Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f

[Peter Zijlstra]

On Wed, Feb 07, 2018 at 11:01:29AM -0800, Linus Torvalds wrote:
> On Wed, Feb 7, 2018 at 10:38 AM, Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
> > On 02/07/2018 10:13 AM, Linus Torvalds wrote:
> >>
> >> That said, intel only _documents_ UD2 (0f 0b).
> >
> > Intel Order Number: 325383-064US, October 2017, documents UD0, UD1, and UD2.
> > Section A.2.5, Table A-1, says:
> 
> Ahh, I had an older version.
> 
> Looking at the latest one I can find (325462-065US), it does specify
> that it has a modrm byte:
> 
>   0F FF /r   UD0 1 r32, r/m32
> 
> so I think that our opcode maps are wrong, and it's a bit dangerous to
> put random constants right after the UD0.
> 
> Maybe we should make our use of UD0 have a third byte: add a harmless
> modrm byte before the warning constants?
> 
> But yes, at least my objdump just thinks it's a bad 2-byte sequence,
> and doesn't look at any modrm bytes at all.

Look what my objdump does:

$ objdump --version
GNU objdump (GNU Binutils for Debian) 2.29.90.20180122

$ objdump -dr defconfig-build/kernel/sched/core.o | grep ud0
    183a:       0f ff 65 48             ud0    0x48(%rbp),%esp
    1881:       0f ff c3                ud0    %ebx,%eax
    18b1:       0f ff 89 c0 48 0f a3    ud0    -0x5cf0b740(%rcx),%ecx
    1940:       0f ff 89 c0 48 0f a3    ud0    -0x5cf0b740(%rcx),%ecx
    19c2:       0f ff 89 c0 48 0f a3    ud0    -0x5cf0b740(%rcx),%ecx

So yeah, we're screwed :-(

Adding a harmless modr/m to our UD0 would grow the kernel image. ARGGH