Messages in this thread | | | Subject | Re: [PATCH 5/8] arm64: kprobes instruction simulation support | From | David Long <> | Date | Wed, 24 Feb 2016 10:07:32 -0500 |
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On 02/24/2016 04:05 AM, Marc Zyngier wrote: > On Wed, 24 Feb 2016 01:56:52 -0500 > David Long <dave.long@linaro.org> wrote: > >> On 02/19/2016 09:04 AM, Marc Zyngier wrote: >>> Hi David, >>> >>> On 18/02/16 23:48, David Long wrote: >>>> From: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com> >>>> >>>> Kprobes needs simulation of instructions that cannot be stepped >>>> from different memory location, e.g.: those instructions >>>> that uses PC-relative addressing. In simulation, the behaviour >>>> of the instruction is implemented using a copy of pt_regs. >>>> >>>> Following instruction catagories are simulated: >>>> - All branching instructions(conditional, register, and immediate) >>>> - Literal access instructions(load-literal, adr/adrp) >>>> >>>> Conditional execution is limited to branching instructions in >>>> ARM v8. If conditions at PSTATE do not match the condition fields >>>> of opcode, the instruction is effectively NOP. Kprobes considers >>>> this case as 'miss'. >>>> >>>> This code also replaces the use of arch/arm/opcodes.c for >>>> arm_check_condition(). >>>> >>>> Thanks to Will Cohen for assorted suggested changes. >>>> >>>> Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com> >>>> Signed-off-by: William Cohen <wcohen@redhat.com> >>>> Signed-off-by: David A. Long <dave.long@linaro.org> > > [...] > >>>> +}; >>>> + >>>> +asmlinkage unsigned int __kprobes arm_check_condition(u32 opcode, u32 psr) >>> >>> Why asmlinkage? This function is never called from assembly code on arm64. >>> >> >> This comes from the 32-bit ARM code that tests the condition from >> entry.S. We include arch/arm/include/asm/opcodes.h in >> arch/arm64/include/asm/opcodes.h so it gets declared there with >> asmlinkage. I can remove the asmlinkage in the actual function >> definition and it still compiles but I'm not sure that is kosher. > > asmlinkage is only meaningful if you're calling it from assembly code. > As you seem to only call it from C code, having asmlinkage is both > pointless and confusing. > >> Will Deacon was advocating getting rid of the include of the 32-bit header >> file but it looked to me like this would mean a lot of duplicated >> defines and the work would be mostly unrelated to kprobes. > > Arguably, arm_check_condition() (which only matters to 32bit code, > hence userspace) is also completely unrelated to kprobes. I still think > Will's point stands. >
Yes, I would not argue about that cross-architecture include needing to be fixed. Can I assume you agree that need not be a part of this kprobes patch though, nor a prerequisite patch for it?
>> >>>> +{ >>>> + u32 cc_bits = opcode >> 28; >>>> + >>>> + if (cc_bits != ARM_OPCODE_CONDITION_UNCOND) { >>>> + if ((*opcode_condition_checks[cc_bits])(psr)) >>>> + return ARM_OPCODE_CONDTEST_PASS; >>>> + else >>>> + return ARM_OPCODE_CONDTEST_FAIL; >>>> + } >>>> + return ARM_OPCODE_CONDTEST_UNCOND; >>>> +} >>>> +EXPORT_SYMBOL_GPL(arm_check_condition); >>> >>> Why do we need this to be exported at all? Also, it'd be better located >>> together with the deprecated instruction handling, possibly in a >>> separate patch (nothing uses this function in this patch). >>> >> >> I've made the function static and moved it to armv8_deprecated. I have >> to leave the static functions that test the individual conditions and >> the global array of pointers to them outside of the conditionally >> compiled armv8_deprecated.c as they have to always be present for >> kprobes to simulate a conditional branch. > > I think that's fine. > > Thanks, > > M. >
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