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

On 8/3/20 3:08 AM, David Laight wrote:

From: Pavel Machek <redacted>

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Sent: 02 August 2020 12:56
Hi!

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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.

Do you which ones don't allow you to copy %pc?

Some of the architctures do not have PC-relative data references.
If they do not allow you to copy the PC into a general purpose
register, then there is no way to implement the statically defined
trampoline that has been discussed so far. In these cases, the
trampoline has to be generate at runtime.

Thanks.

Madhavan