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SubjectRe: [v8,02/12] objtool: Allow alternatives to be ignored
On Mon, Jan 29, 2018 at 09:15:26AM -0800, Guenter Roeck wrote:
> On Sun, Jan 28, 2018 at 03:06:42PM -0600, 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>:
> > > >    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.
> >
> > 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
> >
>
> 'objtool: Improve retpoline alternative handling' works for me.

Thanks! I'll give you a

Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>

--
Josh

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