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Subjectgcc-8 objtool warnings
Hi Josh,

I upgraded the compiler to gcc-8.0.0 (from earlier this week) and into one
configuration that produces countless objtool warnings, starting with

kernel/panic.o: warning: objtool: nmi_panic()+0x31: sibling call from
callable instruction with modified stack frame
kernel/panic.o: warning: objtool: __warn()+0x26: sibling call from
callable instruction with modified stack frame
kernel/panic.o: warning: objtool: nmi_panic.cold.0()+0x0: call without
frame pointer save/setup
kernel/panic.o: warning: objtool: __warn.cold.1()+0x0: call without
frame pointer save/setup
arch/x86/kernel/irq_64.o: warning: objtool: handle_irq()+0x8a: sibling
call from callable instruction with modified stack frame
arch/x86/kernel/dumpstack.o: warning: objtool: oops_end()+0x83:
sibling call from callable instruction with modified stack frame
kernel/exit.o: warning: objtool: do_exit()+0x39: sibling call from
callable instruction with modified stack frame
kernel/exit.o: warning: objtool: do_exit.cold.1()+0x0: call without
frame pointer save/setup
kernel/cred.o: warning: objtool: put_cred_rcu()+0x26: sibling call
from callable instruction with modified stack frame
kernel/cred.o: warning: objtool: put_cred_rcu.cold.1()+0x0: call
without frame pointer save/setup
mm/oom_kill.o: warning: objtool: oom_kill_process.isra.4()+0xfe:
sibling call from callable instruction with modified stack frame
mm/oom_kill.o: warning: objtool: out_of_memory()+0x556: sibling call
from callable instruction with modified stack frame
mm/oom_kill.o: warning: objtool: oom_kill_process.isra.4.cold.5()+0x0:
call without frame pointer save/setup
mm/oom_kill.o: warning: objtool: out_of_memory.cold.6()+0x0: call
without frame pointer save/setup
mm/page_alloc.o: warning: objtool: warn_alloc()+0x2f: sibling call
from callable instruction with modified stack frame
mm/page_alloc.o: warning: objtool: warn_alloc.cold.20()+0x9: call
without frame pointer save/setup
arch/x86/kernel/apic/io_apic.o: warning: objtool: mp_save_irq()+0x6c:
sibling call from callable instruction with modified stack frame

See https://pastebin.com/1EehhrcP for the .config file.

I looked at a few examples, and they all seem to involve calling panic()
at the end of a a function in .text.unlikely, e.g.

0000000000000390 <put_cred_rcu>:
390: 55 push %rbp
391: 48 89 e5 mov %rsp,%rbp
394: 41 55 push %r13
396: 41 54 push %r12
398: 53 push %rbx
399: 48 89 fb mov %rdi,%rbx
39c: 4c 8d ab 60 ff ff ff lea -0xa0(%rbx),%r13
3a3: e8 00 00 00 00 callq 3a8 <put_cred_rcu+0x18>
3a4: R_X86_64_PC32 __sanitizer_cov_trace_pc-0x4
3a8: 44 8b a3 70 ff ff ff mov -0x90(%rbx),%r12d
3af: 41 81 fc 44 61 65 44 cmp $0x44656144,%r12d
3b6: 0f 85 00 00 00 00 jne 3bc <put_cred_rcu+0x2c>
3b8: R_X86_64_PC32 .text.unlikely-0x4

...

0000000000000000 <put_cred_rcu.cold.1>:
0: e8 00 00 00 00 callq 5 <put_cred_rcu.cold.1+0x5>
1: R_X86_64_PC32 __sanitizer_cov_trace_pc-0x4
5: 44 8b 8b 64 ff ff ff mov -0x9c(%rbx),%r9d
c: 48 8b 8b 68 ff ff ff mov -0x98(%rbx),%rcx
13: 44 89 e2 mov %r12d,%edx
16: 44 8b 83 60 ff ff ff mov -0xa0(%rbx),%r8d
1d: 4c 89 ee mov %r13,%rsi
20: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
23: R_X86_64_32S .rodata.str1.8+0x28
27: e8 00 00 00 00 callq 2c <__kstrtab_creds_are_invalid+0x3>
28: R_X86_64_PC32 panic-0x4

Arnd

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