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

[Guenter Roeck]

On 01/28/2018 01:06 PM, Josh Poimboeuf wrote:
> On Mon, Jan 22, 2018 at 12:25:22PM -0800, Guenter Roeck wrote:
> > 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>:
> > >  ÂÂÂ0:	e8 04 00 00 00ÂÂÂÂÂÂÂ	callqÂÂ9 <__x86.indirect_thunk+0x9>
> > >  ÂÂÂ5:	f3 90ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ	pause
> > >  ÂÂÂ7:	eb fcÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ	jmpÂÂÂÂ5 <__x86.indirect_thunk+0x5>
> > >  ÂÂÂ9:	48 8d 64 24 08ÂÂÂÂÂÂÂ	leaÂÂÂÂ0x8(%rsp),%rsp
> > >  ÂÂÂe:	c3ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ	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.
>
> I've got a pending fix for this, so that objtool doesn't seg fault, and
> instead prints out a warning:
>
>    quirks.o: warning: objtool: efi_delete_dummy_variable()+0x99: unsupported intra-function call
>    quirks.o: warning: objtool: If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE.
>
> The code is here, along with a few more fixes:
>
>    https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=TODO-objtool-seg-fault

Excellent. I'll give it a try tomorrow.

Thanks for looking into this!

Guenter