Messages in this thread | | | From | Nick Desaulniers <> | Date | Wed, 26 Jun 2019 15:33:36 -0700 | Subject | Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs |
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On Wed, Jun 26, 2019 at 9:31 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Jun 25, 2019 at 11:47:06PM +0200, Thomas Gleixner wrote: > > > On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote: > > > > > but it also makes objtool unhappy: > > > > > arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool: get_fixed_ranges()+0x9b: unreachable instruction > > > I just checked two of them in the disassembly. In both cases it's jump > > label related. Here is one: > > > > asm volatile("1: rdmsr\n" > > 410: b9 59 02 00 00 mov $0x259,%ecx > > 415: 0f 32 rdmsr > > 417: 49 89 c6 mov %rax,%r14 > > 41a: 48 89 d3 mov %rdx,%rbx > > return EAX_EDX_VAL(val, low, high); > > 41d: 48 c1 e3 20 shl $0x20,%rbx > > 421: 48 09 c3 or %rax,%rbx > > 424: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) > > 429: eb 0f jmp 43a <get_fixed_ranges+0xaa> > > do_trace_read_msr(msr, val, 0); > > 42b: bf 59 02 00 00 mov $0x259,%edi <------- "unreachable"
I assume if 0x42b is unreachable, that's bad as $0x259 is never stored in %edi before the call to get_fixed_ranges+0xaa...
> > 430: 48 89 de mov %rbx,%rsi > > 433: 31 d2 xor %edx,%edx > > 435: e8 00 00 00 00 callq 43a <get_fixed_ranges+0xaa> > > 43a: 44 89 35 00 00 00 00 mov %r14d,0x0(%rip) # 441 <get_fixed_ranges+0xb1> > > Thomas provided the actual .o file, and from that we find that the > .rela__jump_table entries look like: > > 000000000010 000100000002 R_X86_64_PC32 0000000000000000 .text + 3e9 > 000000000014 000100000002 R_X86_64_PC32 0000000000000000 .text + 3f0 > 000000000018 006100000018 R_X86_64_PC64 0000000000000000 __tracepoint_read_msr + 8
I assume these relocations come from arch_static_branch() (and thus appear in triples?)
21 static __always_inline bool arch_static_branch(struct static_key *key, bool branch) 22 { 23 asm_volatile_goto("1:" 24 ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" 25 ".pushsection __jump_table, \"aw\" \n\t" 26 _ASM_ALIGN "\n\t" 27 ".long 1b - ., %l[l_yes] - . \n\t" // 1, 2 28 _ASM_PTR "%c0 + %c1 - .\n\t" // 3 29 ".popsection \n\t" 30 : : "i" (key), "i" (branch) : : l_yes);
> 000000000020 000100000002 R_X86_64_PC32 0000000000000000 .text + 424 > 000000000024 000100000002 R_X86_64_PC32 0000000000000000 .text + 3f0 > 000000000028 006100000018 R_X86_64_PC64 0000000000000000 __tracepoint_read_msr + 8 > > From this we find that the jump target that goes with the NOP at +424 is > +3f0, not +42b as one would expect. > > And as Josh noted, it is also 'weird' that this +3f0 is the very same as > the target for the previous entry.
(Ok, I think I did talk to Josh about this, and I think he did mention something about the jump targets, but I didn't really understand the issue well at the time).
> > When we compare the code at both sites, we find: > > 3f0: bf 58 02 00 00 mov $0x258,%edi > 3f5: 48 89 de mov %rbx,%rsi > 3f8: 31 d2 xor %edx,%edx > 3fa: e8 00 00 00 00 callq 3ff <get_fixed_ranges+0x6f> > 3fb: R_X86_64_PC32 do_trace_read_msr-0x4 > > vs > > 42b: bf 59 02 00 00 mov $0x259,%edi > 430: 48 89 de mov %rbx,%rsi > 433: 31 d2 xor %edx,%edx > 435: e8 00 00 00 00 callq 43a <get_fixed_ranges+0xaa> > 436: R_X86_64_PC32 do_trace_read_msr-0x4 > > Which is not in fact the same code. > > So for some reason the .rela__jump_table are buggy on this clang build.
So that sounds like a correctness bug then. (I'd been doing testing with the STATIC_KEYS_SELFTEST, which I guess doesn't expose this). I'm kind of surprised we can boot and pass STATIC_KEYS_SELFTEST. Any way you can help us pare down a test case? -- Thanks, ~Nick Desaulniers
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