Re: Problems compiling with KCFLAGS="-frecord-gcc-switches"

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Oct 03, 2017 at 01:00:23PM -0500, Josh Poimboeuf wrote:
> On Tue, Oct 03, 2017 at 10:54:57AM -0700, Guenter Roeck wrote:
> > Hi Josh,
> > 
> > On Fri, Sep 29, 2017 at 03:46:09PM -0500, Josh Poimboeuf wrote:
> > > On Fri, Sep 29, 2017 at 01:00:56PM -0700, Guenter Roeck wrote:
> > > > Hi Josh,
> > > > 
> > > > when trying to compile an image with KCFLAGS="-frecord-gcc-switches",
> > > > I get the folllowing build warning/error.
> > > > 
> > > > make allmodconfig
> > > > KCFLAGS="-frecord-gcc-switches" make arch/x86/kvm/emulate.o
> > > > ./tools/objtool/objtool check --no-unreachable "arch/x86/kvm/emulate.o"
> > > > 
> > > > arch/x86/kvm/emulate.o: warning:
> > > > 	objtool: .GCC.command.line+0x0: special: can't find new instruction
> > > > 
> > > > Building a full image aborts with:
> > > > 
> > > > WARNING: arch/x86/kvm/kvm.o(__ex_table+0x4c): Section mismatch in reference
> > > > 	from the (unknown reference) (unknown)
> > > > 	to the variable .GCC.command.line:kvm_fastop_exception
> > > > FATAL: The relocation at __ex_table+0x4c references
> > > > section ".GCC.command.line" which is not executable, IOW
> > > > the kernel will fault if it ever tries to
> > > > jump to it.  Something is seriously wrong
> > > > and should be fixed.
> > > > make[2]: *** [arch/x86/kvm/kvm.o] Error 1
> > > > 
> > > > Any idea what might cause this problem ?
> > > 
> > > I think this is a GCC bug with the -frecord-gcc-switches feature.  Due
> > > to some ambiguity in some inline asm in emulate.c, it's placing some
> > > kernel code in the .GCC.command.line section.
> > > 
> > > This seems to fix it:
> > > 
> > > 
> > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > > index a36254cbf776..d90cdc77e077 100644
> > > --- a/arch/x86/kvm/emulate.c
> > > +++ b/arch/x86/kvm/emulate.c
> > > @@ -425,8 +425,10 @@ static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *));
> > >  	#op " %al \n\t" \
> > >  	FOP_RET
> > >  
> > > -asm(".global kvm_fastop_exception \n"
> > > -    "kvm_fastop_exception: xor %esi, %esi; ret");
> > > +asm(".pushsection .fixup, \"ax\"\n"
> > > +    ".global kvm_fastop_exception \n"
> > > +    "kvm_fastop_exception: xor %esi, %esi; ret\n"
> > > +    ".popsection");
> > >  
> > >  FOP_START(setcc)
> > >  FOP_SETCC(seto)
> > 
> > Do you plan to submit a patch for this, or do you want me to do it ?
> 
> Sorry, I've been juggling a bunch of other bugs.  I'll submit it soon.
> 
Welcome to the club ...

Thanks!
Guenter