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Subject: Re: Live patching on ARM64
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org,
        Mark Brown <broonie@kernel.org>,
        Julien Thierry <jthierry@redhat.com>, jpoimboe@redhat.com,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
References: <f3fe6a60-9ac2-591d-1b83-9113c50dc492@linux.microsoft.com>
 <20210115123347.GB39776@C02TD0UTHF1T.local>
From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Message-ID: <a5f22237-a18d-3905-0521-f0d0f9c253ea@linux.microsoft.com>
Date:   Sun, 17 Jan 2021 11:25:41 -0600
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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.
> 

Thanks for the information.

Madhavan