Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

From: Florian Weimer <[email-masked]>
Date: Wed, 29 Jul 2020 15:29:31 +0200

* Andy Lutomirski:

> This is quite clever, but now I’m wondering just how much kernel help
> is really needed. In your series, the trampoline is an non-executable
> page.  I can think of at least two alternative approaches, and I'd
> like to know the pros and cons.
>
> 1. Entirely userspace: a return trampoline would be something like:
>
> 1:
> pushq %rax
> pushq %rbc
> pushq %rcx
> ...
> pushq %r15
> movq %rsp, %rdi # pointer to saved regs
> leaq 1b(%rip), %rsi # pointer to the trampoline itself
> callq trampoline_handler # see below
>
> You would fill a page with a bunch of these, possibly compacted to get
> more per page, and then you would remap as many copies as needed.

libffi does something like this for iOS, I believe.

The only thing you really need is a PC-relative indirect call, with the
target address loaded from a different page.  The trampoline handler can
do all the rest because it can identify the trampoline from the stack.
Having a closure parameter loaded into a register will speed things up,
of course.

I still hope to transition libffi to this model for most Linux targets.
It really simplifies things because you don't have to deal with cache
flushes (on both the data and code aliases for SELinux support).

But the key observation is that efficient trampolines do not need
run-time code generation at all because their code is so regular.

Thanks,
Florian

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