Re: Live patching on ARM64

From: Madhavan T. Venkataraman
Date: Sun Jan 17 2021 - 12:29:42 EST

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.


>> 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.


> 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.


> * 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).


> 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.


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.

Thanks for the information.