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Date: Tue, 25 Aug 2020 13:31:46 +0100 From: Julien Thierry <jthierry@...hat.com> To: Josh Poimboeuf <jpoimboe@...hat.com> Cc: linux-kernel@...r.kernel.org, peterz@...radead.org, mbenes@...e.cz, raphael.gault@....com, benh@...nel.crashing.org Subject: Re: [PATCH v2 9/9] objtool: Abstract unwind hint reading On 8/3/20 10:35 PM, Josh Poimboeuf wrote: > On Mon, Aug 03, 2020 at 01:13:14PM +0100, Julien Thierry wrote: >> >> >> On 7/31/20 3:04 PM, Josh Poimboeuf wrote: >>> On Fri, Jul 31, 2020 at 08:00:58AM +0100, Julien Thierry wrote: >>>>>> + cfa->offset = hint->sp_offset; >>>>>> + insn->cfi.hint_type = hint->type; >>>>>> + insn->cfi.end = hint->end; >>>>>> + >>>>>> + insn->cfi.sp_only = hint->type == ORC_TYPE_REGS || hint->type == ORC_TYPE_REGS_IRET; >>>>> >>>>> What does "sp" mean here in sp_only? >>>>> >>>> >>>> Stack pointer, like in CFI_SP. When objtool encounters one of these hints, >>>> it starts to only track the stack frame with the stack pointer (no BP, no >>>> drap register, no move to temporary registers). Just trying to make some >>>> sense of this corner case. >>> >>> I think that's not quite right, because ORC_TYPE_CALL could also be >>> "sp_only" in some cases, by that definition. >>> >> >> But in that case the code will still track when/if the CFI becomes pointed >> to by BP. >> >>> The call to update_cfi_state_regs() is really regs-specific, not >>> sp-specific. >>> >> >> I must admit I don't really understand what "regs" is and why exactly such >> an exception in stack state tracking is made where only operations to SP are >> taken into account. > > "regs" is a special type of stack frame, usually for asm entry code, > where the frame is actually an instance of 'struct pt_regs'. So if > there's a variable associated it with it, maybe it should have "regs" in > the name. > > Though I think non-x86 arches will also have regs frames, so would it > make sense to just make the unwind hint types a global multiarch thing? > They could be renamed to UNWIND_HINT_TYPE_REGS{_PARTIAL}. Then there > wouldn't really be a need for the "sp_only" thing. > If having regs frame means having a pt_regs on the stack when procedure calls/return, then yes this will probably be the case on most archs (it is for arm64 at least. However in that case, arm64 still builds a stack frame and sets the frame pointer, so only handling SP operations doesn't make much sense for arm64. Also, things like ORC_TYPE_REGS_IRET don't have a use for arm64 (but maybe for other non-x86 arches it does?) In the end that's why I left the unwind hint types as arch defined. It seems like every arch will have their specific semantics they might want to let objtool know about. -- Julien Thierry
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