RE: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f

[David Laight <David.Laight@ACULAB.COM>]

From: Peter Zijlstra
> Sent: 08 February 2018 09:13
...
> > > Yeah, note says UD0 didn't eat a ModRM byte on old CPUs. But then that
> > > changed too. Fun stuff changing insn encoding underway.
> > >
> > > So if we opt for adding a ModRM byte, could a 0x90 NOP work so that it
> > > doesn't shit itself on those old CPUs?
> >
> > We could just also decide that the only thing that the modrm bytes of
> > UD0 actually *affect* is how the CPU might act for a page-crossing
> > instruction.
> >
> > Because I think that's the only semantic difference: if it's a
> > page-crosser, the instruction could take a page fault before raising
> > the #UD.
> >
> > Is there any other decode issue we might want to look out for?
> 
> _The_ problem is that new binutils cannot sanely decode any function
> that has a WARN in (this very much includes perf annotate):
> 
> old:
> 
> 00000000000016a0 <copy_overflow>:
>     16a0:       48 89 f2                mov    %rsi,%rdx
>     16a3:       89 fe                   mov    %edi,%esi
>     16a5:       48 c7 c7 00 00 00 00    mov    $0x0,%rdi
>                         16a8: R_X86_64_32S      .rodata.str1.8+0x288
>     16ac:       e8 00 00 00 00          callq  16b1 <copy_overflow+0x11>
>                         16ad: R_X86_64_PC32     __warn_printk-0x4
>     16b1:       0f ff                   (bad)
>     16b3:       c3                      retq
>     16b4:       66 90                   xchg   %ax,%ax
>     16b6:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
>     16bd:       00 00 00
> 
> new:
> 
> 00000000000016a0 <copy_overflow>:
>     16a0:       48 89 f2                mov    %rsi,%rdx
>     16a3:       89 fe                   mov    %edi,%esi
>     16a5:       48 c7 c7 00 00 00 00    mov    $0x0,%rdi
>                         16a8: R_X86_64_32S      .rodata.str1.8+0x288
>     16ac:       e8 00 00 00 00          callq  16b1 <copy_overflow+0x11>
>                         16ad: R_X86_64_PC32     __warn_printk-0x4
>     16b1:       0f ff c3                ud0    %ebx,%eax
>     16b4:       66 90                   xchg   %ax,%ax
>     16b6:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
>     16bd:       00 00 00
> 
> 
> I went through the register opcodes and matched it against the ModR/M
> encoding, and the best option I've found so far is using 0xd6 as the
> next byte.

Wouldn't 0xc3 work as well.
A retq is probably better than an extra (bad).

Actually objdump ought to be more explicit than (bad) for the explicit UD0/1

	David