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Subjectwarning: objtool: mce_panic()+0x11b: unreachable instruction
Hey Josh,

I'm seeing

arch/x86/kernel/cpu/mce/core.o: warning: objtool: mce_panic()+0x11b: unreachable instruction

on a brand new debian install here with gcc9: gcc (Debian 9.2.1-4) 9.2.1 20190821

and thought should run it by you, you might've seen it already.

So mce_panic is at ffffffff8102f390, which makes the offset
0xffffffff8102f4ab and at that I have:

ffffffff8102f497: 48 83 c4 08 add $0x8,%rsp
ffffffff8102f49b: 5b pop %rbx
ffffffff8102f49c: 5d pop %rbp
ffffffff8102f49d: 41 5c pop %r12
ffffffff8102f49f: 41 5d pop %r13
ffffffff8102f4a1: 41 5e pop %r14
ffffffff8102f4a3: 41 5f pop %r15
ffffffff8102f4a5: c3 retq


ffffffff8102f4a6: e8 b5 fe ff ff callq ffffffff8102f360 <wait_for_panic>
ffffffff8102f4ab: e9 23 ff ff ff jmpq ffffffff8102f3d3 <mce_panic+0x43>

ffffffff8102f4b0 <mce_timed_out>:
ffffffff8102f4b0: e8 eb 21 7d 00 callq ffffffff818016a0 <__fentry__>
ffffffff8102f4b5: 55 push %rbp
ffffffff8102f4b6: 48 89 f5 mov %rsi,%rbp

which is two instructions which gcc has put after the RET. Looking at
gcc8 output, I have that too:

ffffffff8102e39b: 0f 8e 72 ff ff ff jle ffffffff8102e313 <mce_panic+0x73>
ffffffff8102e3a1: 48 8b 5c 24 10 mov 0x10(%rsp),%rbx
ffffffff8102e3a6: 48 8b 6c 24 18 mov 0x18(%rsp),%rbp
ffffffff8102e3ab: 4c 8b 64 24 20 mov 0x20(%rsp),%r12
ffffffff8102e3b0: 4c 8b 6c 24 28 mov 0x28(%rsp),%r13
ffffffff8102e3b5: 4c 8b 74 24 30 mov 0x30(%rsp),%r14
ffffffff8102e3ba: 4c 8b 7c 24 38 mov 0x38(%rsp),%r15
ffffffff8102e3bf: 48 83 c4 40 add $0x40,%rsp
ffffffff8102e3c3: c3 retq


ffffffff8102e3c4: 49 3b 16 cmp (%r14),%rdx
ffffffff8102e3c7: 75 9e jne ffffffff8102e367 <mce_panic+0xc7>
ffffffff8102e3c9: 49 8b 4e 10 mov 0x10(%r14),%rcx
ffffffff8102e3cd: 48 39 4d 18 cmp %rcx,0x18(%rbp)
ffffffff8102e3d1: 75 94 jne ffffffff8102e367 <mce_panic+0xc7>
ffffffff8102e3d3: 49 8b 4e 08 mov 0x8(%r14),%rcx
ffffffff8102e3d7: 48 39 4d 10 cmp %rcx,0x10(%rbp)
ffffffff8102e3db: 75 8a jne ffffffff8102e367 <mce_panic+0xc7>
ffffffff8102e3dd: eb a2 jmp ffffffff8102e381 <mce_panic+0xe1>
ffffffff8102e3df: e8 8c fe ff ff callq ffffffff8102e270 <wait_for_panic>
ffffffff8102e3e4: 4d 85 f6 test %r14,%r14
ffffffff8102e3e7: 0f 85 4c 26 00 00 jne ffffffff81030a39 <mce_panic.cold.50+0xad>
ffffffff8102e3ed: 0f 1f 00 nopl (%rax)
ffffffff8102e3f0: e9 b2 25 00 00 jmpq ffffffff810309a7 <mce_panic.cold.50+0x1b>
ffffffff8102e3f5: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
ffffffff8102e3fc: 00 00 00 00

ffffffff8102e400 <__start_timer>:
ffffffff8102e400: e8 eb 2f 7d 00 callq ffffffff818013f0 <__fentry__>
ffffffff8102e405: 48 83 ec 10 sub $0x10,%rsp
ffffffff8102e409: 48 8b 05 f0 6b fd 00 mov 0xfd6bf0(%rip),%rax # ffffffff82005000 <jiffies>

but objtool doesn't complain there, for some reason.


Btw, I have a couple of those warnings on gcc9:

arch/x86/kernel/cpu/mce/core.o: warning: objtool: mce_panic()+0x11b: unreachable instruction
kernel/exit.o: warning: objtool: __x64_sys_exit_group()+0x14: unreachable instruction
fs/btrfs/extent_io.o: warning: objtool: __set_extent_bit.cold()+0xd: unreachable instruction
fs/btrfs/relocation.o: warning: objtool: add_tree_block.isra.0.cold()+0xc: unreachable instruction
net/core/skbuff.o: warning: objtool: skb_push.cold()+0x15: unreachable instruction



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