[llvm-dev] Quick question: How to BuildMI mov64mi32 arbitrary MMB address to memory

[K Jelesnianski via llvm-dev]

I also confirm I am building my executables and libraries with "-fPIC -pie"
CFLAGS to be able to take advantage of ASLR.

On Sat, Sep 22, 2018 at 5:20 PM K Jelesnianski <kjski at vt.edu> wrote:

> Dear Mr. Northover,
>
> Thank you for the quick reply. You are correct about the address-mode
> operands :) . I guess an important detail left out was that the basic block
> (call it A) that wants to calculate the address of the target stationary
> trampoline basic block (call it B) will be moved around in memory during
> run-time. Our earlier solution, before the feature was implemented to move
> around (A) is exactly as you explained using the following with a scratch
> reg:
>
> $ bin/llc -relocation-model=pic simple.ll -o -
>> [...]
>>         leaq    .Ltmp0(%rip), %rax
>>         movq    %rax, (%rdi)
>>
>
> We now run into the problem that with this feature enabled, if we try to
> perform LEA backwards to the trampoline, after A has been moved the %rip
> relative offset that was put in by LLVM is no longer valid. Also if we
> perform LEA forwards to the target address in A that trampoline B is
> supposed to trampoline us too, that address will also be invalidated once A
> has been moved. Thus calculating forwards is most likely impossible. This
> leaves calculating LEA backwards to the trampoline BB (B) since we know
> that the trampoline BB will remain stationary throughout execution. That is
> why I would "like" to somehow store (B)'s address.
>
> I am looking for a work around to accommodate this feature. I have never
> attempted to make my own section/symbols using LLVM, but I assume this is
> the route I should take? Pairing each trampoline BB to a symbol should make
> it visible and this MOVQ instruction I want possible?
> With that approach, my questions are:
> Do I need to make these symbols for the trampoline BBs as an IR opt pass,
> can I get away with it using a MachineModule Pass to add the trampolines
> per module (file) (so far I have only created BasicBlock,
> MachineBasicBlock, and MachineFunction passes)??
> Do I need to make a separate custom section for these trampolines symbols,
> or can I just add them to the .text section?
>
> Thanks again for your reply.
> Sincerely,
>
> K Jelesnianski
>
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