Messages in this thread | | | Date | Thu, 2 Apr 2020 09:50:36 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET |
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On Thu, Apr 02, 2020 at 07:41:46AM +0100, Julien Thierry wrote: > On 4/1/20 6:09 PM, Peter Zijlstra wrote:
> > The code in question (x86's sync_core()), is an exception return to > > self. It pushes an exception frame that points to right after the > > exception return instruction. > > > > This is the only usage of IRET in STT_FUNC symbols. > > > > So rather than teaching objtool how to interpret the whole > > push;push;push;push;push;iret sequence, teach it how big the frame is > > (arch_exception_frame_size) and let it continue. > > > > All the other (real) IRETs are in STT_NOTYPE in the entry assembly. > > > > Right, I see.. However I'm not completely convinced by this. I must admit I > haven't followed the whole conversation, but what was the issue with the > HINT_IRET_SELF? It seemed more elegant, but I might be missing some context.
https://lkml.kernel.org/r/20200331211755.pb7f3wa6oxzjnswc@treble
Josh didn't think it was worth it, I think.
> Otherwise, it might be worth having a comment in the code to point that this > only handles the sync_core() case.
I can certainly do that. Does ARM have any ERETs sprinkled around in places it should not have? That is, is this going to be a problem for you?
> Also, instead of adding a special "arch_exception_frame_size", I could > suggest: > - Picking this patch [1] from a completely arbitrary source > - Getting rid of INSN_STACK type, any instruction could then include stack > ops on top of their existing semantics, they can just have an empty list if > they don't touch SP/BP > - x86 decoder adds a stack_op to the iret to modify the stack pointer by the > right amount
That's not the worst idea, lemme try that.
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