Re: 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