Re: [PATCH 1/3] retpolines: Only enable retpoline when compiler support it

[Zhenzhong Duan]

On 2018/10/30 18:09, Peter Zijlstra wrote:
> On Tue, Oct 30, 2018 at 06:39:24PM +0900, Masahiro Yamada wrote:
> > Hi,
> >
> >
> >
> > On Tue, Oct 30, 2018 at 3:57 PM Zhenzhong Duan
> > <zhenzhong.duan@xxxxxxxxxx> wrote:
> > > Since retpoline capable compilers are widely available, make
> > > CONFIG_RETPOLINE hard depend on it.
> > >
> > > Change KBUILD to use CONFIG_RETPOLINE_SUPPORT to avoid conflict with
> > > CONFIG_RETPOLINE which is used by kernel.
> > >
> > > With all that stuff, the check of RETPOLINE is changed to
> > > CONFIG_RETPOLINE.
> > >
> > > This change is based on suggestion in https://lkml.org/lkml/2018/9/18/1016
> > >
> > > Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Borislav Petkov <bp@xxxxxxx>
> > > Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > > Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > > Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > > Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
> > > ---
> >
> >
> > Instead of adding another CONFIG option,
> > does it make sense to add compiler support checks
> > to 'depends on' syntax ?
> >
> >
> > config RETPOLINE
> >           bool "Avoid speculative indirect branches in kernel"
> >           depends on $(cc-option,-mindirect-branch=thunk-extern
> > -mindirect-branch-register) || \
> >                      $(cc-option,-mretpoline-external-thunk)
> >           default y
> >           select STACK_VALIDATION if HAVE_STACK_VALIDATION

Looks better, thanks for suggestion.

> That seems to be what we did for stackprotector, which is similar in
> that it used to fail the build. So yes, this seems sane.

Should I add a scripts/gcc-x86_64-has-retpoline.sh like what 
stackprotector does as below or there is a simpler way?

config CC_HAS_SANE_STACKPROTECTOR
        bool
        default 
$(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC)) if 
64BIT
        default 
$(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
        help
           We have to make sure stack protector is unconditionally 
disabled if
           the compiler produces broken code.

Thanks
Zhenzhong