Re: [PATCH v2 09/12] arm64: traps: Fix inconsistent faulting instruction skipping

[Dave Martin]

On Tue, Oct 15, 2019 at 05:49:05PM +0100, Dave Martin wrote:
> On Tue, Oct 15, 2019 at 05:42:04PM +0100, Mark Rutland wrote:
> > On Tue, Oct 15, 2019 at 04:21:09PM +0100, Dave Martin wrote:
> > > On Fri, Oct 11, 2019 at 04:24:53PM +0100, Mark Rutland wrote:
> > > > On Thu, Oct 10, 2019 at 07:44:37PM +0100, Dave Martin wrote:
> > > > > Correct skipping of an instruction on AArch32 works a bit
> > > > > differently from AArch64, mainly due to the different CPSR/PSTATE
> > > > > semantics.
> > > > > 
> > > > > There have been various attempts to get this right.  Currenty
> > > > > arm64_skip_faulting_instruction() mostly does the right thing, but
> > > > > does not advance the IT state machine for the AArch32 case.
> > > > > 
> > > > > arm64_compat_skip_faulting_instruction() handles the IT state
> > > > > machine but is local to traps.c, and porting other code to use it
> > > > > will make a mess since there are some call sites that apply for
> > > > > both the compat and native cases.
> > > > > 
> > > > > Since manual instruction skipping implies a trap, it's a relatively
> > > > > slow path.
> > > > > 
> > > > > So, make arm64_skip_faulting_instruction() handle both compat and
> > > > > native, and get rid of the arm64_compat_skip_faulting_instruction()
> > > > > special case.
> > > > > 
> > > > > Fixes: 32a3e635fb0e ("arm64: compat: Add CNTFRQ trap handler")
> > > > > Fixes: 1f1c014035a8 ("arm64: compat: Add condition code checks and IT advance")
> > > > > Fixes: 6436beeee572 ("arm64: Fix single stepping in kernel traps")
> > > > > Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm")
> > > > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> > > > > ---
> > > > >  arch/arm64/kernel/traps.c | 18 ++++++++----------
> > > > >  1 file changed, 8 insertions(+), 10 deletions(-)
> > > > 
> > > > This looks good to me; it's certainly easier to reason about.
> > > > 
> > > > I couldn't spot a place where we do the wrong thing today, given AFAICT
> > > > all the instances in arch/arm64/kernel/armv8_deprecated.c would be
> > > > UNPREDICTABLE within an IT block.
> > > > 
> > > > It might be worth calling out an example in the commit message to
> > > > justify the fixes tags.
> > > 
> > > IIRC I found no bug here; rather we have pointlessly fragmented code,
> > > so I followed the "if fixing the same bug in multiple places, merge
> > > those places so you need only fix it in one place next time" rule.
> > 
> > Sure thing, that makes sense to me.
> > 
> > > Since arm64_skip_faulting_instruction() is most of the way to being
> > > generically usable anyway, this series merges all the special-case
> > > handling into it.
> > > 
> > > I could add something like
> > > 
> > > --8<--
> > > 
> > > This allows this fiddly operation to be maintained in a single
> > > place rather than trying to maintain fragmented versions spread
> > > around arch/arm64.
> > > 
> > > -->8--
> > > 
> > > Any good?
> > 
> > My big concern is that the commit message reads as a fix, implying that
> > there's an existing correctness bug. I think that simplifying it to make
> > it clearer that it's a cleanup/improvement would be preferable.
> > 
> > How about:
> > 
> > | arm64: unify native/compat instruction skipping
> > |
> > | Skipping of an instruction on AArch32 works a bit differently from
> > | AArch64, mainly due to the different CPSR/PSTATE semantics.
> > |
> > | Currently arm64_skip_faulting_instruction() is only suitable for
> > | AArch64, and arm64_compat_skip_faulting_instruction() handles the IT
> > | state machine but is local to traps.c.
> > | 
> > | Since manual instruction skipping implies a trap, it's a relatively
> > | slow path.
> > | 
> > | So, make arm64_skip_faulting_instruction() handle both compat and
> > | native, and get rid of the arm64_compat_skip_faulting_instruction()
> > | special case.
> > |
> > | Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> 
> And drop the Fixes tags.  Yes, I think that's reasonable.
> 
> I think I was probably glossing over the fact that we don't need to get
> the ITSTATE machine advance correct for the compat insn emulation; as
> you say, I can't see what else this patch "fixes".
> 
> > With that, feel free to add:
> >
> > Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> Thanks!
> 
> > We could even point out that the armv8_deprecated cases are
> > UNPREDICTABLE in an IT block, and correctly emulated either way.
> 
> Yes, I'll add something along those lines.

Taking another look, I now can't track down where e.g., SWP in an IT
block is specified to be UNPREDICTABLE.  I only see e.g.,
ARM DDI 0487E.a Section 1.8.2 ("F1.8.2 Partial deprecation of IT"),
which only deprecates the affected instructions.

The legacy AArch32 SWP{B} insn is obsoleted by ARMv8, but the whole
point of the armv8_deprecated stuff is to provide some backwards
compatiblity with v7.


So, this needs a closer look.

I'll leave the Fixes tags for now, so that the archaeology doesn't need
to redone if we decide that this patch does fix incorrect behaviour.

Cheers
---Dave