Re: [PATCH 1/5] x86: Put trampoline in .init.data section

[Jan Beulich <jbeulich@suse.com>]

On 27.08.2024 16:56, Frediano Ziglio wrote:
> On Mon, Aug 19, 2024 at 4:50 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 19.08.2024 17:30, Frediano Ziglio wrote:
>>> On Mon, Aug 19, 2024 at 3:30 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>> On 19.08.2024 16:16, Frediano Ziglio wrote:
>>>>> Could I ouput the trampoline in a code section ("ax" instead of "aw")
>>>>> and then later move it into .init.data section assuring .init.data is
>>>>> writeable but not executable?
>>>>
>>>> Could you go into a little more detail on what you mean here? At the
>>>> first glance my reaction is "yes, sure, why not", but much depends on
>>>> what exactly is meant.
>>>
>>> For instance you could put the trampoline into a
>>>     .section .init.trampoline, "awx", @progbits
>>> section (having the "x" will be disassembled by objdump -d head.o).
>>> Then in xen/arch/x86/xen.lds.S in the .init.data section having something like
>>> ...
>>>   DECL_SECTION(.init.data) {
>>>        *(.init.bss.stack_aligned)
>>>       (.init.trampoline)
>>>    ...
>>> this will put the trampoline in .init.data section of the final
>>> object. At this point the .init.data containing code will have execute
>>> permission that you would have to fix using objcopy command.
>>> The final trampoline will be in a data section not executable so to
>>> use objdump you will need the -D option, but not disassembling head.o.
>>> In theory we could keep the temporary object file before the objcopy
>>> adjustment to avoid the -D but I don't think it would save a lot of
>>> burdain.
>>
>> Part of my "want to be able to disassemble" also applies to the final
>> binaries. Since iirc one can disassemble individual sections, an option
>> may be to have .trampoline be its own section even in the final (PE)
>> binary?
>>
>> In any event, especially as long as there is no really good option, I
>> think I'd like to have input from Andrew and/or Roger as well.
> 
> what about having an extra copy in .init.data. That is, compile the
> trampoline in .init.text having it readable/executable in the final
> PE, at the beginning copying into .init.data and handle the changes
> there then when we can allocate the final memory allocate it and copy
> the temporary writable copy into final lower memory area.

I dislike such duplication, and I'd fear it may cause more confusion
than to actually help. Plus you'd need to sort the symbol name clashes.

> On a related but not too much topic, I noted there's no .init.bss.
> Maybe we could allocate .init.bss (and .init.bss.stack_aligned or
> whatever .init.bss.*) at the end of normal .bss and "cut" it while we
> remove .init.text and .init.data.

How would anything (in C) use .init.bss? __attribute__((section(...)))
doesn't make @nobits sections (last I checked), and .init.bss that's
@progbits is not really different from .init.data.

Jan