Re: [RFC PATCH 14/28] x86/rethook: Use RIP-relative reference for return address

[Jan Beulich <jbeulich@suse.com>]

On 25.09.2024 18:51, Andrew Cooper wrote:
> On 25/09/2024 5:45 pm, Ard Biesheuvel wrote:
>> On Wed, 25 Sept 2024 at 18:39, Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>>> And we do have special calling conventions that aren't the regular
>>> ones, so %rdi might actually be used elsewhere. For example,
>>> __get_user_X and __put_user_X all have magical calling conventions:
>>> they don't actually use %rdi, but part of the calling convention is
>>> that the unused registers aren't modified.
>>>
>>> Of course, I'm not actually sure you can probe those and trigger this
>>> issue, but it all makes me think it's broken.
>>>
>>> And it's entirely possible that I'm wrong for some reason, but this
>>> just _looks_ very very wrong to me.
>>>
>>> I think you can do this with a "pushq mem" instead, and put the
>>> relocation into the memory location.
>>>
>> I'll change this into
>>
>>   pushq arch_rethook_trampoline@GOTPCREL(%rip)
>>
>> which I had originally. I was trying to avoid the load from memory,
>> but that obviously only works if the register is not live.
> 
> But does that work?  Won't that will push the 8 bytes from the start of
> arch_rethook_trampoline, when what's wanted is simply the address of
> arch_rethook_trampoline itself.

What you describe is

   pushq arch_rethook_trampoline(%rip)

The @GOTPCREL makes the PUSH access an item from the GOT, and that item
is arch_rethook_trampoline's address.

Jan