Subject: [PATCH v3 2/2] arm64: Fix single stepping in kernel traps

From: [email-masked] (Will Deacon)
Date: Tue, 24 Oct 2017 15:27:02 +0100

Hi Julien,

On Thu, Oct 12, 2017 at 02:43:03PM +0100, Julien Thierry wrote:
> Software Step exception is missing after stepping a trapped instruction.
> 
> Ensure SPSR.SS gets set to 0 after emulating/skipping a trapped instruction
> before doing ERET.

Curious, but how did you spot this?

> Signed-off-by: Julien Thierry <[email-masked]>
> Reviewed-by: Alex Benn?e <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Mark Rutland <[email-masked]>
> ---
>  arch/arm64/include/asm/insn.h        |  5 +++++
>  arch/arm64/include/asm/traps.h       |  6 ++++++
>  arch/arm64/kernel/armv8_deprecated.c |  8 ++++----
>  arch/arm64/kernel/cpufeature.c       |  2 +-
>  arch/arm64/kernel/traps.c            | 21 ++++++++++++++++-----
>  5 files changed, 32 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
> index 4214c38..de5e31a 100644
> --- a/arch/arm64/include/asm/insn.h
> +++ b/arch/arm64/include/asm/insn.h
> @@ -22,6 +22,11 @@
> 
>  /* A64 instructions are always 32 bits. */
>  #define	AARCH64_INSN_SIZE		4
> +#define	AARCH32_INSN_SIZE		4
> +
> +/* Thumb/Thumb2 instruction sizes */
> +#define	AARCH32_T32_INSN_SIZE		4
> +#define	AARCH32_T16_INSN_SIZE		2

The naming here is a bit misleading, since T32 is an instruction set
consisting of both 32-bit and 16-bit instructions. For now, you might just
be better off having the caller pass in an immediate directly rather than
add these confusing #defines.

>  #ifndef __ASSEMBLY__
>  /*
> diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
> index d131501..67b971e 100644
> --- a/arch/arm64/include/asm/traps.h
> +++ b/arch/arm64/include/asm/traps.h
> @@ -37,6 +37,12 @@ struct undef_hook {
> 
>  void arm64_notify_segfault(struct pt_regs *regs, unsigned long addr);
> 
> +/*
> + * Move regs->pc to next instruction and do necessary setup before it
> + * is executed.
> + */
> +void arm64_setup_next_instr(struct pt_regs *regs, unsigned long size);
> +
>  static inline int __in_irqentry_text(unsigned long ptr)
>  {
>  	return ptr >= (unsigned long)&__irqentry_text_start &&
> diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
> index d06fbe4..2808e56 100644
> --- a/arch/arm64/kernel/armv8_deprecated.c
> +++ b/arch/arm64/kernel/armv8_deprecated.c
> @@ -431,7 +431,7 @@ static int swp_handler(struct pt_regs *regs, u32 instr)
>  	pr_warn_ratelimited("\"%s\" (%ld) uses obsolete SWP{B} instruction at 0x%llx\n",
>  			current->comm, (unsigned long)current->pid, regs->pc);
> 
> -	regs->pc += 4;
> +	arm64_setup_next_instr(regs, AARCH32_INSN_SIZE);
>  	return 0;
> 
>  fault:
> @@ -512,7 +512,7 @@ static int cp15barrier_handler(struct pt_regs *regs, u32 instr)
>  	pr_warn_ratelimited("\"%s\" (%ld) uses deprecated CP15 Barrier instruction at 0x%llx\n",
>  			current->comm, (unsigned long)current->pid, regs->pc);
> 
> -	regs->pc += 4;
> +	arm64_setup_next_instr(regs, AARCH32_INSN_SIZE);
>  	return 0;
>  }
> 
> @@ -586,14 +586,14 @@ static int compat_setend_handler(struct pt_regs *regs, u32 big_endian)
>  static int a32_setend_handler(struct pt_regs *regs, u32 instr)
>  {
>  	int rc = compat_setend_handler(regs, (instr >> 9) & 1);
> -	regs->pc += 4;
> +	arm64_setup_next_instr(regs, AARCH32_INSN_SIZE);
>  	return rc;
>  }
> 
>  static int t16_setend_handler(struct pt_regs *regs, u32 instr)
>  {
>  	int rc = compat_setend_handler(regs, (instr >> 3) & 1);
> -	regs->pc += 2;
> +	arm64_setup_next_instr(regs, AARCH32_T16_INSN_SIZE);
>  	return rc;
>  }
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 21e2c95..235834e 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1287,7 +1287,7 @@ static int emulate_mrs(struct pt_regs *regs, u32 insn)
>  	if (!rc) {
>  		dst = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RT, insn);
>  		pt_regs_write_reg(regs, dst, val);
> -		regs->pc += 4;
> +		arm64_setup_next_instr(regs, AARCH64_INSN_SIZE);
>  	}
> 
>  	return rc;
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 5ea4b85..f93b33f 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -293,6 +293,17 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
>  	}
>  }
> 
> +void arm64_setup_next_instr(struct pt_regs *regs, unsigned long size)
> +{

This strikes me as a pretty broadly named function to be exposing like this.
How about: arm64_skip_faulting_instruction instead?

Will