Live patching on ARM64

[Julien Thierry]

Hi Madhavan,

On 1/17/21 6:25 PM, Madhavan T. Venkataraman wrote:
> 
> 
> On 1/15/21 6:33 AM, Mark Rutland wrote:
> 
>>> It looks like the most recent work in this area has been from the
>>> following folks:
>>>
>>> Mark Brown and Mark Rutland:
>>> 	Kernel changes to providing reliable stack traces.
>>>
>>> Julien Thierry:
>>> 	Providing ARM64 support in objtool.
>>>
>>> Torsten Duwe:
>>> 	Ftrace with regs.
>>
>> IIRC that's about right. I'm also trying to make arm64 patch-safe (more
>> on that below), and there's a long tail of work there for anyone
>> interested.
>>
> 
> OK.
> 
>>> I apologize if I have missed anyone else who is working on Live Patching
>>> for ARM64. Do let me know.
>>>
>>> Is there any work I can help with? Any areas that need investigation, any code
>>> that needs to be written, any work that needs to be reviewed, any testing that
>>> needs to done? You folks are probably super busy and would not mind an extra
>>> hand.
>>
>> One general thing that I believe we'll need to do is to rework code to
>> be patch-safe (which implies being noinstr-safe too). For example, we'll
>> need to rework the instruction patching code such that this cannot end
>> up patching itself (or anything that has instrumented it) in an unsafe
>> way.
>>
> 
> OK.
> 
>> Once we have objtool it should be possible to identify those cases
>> automatically. Currently I'm aware that we'll need to do something in at
>> least the following places:
>>
>> * The entry code -- I'm currently chipping away at this.
>>
> 
> OK.
> 
>> * The insn framework (which is used by some patching code), since the
>>    bulk of it lives in arch/arm64/kernel/insn.c and isn't marked noinstr.
>>    
>>    We can probably shift the bulk of the aarch64_insn_gen_*() and
>>    aarch64_get_*() helpers into a header as __always_inline functions,
>>    which would allow them to be used in noinstr code. As those are
>>    typically invoked with a number of constant arguments that the
>>    compiler can fold, this /might/ work out as an optimization if the
>>    compiler can elide the error paths.
>>
>> * The alternatives code, since we call instrumentable and patchable
>>    functions between updating instructions and performing all the
>>    necessary maintenance. There are a number of cases within
>>    __apply_alternatives(), e.g.
>>
>>    - test_bit()
>>    - cpus_have_cap()
>>    - pr_info_once()
>>    - lm_alias()
>>    - alt_cb, if the callback is not marked as noinstr, or if it calls
>>      instrumentable code (e.g. from the insn framework).
>>    - clean_dcache_range_nopatch(), as read_sanitised_ftr_reg() and
>>      related code can be instrumented.
>>
>>    This might need some underlying rework elsewhere (e.g. in the
>>    cpufeature code, or atomics framework).
>>
> 
> OK.
> 
>> So on the kernel side, maybe a first step would be to try to headerize
>> the insn generation code as __always_inline, and see whether that looks
>> ok? With that out of the way it'd be a bit easier to rework patching
>> code depending on the insn framework.
>>
> 
> OK.
> 
> I have an understanding of some of the above already. I will come up to
> speed on the others. I will email you any questions I might have.
> 
>> I'm not sure about the objtool side, so I'll leave that to Julien and co
>> to answer.
>>

Sorry for the late reply. The last RFC for arm64 support in objtool is a 
bit old because it was preferable to split things into smaller series.

I touched it much lately, so I'm picking it back up and will try to get 
a git branch into shape on a recent mainline (a few things need fixing 
since the last time I rebased it).

I'll update you once I have something at least usable/presentable.

Cheers,

-- 
Julien Thierry