gcc-8 objtool warnings

[Arnd Bergmann]

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