Live patching on ARM64

[Mark Brown]

On Fri, Jan 15, 2021 at 12:33:47PM +0000, Mark Rutland wrote:
> On Thu, Jan 14, 2021 at 04:07:55PM -0600, Madhavan T. Venkataraman wrote:
> > Hi all,
> > 
> > My name is Madhavan Venkataraman.
> 
> Hi Madhavan,
> 
> > Microsoft is very interested in Live Patching support for ARM64.
> > On behalf of Microsoft, I would like to contribute.
> > 
> > I would like to get in touch with the people who are currently working
> > in this area, find out what exactly they are working on and see if they
> > could use an extra pair of eyes/hands with what they are working on.
> > 
> > It looks like the most recent work in this area has been from the
> > following folks:

Also copying in Bill Wendling who has also expressed an interest in
this.  Not deleting context for his benefit.

> > 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'm not sure about the objtool side, so I'll leave that to Julien and co
> to answer.
> 
> Thanks,
> Mark.
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