From: Pavel Machek <redacted>

Sent: 02 August 2020 12:56
Hi!

quoted

quoted

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

For nested calls (where the trampoline needs to pass the
original stack frame to the nested function) I think you
just need a page full of:
	mov	$0, scratch_reg; jmp trampoline_handler

I believe you could do with mov %pc, scratch_reg; jmp ...

That has advantage of being able to share single physical
page across multiple virtual pages...

A lot of architecture don't let you copy %pc that way so you would
have to use 'call' - but that trashes the return address cache.
It also needs the trampoline handler to know the addresses
of the trampolines.

	David

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