Messages in this thread | | | Date | Mon, 30 Mar 2020 14:02:05 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case |
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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
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