Messages in this thread | | | Subject | Re: [PATCH V2 5/9] objtool: Add support for intra-function calls | From | Alexandre Chartre <> | Date | Wed, 8 Apr 2020 19:06:50 +0200 |
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On 4/8/20 6:04 PM, Julien Thierry wrote: > > > On 4/8/20 5:03 PM, Alexandre Chartre wrote: >> >> >> On 4/8/20 4:19 PM, Julien Thierry wrote: >>> >>> >>> On 4/8/20 3:06 PM, Alexandre Chartre wrote: >>>> >>>> >>>> On 4/7/20 3:28 PM, Alexandre Chartre wrote: >>>>> >>>>> On 4/7/20 3:07 PM, Peter Zijlstra wrote: >>>>>> On Tue, Apr 07, 2020 at 09:31:38AM +0200, Alexandre Chartre wrote: >>>>>> >>>>>>> index a62e032863a8..7ee1561bf7ad 100644 >>>>>>> --- a/tools/objtool/arch/x86/decode.c >>>>>>> +++ b/tools/objtool/arch/x86/decode.c >>>>>>> @@ -497,3 +497,15 @@ void arch_initial_func_cfi_state(struct cfi_state *state) >>>>>>> state->regs[16].base = CFI_CFA; >>>>>>> state->regs[16].offset = -8; >>>>>>> } >>>>>>> + >>>>>>> + >>>>>>> +void arch_configure_intra_function_call(struct stack_op *op) >>>>>>> +{ >>>>>>> + /* >>>>>>> + * For the impact on the stack, make an intra-function >>>>>>> + * call behaves like a push of an immediate value (the >>>>>>> + * return address). >>>>>>> + */ >>>>>>> + op->src.type = OP_SRC_CONST; >>>>>>> + op->dest.type = OP_DEST_PUSH; >>>>>>> +} >>>>>> >>>>>> An alternative is to always set up stack ops for CALL/RET on decode, but >>>>>> conditionally run update_insn_state() for them. >>>>>> >>>>>> Not sure that makes more logical sense, but the patch would be simpler I >>>>>> think. >>>>> >>>>> Right, this would avoid adding a new arch dependent function and the patch >>>>> will be simpler. This probably makes sense as the stack impact is the same >>>>> for all calls (but objtool will use it only for intra-function calls). >>>>> >>>> >>>> Actually the processing of the ret instruction is more complicated than I >>>> anticipated with intra-function calls, and so my implementation is not >>>> complete at the moment. >>>> >>>> The issue is to correctly handle how the ret is going to behave depending how >>>> the stack (or register on arm) is modified before the ret. Adjusting the stack >>>> offset makes the stack state correct, but objtool still needs to correctly >>>> figure out where the ret is going to return and where the code flow continues. >>>> >>> >>> A hint indicating the target "jump" address could be useful. It could >>> be used to add the information on some call/jump dynamic that aren't >>> associated with jump tables. Currently when objtool finds a jump >>> dynamic, if no branches were added to it, it will just return. >>> >>> Having such a hint could help make additional links (at least on >>> arm64). I don't know what Peter and Josh would think of that (if that >>> helps in your case of course). >>> >> >> Yes, I am thinking about tracking intra-function call return address, >> and having hints to specify a return address changes. For example, >> on x86, when we push the branch address on the stack we overwrite the >> last return address (the return address of the last intra-function call). >> Then the return instruction can figure out where to branch. > > I see, I was thinking about a more generic hint, that would just > indicate "this instruction actually jumps here". So in your case it > would just point that a certain return instruction causes to branch > somewhere.
I thought about doing that but the problem is that on x86 the same retpoline code can branch differently depending on how it is used. Basically we have a return instruction that will branch differently based on what's on the stack. So we can just tell that this ret instruction will branch/return there.
alex.
> This way the hint could also be used for other instructions (e.g. > INSN_JUMP_DYNAMIC). > > >
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