Re: [RFC PATCH v2 02/13] x86/um: nommu: elf loader for fdpic

[Greg Ungerer <gerg@linux-m68k.org>]

Hi Hajime,

On 14/11/24 09:32, Hajime Tazaki wrote:
> On Wed, 13 Nov 2024 22:55:02 +0900,
> Geert Uytterhoeven wrote:
>> On Wed, Nov 13, 2024 at 2:17 PM Hajime Tazaki <thehajime@gmail.com> wrote:
>>> On Wed, 13 Nov 2024 19:27:08 +0900,
>>> Geert Uytterhoeven wrote:
>>>> On Wed, Nov 13, 2024 at 9:37 AM Johannes Berg <johannes@sipsolutions.net> wrote:
>>>>> On Wed, 2024-11-13 at 09:36 +0100, Johannes Berg wrote:
>>>>>> On Wed, 2024-11-13 at 09:19 +0100, Geert Uytterhoeven wrote:
>>>>>>>
>>>>>>>>>> -       depends on ARM || ((M68K || RISCV || SUPERH || XTENSA) && !MMU)
>>>>>>>>>> +       depends on ARM || ((M68K || RISCV || SUPERH || UML || XTENSA) && !MMU)
>>>>>>>>>
>>>>>>>>> s/UML/X86/?
>>>>>>>>
>>>>>>>> I guess the fdpic loader can be used to X86, but this patchset only
>>>>>>>> adds UML to be able to select it.  I intended to add UML into nommu
>>>>>>>> family.
>>>>>>>
>>>>>>> While currently x86-nommu is supported for UML only, this is really
>>>>>>> x86-specific. I still hope UML will get support for other architectures
>>>>>>> one day, at which point a dependency on UML here will become wrong...
>>>>>>>
>>>>>>
>>>>>> X86 isn't set for UML, X64_32 and X64_64 are though.
>>>>>>
>>>>>> Given that the no-MMU UM support even is 64-bit only, that probably
>>>>>> should then really be (UML && X86_64).
>>>>>>
>>>>>> But it already has !MMU, so can't be selected otherwise, and it seems
>>>>>> that non-X86 UML
>>>>>
>>>>> ... would require far more changes in all kinds of places, so not sure
>>>>> I'd be too concerned about it here.
>>>>
>>>> OK, up to you...
>>>
>>> Indeed, this particular patch [02/13] intends to support the fdpic
>>> loader under the condition 1) x86_64 ELF binaries (w/ PIE), 2) on UML,
>>> 3) and with) !MMU configured.  Given that situation, the strict check
>>> should be like:
>>>
>>>     depends on ARM || ((M68K || RISCV || SUPERH || (UML && X86_64) || XTENSA) && !MMU)
>>>
>>> (as Johannes mentioned).
>>>
>>> on the other hand, the fdpic loader works (afaik) on MMU environment so,
>>>
>>>     depends on ARM || (UML && X86_64) || ((M68K || RISCV || SUPERH || XTENSA) && !MMU)
>>>
>>> should also works, but this might be too broad for this patchset (and
>>> not sure if this makes a new use case).
>>
>> AFAIK that depends on the architecture's MMU context structure, cfr.
>> the comment in commit 782f4c5c44e7d99d ("m68knommu: allow elf_fdpic
>> loader to be selected"), which restricts it to nommu on m68k.  If it
>> does work on X86_64, you can drop the dependency on UML, and we're
>> (almost) back to my initial comment ;-)
> 
> I checked and it doesn't work as-is with (UML_X86_64 && MMU).
> restricting nommu with UML might be a good to for this patch.
> 
> even if it works, I would like to focus on UML && !MMU for this patch
> series since I wish to make the (initial) patchset as small as
> possible.  If we would like to make it broadly available on x86, that
> would be a different patch.

Makes sense.

I was only interested in the ability to run ELF based static/PIE binaries
when I did 782f4c5c44e7d99d ("m68knommu: allow elf_fdpic loader to be selected").
I did the same thing for RISC-V in commit 9549fb354ef1 ("riscv: support the
elf-fdpic binfmt loader"), limiting it to !MMU configurations only.

There is no need for binfmt_fdpic in MMU configurations if all you want to
do is run ELF PIE binaries. The normal binfmt_elf loader can load and run
those already.

Regards
Greg



>>> anyway, thank you for the comment.
>>> # I really wanted to have comments from nommu folks.
>>
>> I've added some in CC...
> 
> Thanks,
> 
> -- Hajime