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

[David Laight <David.Laight@ACULAB.COM>]

From: Pavel Machek <pavel@ucw.cz>
> Sent: 02 August 2020 12:56
> Hi!
> 
> > > 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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