Re: [PATCH 5/7] x86/mkreloc: remove warning about relocations to RO section

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.03.2025 09:14, Roger Pau Monné wrote:
> On Wed, Mar 19, 2025 at 11:46:22AM +0100, Jan Beulich wrote:
>> On 19.03.2025 11:32, Jan Beulich wrote:
>>> On 18.03.2025 18:35, Roger Pau Monne wrote:
>>>> Relocations are now applied after having moved the trampoline,
>>>
>>> That's two entirely different sets of relocations, isn't it? 
> 
> Right, this is the plain .reloc, while the trampoline one is
> .trampoline_{rel,seg}
> 
>>> What we generate
>>> here is what is to be encoded in the PE binary's .reloc section, for the PE
>>> loader to process. And for us to then process again once we move Xen back to
>>> its linked position (by virtue of leaving physical mode). Therefore what
>>> matters here is whether these relocations are still carried out while on the
>>> page tables to boot loader created, or when already on page tables we 
>>> control.
>>> In the former case any relocation to a non-writable section would be liable
>>> to fault when applied.
>>
>> And yes - both calls to efi_arch_relocate_image() are ahead of switching page
>> tables. The first call is benign - no writes occur there. The second call
>> would cause #PF though for any relocs applied to .text or .rodata or 
>> .init.text
>> or whatever else is non-writable.
> 
> I wonder how this worked then, as I've tested with the xen.efi smoke
> test in gitlab CI.  Maybe ovmf doesn't acknowledge the RX sections and
> unconditionally sets all mappings as writable?

Possible. And that would be in line with the mode being call "physical mode":
There are no permissions to enforce there. It just so happens that x86-64
requires paging to be enabled to be able to run 64-bit code.

My experience with OVMF has been that it's hard to find where certain code
lives. Perhaps I should try whether I can find respective code there. Then
again if I find nothing, there wouldn't be any guarantee that I merely didn't
spot the right place.

Jan