 10:33, Frediano Ziglio wrote:
>>> On Thu, Aug 8, 2024 at 8:49 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>> 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)?
>>
> 
> Yes, that works. But is a bit painfull, I mean, the %esi will point to
> the correct address, but still you will use something like
> syms_esi(foo) expecting to work but it won't as there will be applied
> a relocation offset.

I find your reply contradictory in itself. You first say this works, to
then say it can't work. The underlying idea has to be to establish %esi
such that it works uniformly.

> On 32bit PIC code you could use something like
> foo@GOTOFF(%esi), assuing %esi is pointing to the global offset table.
> I was trying to use that but linker is complaining a bit as generating
> a 64bit relocation. The x64 architecture supports such relocation as
> 32bit but I didn't find a way to tell assembler to use the 32bit
> version instead of the 64bit one. Also I didn't find a way to set
> _GLOBAL_OFFSET_TABLE_ where I want it to be, it looks like that if the
> linker is not generating it is not picking up the forcedly set symbol.

Even if the toolchain permitted this: We don't have and don't want to
have any GOT. Note how the linker script actually has an assertion for
.got to be empty (plus a few more ones for other sections).

Jan