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

From: Mark Rutland <[email-masked]>
Date: Fri, 11 Oct 2019 16:24:53 +0100

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 <[email-masked]>
> ---
>  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.

Thanks,
Mark.

> 
> 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
> 

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