[PATCH v4 02/14] arm64: Allow mismatched 32-bit EL0 support

[Qais Yousef]

On 12/01/20 16:56, Will Deacon wrote:
> On Fri, Nov 27, 2020 at 01:09:41PM +0000, Qais Yousef wrote:
> > On 11/24/20 15:50, Will Deacon wrote:
> > > When confronted with a mixture of CPUs, some of which support 32-bit
> > 
> > Confronted made me laugh, well chosen word! :D
> > 
> > For some reason made me think of this :p
> > 
> > 	https://www.youtube.com/watch?v=NJbXPzSPzxc&t=1m33s
> 
> I think it just about sums it up!
> 
> > > applications and others which don't, we quite sensibly treat the system
> > > as 64-bit only for userspace and prevent execve() of 32-bit binaries.
> > > 
> > > Unfortunately, some crazy folks have decided to build systems like this
> > > with the intention of running 32-bit applications, so relax our
> > > sanitisation logic to continue to advertise 32-bit support to userspace
> > > on these systems and track the real 32-bit capable cores in a cpumask
> > > instead. For now, the default behaviour remains but will be tied to
> > > a command-line option in a later patch.
> > > 
> > > Signed-off-by: Will Deacon <will at kernel.org>
> > > ---
> > >  arch/arm64/include/asm/cpucaps.h    |   2 +-
> > >  arch/arm64/include/asm/cpufeature.h |   8 ++-
> > >  arch/arm64/kernel/cpufeature.c      | 106 ++++++++++++++++++++++++++--
> > >  3 files changed, 107 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> > > index e7d98997c09c..e6f0eb4643a0 100644
> > > --- a/arch/arm64/include/asm/cpucaps.h
> > > +++ b/arch/arm64/include/asm/cpucaps.h
> > > @@ -20,7 +20,7 @@
> > >  #define ARM64_ALT_PAN_NOT_UAO			10
> > >  #define ARM64_HAS_VIRT_HOST_EXTN		11
> > >  #define ARM64_WORKAROUND_CAVIUM_27456		12
> > > -#define ARM64_HAS_32BIT_EL0			13
> > > +#define ARM64_HAS_32BIT_EL0_DO_NOT_USE		13
> > 
> > nit: would UNUSED be better here? Worth adding a comment as to why too?
> 
> UNUSED sounds like you could delete it, but I'll add a comment.

+1, thanks.

> 
> > >  #define ARM64_HARDEN_EL2_VECTORS		14
> > >  #define ARM64_HAS_CNP				15
> > >  #define ARM64_HAS_NO_FPSIMD			16
> > 
> > [...]
> > 
> > > +static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
> > > +{
> > > +	if (!has_cpuid_feature(entry, scope))
> > > +		return allow_mismatched_32bit_el0;
> > 
> > If a user passes the command line by mistake on a 64bit only system, this will
> > return true. I'll be honest, I'm not entirely sure what the impact is. I get
> > lost in the features maze. It is nicely encapsulated, but hard to navigate for
> > the none initiated :-)
> 
> The thing is, we can't generally detect a 64-bit-only system because a
> 32-bit-capable CPU could be hotplugged on late. So passing this option
> just controls what the behaviour is at the point that the 32-bit-capable
> CPU appears. If one doesn't appear, then there won't be a difference.

Okay, thanks for confirming.

Cheers

--
Qais Yousef