Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Mar 30, 2020 at 07:02:00PM +0200, Peter Zijlstra wrote:
> Subject: objtool: Implement RET_TAIL hint
> 
> This replaces the SAVE/RESTORE hints with a RET_TAIL hint that applies to:
> 
>  - regular RETURN and sibling calls (which are also function exists)
>    it allows the stack-frame to be off by one word, ie. it allows a
>    return-tail-call.
> 
>  - EXCEPTION_RETURN (a new INSN_type that splits IRET out of
>    CONTEXT_SWITCH) and here it denotes a return to self by having it
>    consume arch_exception_frame_size bytes off the stack and continuing.
> 
> Apply this hint to ftrace_64.S and sync_core(), the two existing users
> of the SAVE/RESTORE hints.
> 
> For ftrace_64.S we split the return path and make sure the
> ftrace_epilogue call is seen as a sibling/tail-call turning it into it's
> own function.
> 
> By splitting the return path every instruction has a unique stack setup
> and ORC can generate correct unwinds (XXX check if/how the ftrace
> trampolines map into the ORC). Then employ the RET_TAIL hint to the
> tail-call exit that has the direct-call (orig_eax) return-tail-call on.
> 
> For sync_core() annotate the IRET with RET_TAIL to mark it as a
> control-flow NOP that consumes the exception frame.

I do like the idea to get rid of SAVE/RESTORE altogether.  And it's nice
to make that ftrace code unwinder-deterministic.

However sync_core() and ftrace_regs_caller() are very different from
each other and I find the RET_TAIL hint usage to be extremely confusing.

For example, IRETQ isn't even a tail cail.

And the need for the hint to come *before* the insn which changes the
state is different from the other hints.

And now objtool has to know the arch exception stack size because of a
single code site.

And for a proper tail call, the stack should be empty.  I don't
understand the +8 thing in has_modified_stack_frame().  It seems
hard-coded for the weird ftrace case, rather than for tail calls in
general (which should already work as designed).

How about a more general hint like UNWIND_HINT_ADJUST?

For sync_core(), after the IRETQ:

  UNWIND_HINT_ADJUST sp_add=40

And ftrace_regs_caller_ret could have:

  UNWIND_HINT_ADJUST sp_add=8
  
-- 
Josh