Messages in this thread | | | Date | Wed, 21 Jan 2015 13:02:55 -0500 | From | David Long <> | Subject | Re: [PATCH v4 3/6] arm64: Kprobes with single stepping support |
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On 01/19/15 04:03, Pratyush Anand wrote: > > > On Saturday 17 January 2015 12:58 AM, David Long wrote: >>>> +static bool aarch64_insn_is_steppable(u32 insn) >>>> +{ >>>> + if (aarch64_get_insn_class(insn) == AARCH64_INSN_CLS_BR_SYS) { >>>> + if (aarch64_insn_is_branch(insn)) >>>> + return false; >>>> + >>>> + /* modification of daif creates issues */ >>>> + if (aarch64_insn_is_msr_daif(insn)) >>>> + return false; >>>> + >>>> + if (aarch64_insn_is_hint(insn)) >>>> + return aarch64_insn_is_nop(insn); >>>> + >>>> + return true; >>>> + } >>>> + >>>> + if (aarch64_insn_uses_literal(insn)) >>>> + return false; >>>> + >>>> + if (aarch64_insn_is_exclusive(insn)) >>>> + return false; >>>> + >>>> + return true; >>> >>> Default true return may not be a good idea until we are sure that we >>> are returning false for all possible >>> simulation and rejection cases. In my opinion, its better to return >>> true only for steppable and false for >>> all remaining. >>> >> >> I struggled a little with this when I did it but I decided if the >> question was: "should we have to recognize every instruction before >> deciding it was single-steppable or should we only recognize >> instructions that are *not* single-steppable", maybe it was OK to do the >> latter while recognizing extensions to the instruction set *could* end >> up (temporarly) allowing us to try and fail (badly) at single-stepping >> any problematic new instructions. Certainly opinions could differ. If > > Lets see what others say, but I see that this approach will result in > undesired behavior. For example: a probe has been tried to insert to svc > instruction. SVC or any other exception generation instruction is > expected to be rejected. But, current aarch64_insn_is_steppable will > return true for it and then kprobe/uprobe code will allow to insert > probe at that instruction, which will be wrong, no? I mean, I do not see > a way to get into last else (INSN_REJECTED) of arm_kprobe_decode_insn. > > So, if we go with this approach we need to insure that we cover all > simulation-able and reject-able cases in aarch64_insn_is_steppable. >
yes, of course. Any case that's missing in the current code needs to be fixed. If the result starts to look less practical than the table-driven code then the new approach needs to be discarded.
> ~Pratyush > > > >> the consensus is that we can't allow this to ever happen (because old >> kprobe code is running on new hardware) then I think the only choice is >> to return to parsing binary tables. Hopefully I could still find a way >> to leverage insn.c in that case.
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