[PATCH v3 09/12] arm64: traps: Fix inconsistent faulting instruction skipping

[Dave Martin]

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>

---

**NOTE**

Despite discussions on the v2 series to the effect that the prior
behaviour is not broken, I'm now not so sure:

Taking another look, I now can't track down for example where 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 looks like it needs a closer look.

I'll leave the Fixes tags for now, so that the archaeology doesn't need
to be repeated if we conclude that this patch really is a fix.
---
 arch/arm64/kernel/traps.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 15e3c4f..44c91d4 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -268,6 +268,8 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
 	}
 }
 
+static void advance_itstate(struct pt_regs *regs);
+
 void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
 {
 	regs->pc += size;
@@ -278,6 +280,9 @@ void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
 	 */
 	if (user_mode(regs))
 		user_fastforward_single_step(current);
+
+	if (regs->pstate & PSR_MODE32_BIT)
+		advance_itstate(regs);
 }
 
 static LIST_HEAD(undef_hook);
@@ -629,19 +634,12 @@ static void advance_itstate(struct pt_regs *regs)
 	compat_set_it_state(regs, it);
 }
 
-static void arm64_compat_skip_faulting_instruction(struct pt_regs *regs,
-						   unsigned int sz)
-{
-	advance_itstate(regs);
-	arm64_skip_faulting_instruction(regs, sz);
-}
-
 static void compat_cntfrq_read_handler(unsigned int esr, struct pt_regs *regs)
 {
 	int reg = (esr & ESR_ELx_CP15_32_ISS_RT_MASK) >> ESR_ELx_CP15_32_ISS_RT_SHIFT;
 
 	pt_regs_write_reg(regs, reg, arch_timer_get_rate());
-	arm64_compat_skip_faulting_instruction(regs, 4);
+	arm64_skip_faulting_instruction(regs, 4);
 }
 
 static const struct sys64_hook cp15_32_hooks[] = {
@@ -661,7 +659,7 @@ static void compat_cntvct_read_handler(unsigned int esr, struct pt_regs *regs)
 
 	pt_regs_write_reg(regs, rt, lower_32_bits(val));
 	pt_regs_write_reg(regs, rt2, upper_32_bits(val));
-	arm64_compat_skip_faulting_instruction(regs, 4);
+	arm64_skip_faulting_instruction(regs, 4);
 }
 
 static const struct sys64_hook cp15_64_hooks[] = {
@@ -682,7 +680,7 @@ asmlinkage void __exception do_cp15instr(unsigned int esr, struct pt_regs *regs)
 		 * There is no T16 variant of a CP access, so we
 		 * always advance PC by 4 bytes.
 		 */
-		arm64_compat_skip_faulting_instruction(regs, 4);
+		arm64_skip_faulting_instruction(regs, 4);
 		return;
 	}
 
-- 
2.1.4