Ping. On 21.02.2019 03:08, Kamil Rytarowski wrote: > On 20.02.2019 18:29, Paolo Bonzini wrote: >> On 20/02/19 12:59, Kamil Rytarowski wrote: >>> Ping, still valid. >> >> Sorry, I missed your email. >> >>> On 15.02.2019 00:38, Kamil Rytarowski wrote: >>>> I consider it as fragile hack and certainly not something to depend on. >>>> Also in some circumstances of such code, especially "if (zero0)" we want >>>> to enable disabled code under a debugger. >> >> That's a good objection, but certainly does not apply to KVM on NetBSD. >> > > There is KVM for Darwin (experimental and rather toy project) and it > might be ported to NetBSD (I have actually forked it on GitHub > recently), but I doubt that someone would enable KVM on any platform > under a debugger this way and expect something to work. > >>>> There were also kernel backdoors due to this optimization. >> >> Citation please? >> > > I saw an exploit for such case with a .txt writeup on ftp of grsecurity > but that service seems to be gone (probably long time ago), so please > defer discussion on it. If someone is interested to find it out, there > are enough pointers to dig it (assuming that this is still possible). > >>>> Requested cpu.i (hopefully correctly generated) >>>> >>>> http://netbsd.org/~kamil/qemu/cpu.i.bz2 >> >> So, first thing first I can reproduce clang's behavior with this .i file >> and also with this reduced test case. >> >> extern void f(void); >> int i, j; >> int main() >> { >> if (0 && i) f(); >> if (j && 0) f(); >> } >> >> The first is eliminated but the second is not, just like in QEMU where >> this works: >> >> if (kvm_enabled() && cpu->enable_pmu) { >> KVMState *s = cs->kvm_state; >> >> *eax = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EAX); >> *ebx = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EBX); >> *ecx = kvm_arch_get_supported_cpuid(s, 0xA, count, R_ECX); >> *edx = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EDX); >> } else if (hvf_enabled() && cpu->enable_pmu) { >> *eax = hvf_get_supported_cpuid(0xA, count, R_EAX); >> *ebx = hvf_get_supported_cpuid(0xA, count, R_EBX); >> *ecx = hvf_get_supported_cpuid(0xA, count, R_ECX); >> *edx = hvf_get_supported_cpuid(0xA, count, R_EDX); >> >> while this doesn't: >> >> if ((env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_INTEL_PT) && >> kvm_enabled()) { >> KVMState *s = CPU(cpu)->kvm_state; >> uint32_t eax_0 = kvm_arch_get_supported_cpuid(s, 0x14, 0, R_EAX); >> uint32_t ebx_0 = kvm_arch_get_supported_cpuid(s, 0x14, 0, R_EBX); >> uint32_t ecx_0 = kvm_arch_get_supported_cpuid(s, 0x14, 0, R_ECX); >> uint32_t eax_1 = kvm_arch_get_supported_cpuid(s, 0x14, 1, R_EAX); >> uint32_t ebx_1 = kvm_arch_get_supported_cpuid(s, 0x14, 1, R_EBX); >> >> But, that's okay, it's -O0 so we give clang a pass for that Note that >> clang does do the optimization even in more complex cases like >> >> extern _Bool f(void); >> int main() >> { >> if (!0) return 0; >> if (!f()) return 0; >> } >> >> The problem is that there is a kvm-stub.c entry for that, and in fact >> my compilation passes and the symbol is resolved correctly: >> >> $ nm target/i386/cpu.o |grep kvm_.*get_sup >> U kvm_arch_get_supported_cpuid >> $ nm target/i386/kvm-stub.o|grep kvm_.*get_sup >> 0000000000000030 T kvm_arch_get_supported_cpuid >> $ nm qemu-system-x86_64 |grep kvm_.*get_sup >> 000000000046eab0 T kvm_arch_get_supported_cpuid >> >> As expected, something much less obvious is going on for you, in >> particular __OPTIMIZE__seems not to be working properly. However, >> that would also be very surprising. >> >> Please: >> >> 1) run the last two "nm" commands on your build (wthout grep). >> > > I cannot run nm(1) on qemu-system-x86_64 as it's not linkable. > > I'm getting the same result for target/i386/cpu.o and > target/i386/kvm-stub.o. > > $ nm ./i386-softmmu/target/i386/kvm-stub.o > U abort > 0000000000000000 T kvm_allows_irq0_override > 0000000000000030 T kvm_arch_get_supported_cpuid > 0000000000000020 T kvm_enable_x2apic > 0000000000000010 T kvm_has_smm > 0000000000000050 T kvm_hv_vpindex_settable > > grep(1) used, but otherwise I would need to upload results somewhere else. > > $ nm ./i386-bsd-user/target/i386/cpu.o |grep kvm > U kvm_arch_get_supported_cpuid > 0000000000001290 d kvm_default_props > U kvm_state > 0000000000000240 T x86_cpu_change_kvm_default > > Please note that there are 4 types of x86 build: i386, x86_64 and two > bsd-user (32-bit and 64-bit). > > According to my observations of repeated attempts both builds of > bsd-user are affected. > > There is also a difference that kvm-stub is twice in !bsd-user > directories and once in bsd-user ones. > > $ find . -name kvm-stub.o|grep x86_64 > ./x86_64-softmmu/accel/stubs/kvm-stub.o > ./x86_64-softmmu/target/i386/kvm-stub.o > ./x86_64-bsd-user/accel/stubs/kvm-stub.o > >> 2) do the same exercise to get a .i for target/i386/kvm-stub.c >> >> 3) try removing the "#ifndef __OPTIMIZE__" and leave everything else as is, >> see if it works. > > Same result as above, it seems that bsd-user ones are affected in the > same way. > >> No need to play with macros, which also goes to show that >> you didn't really understand what's going on---that's fine, but then please >> refrain from making summary judgments which only lengthen the discussion. >> > > I stand with my expression that I find playing with optimizer as too > fragile to depend on it. Relying on a preprocessor to disable unused > code is predictable. But if that is the style in qemu, I can just > acknowledge it. > >> Thanks, >> >> Paolo >> > >
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