On 15/1/21 11:33 pm, 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: >> >> 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. I am quite interested as well, I did some of the work for ppc64le >> >> 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. Do we know how this differs across architectures? Usually kprobe and ftrace unsafe functions are annotated as such, is there more to it? > > 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. Could you please explain, whats bits of the entry code? I suspect we never patch anything in assembly > > * 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. > noinstr is largely kcsan and kasan related, right? > 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, it would be good to see what the expectations from objtool are, I thought only x86 needed it due to variable size instructions and -fomit- frame-pointers Balbir Singh.
