Re: [PATCH v4 1/3] x86: Put trampoline in separate .init.trampoline section

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.09.2024 17:31, Frediano Ziglio wrote:
> On Mon, Sep 23, 2024 at 4:17 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 19.09.2024 10:00, Frediano Ziglio wrote:
>>> This change put the trampoline in a separate, not executable section.
>>> The trampoline contains a mix of code and data (data which
>>> is modified from C code during early start so must be writable).
>>> This is in preparation for W^X patch in order to satisfy UEFI CA
>>> memory mitigation requirements.
>>> At the moment .init.text and .init.data in EFI mode are put together
>>> so they will be in the same final section as before this patch.
>>> Putting in a separate section (even in final executables) allows
>>> to easily disassembly that section. As we need to have a writable
>>> section and as we can't have code and data together to satisfy W^X
>>> requirement we need to have a data section. However tools like objdump
>>> by default do not disassemble data sections. Forcing disassembly of
>>> data sections would result in a very large output and possibly crash
>>> of tools. Putting in a separate section allows to selectively
>>> disassemble that part of code using a command like
>>>
>>>     objdump -m i386 -j .init.trampoline -d xen-syms
>>
>> For xen.efi it won't be quite as neat. One of the reason all .init.*
>> are folded into a single section there is that the longer section names
>> aren't properly represented, because of the linker apparently preferring
>> to truncate them instead of using the "long section names" extension. To
>> disassemble there one will need to remember to use "-j .init.tr". I'll
>> have to check if there's a linker option we fail to enable, but in the
>> absence of that we may want to consider to name the output section just
>> ".trampoline" there, abbreviating to ".trampol" (i.e. at least a little
>> more descriptive).
>>
> 
> Long names are working for me, probably some issues with older binutils tools.
> ".trampol" looks fine for me.

See the patch just sent, including the remark towards the somewhat unexpected /
inconsistent behavior of the linker. No need to drop the .init with that patch
in place then.

Jan