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Tue, 27 Jan 2026 19:15:10 +0000 Message-ID: <960d07f0-d693-4139-ba80-d622b69e6273@intel.com> Date: Tue, 27 Jan 2026 11:15:08 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/7] stop_machine: Introduce stop_machine_nmi() To: Borislav Petkov , kernel test robot CC: , , , , , , , References: <20260125014224.249901-2-chang.seok.bae@intel.com> <202601261901.Yj2kmvgI-lkp@intel.com> <20260127144947.GKaXjQi8R9cE17WYKM@fat_crate.local> Content-Language: en-US From: "Chang S. 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> > function pointer which objtool can't see through. Yes. Under certain config, the 0-day bot tirggers this with CONFIG_NOINSTR_VALIDATION=y. Objtool ends up with the following path: tools/objtool/check.c::validation_call() -> noinstr_call_dest() { /* * We can't deal with indirect function calls at present; * assume they're instrumented. */ if (!func) { if (file->pv_ops) return pv_call_dest(file, insn); return false; } ... } So every indirect call under noinstr is conservatively treated as invalid except for pv_ops which seems to be investigated further. In this series, however, the destination code is to in .noinstr.text. Given this, I considered something to be adjusted on the tool side, e.g. by whitelisting the case along the lines of if (!func) { ... if (!strncmp(insn_func(insn)->name, "stop_machine_nmi_handler", 24)) return true; return false; } The existing pv_ops handling also looks somewhat misaligned with large objects like vmlinux.o. > > So my suggestion would be to do: > > struct nmi_multi_stop_data { > cpu_stop_safe_fn_t fn; > > note the different function pointer type which we will hopefully catch during > review in the sense that only *safe* functions should be callable by the > NMI-specific stomp_machine. > > Along with: > > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c > index 189b4b108d13..55a350048d4c 100644 > --- a/kernel/stop_machine.c > +++ b/kernel/stop_machine.c > @@ -230,7 +230,9 @@ bool noinstr stop_machine_nmi_handler(void) > raw_cpu_write(stop_machine_nmi_ctrl.nmi_enabled, false); > > msdata = raw_cpu_read(stop_machine_nmi_ctrl.msdata); > + instrumentation_begin(); > err = msdata->fn(msdata->data); > + instrumentation_end(); > raw_cpu_write(stop_machine_nmi_ctrl.err, err); > return true; > } I also thought this change, but I was a bit hesitant as it could be seen as lifting up the no-instrumentation. Now I suppose the net effect will be the same with other workaround, e.g. tool-side whitelisting. So I think I can go with this as you suggested (with a clear note on this). > > ofc. > > Unless someone has a better idea ofc... Sure. Thanks, Chang