Re: [v8,02/12] objtool: Allow alternatives to be ignored

[Guenter Roeck]

Hi David,

On Mon, Jan 22, 2018 at 07:34:04PM +0000, David Woodhouse wrote:
> On Thu, 2018-01-18 at 11:41 -0800, Guenter Roeck wrote:
> > 
> > > Not sure, does your gcc have retpolines?  Give me your .o file and I can
> > > diagnose it.
> > > 
> > Yes, it does, only it is the gcc from the Google toolchain which may
> > generate different code than the upstream version.
> > 
> > I attached an affected object file. Please let me know if there is anything else
> > I can do to help.
> Disassembly of section .text.__x86.indirect_thunk:
> 
> 0000000000000000 <__x86.indirect_thunk>:
>    0:	e8 04 00 00 00       	callq  9 <__x86.indirect_thunk+0x9>
>    5:	f3 90                	pause  
>    7:	eb fc                	jmp    5 <__x86.indirect_thunk+0x5>
>    9:	48 8d 64 24 08       	lea    0x8(%rsp),%rsp
>    e:	c3                   	retq   
> 
> That has the old-style CET-incompatible retpoline in a COMDAT section
> in the .o file. What compiler options are being used for that? The
> kernel should only use retpoline if GCC supports both of
> -mindirect-branch=thunk-extern and -mindirect-branch-register, and this
> compiler is doing *neither* of those. 

It uses "-mindirect-branch=thunk -mindirect-branch-loop=pause
-fno-jump-tables", though I don't know if that even exists in
upstream gcc (it is the gcc use for Chrome OS builds). I'll pass
your feedback to our compiler team.

Either case, I think it is less than optimal that objtool crashes
with _any_ object code.

Thanks,
Guenter