From: Pavel Machek <pavel@xxxxxx> > 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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
