On Tue, 20 Feb 2024 11:20:31 +0000,
Joey Gouly <joey.gouly@xxxxxxx> wrote:
>
> On Mon, Feb 19, 2024 at 09:20:02AM +0000, Marc Zyngier wrote:
> > The unsuspecting kernel tinkerer can be easily confused into
> > writing something that looks like this:
> >
> > ikey.lo = __vcpu_sys_reg(vcpu, SYS_APIAKEYLO_EL1);
> >
> > which seems vaguely sensible, until you realise that the second
> > parameter is the encoding of a sysreg, and not the index into
> > the vcpu sysreg file... Debugging what happens in this case is
>
> type safety :(
Are you advocating for making everything a struct? Or something else?
>
> > an interesting exercise in head<->wall interactions.
> >
> > As they often say: "Any resemblance to actual persons, living
> > or dead, or actual events is purely coincidental".
> >
> > In order to save people's time, add some compile-time hardening
> > that will at least weed out the "stupidly out of range" values.
> > This will *not* catch anything that isn't a compile-time constant.
> >
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 9 ++++++++-
> > 1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 181fef12e8e8..a5ec4c7d3966 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -895,7 +895,7 @@ struct kvm_vcpu_arch {
> > * Don't bother with VNCR-based accesses in the nVHE code, it has no
> > * business dealing with NV.
> > */
> > -static inline u64 *__ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
> > +static inline u64 *___ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
>
> When in doubt, add more underscores!
That's the one true way.
>
> > {
> > #if !defined (__KVM_NVHE_HYPERVISOR__)
> > if (unlikely(cpus_have_final_cap(ARM64_HAS_NESTED_VIRT) &&
> > @@ -905,6 +905,13 @@ static inline u64 *__ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
> > return (u64 *)&ctxt->sys_regs[r];
> > }
> >
> > +#define __ctxt_sys_reg(c,r) \
> > + ({ \
> > + BUILD_BUG_ON(__builtin_constant_p(r) && \
> > + (r) >= NR_SYS_REGS); \
> > + ___ctxt_sys_reg(c, r); \
> > + })
>
> I'm assuming the extra macro layer is to try make __builtin_constant_p() as
> effective as possible? Otherwise maybe it relies on the compiler inling the
> ___ctxt_sys_reg() function?
It's not about efficiency. It's about making it *work*. Otherwise,
lack of inlining will screw you over, and you may not check anything.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
