Subject: [PATCH REPOST 2/3] arm64: Fix single stepping in kernel traps

From: [email-masked] (Alex Bennée)
Date: Wed, 04 Oct 2017 13:01:30 +0100

Julien Thierry <[email-masked]> writes:

> On 04/10/17 00:45, Alex Benn?e wrote:
>>
>> Julien Thierry <[email-masked]> writes:
>>
>>> 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.
>>>
>>> Signed-off-by: Julien Thierry <[email-masked]>
>>> Cc: Catalin Marinas <[email-masked]>
>>> Cc: Will Deacon <[email-masked]>
>>> Cc: Mark Rutland <[email-masked]>
>>>
>>> ---
>>>   arch/arm64/include/asm/traps.h       |  2 ++
>>>   arch/arm64/kernel/armv8_deprecated.c |  8 ++++----
>>>   arch/arm64/kernel/cpufeature.c       |  2 +-
>>>   arch/arm64/kernel/traps.c            | 21 ++++++++++++++++-----
>>>   4 files changed, 23 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
>>> index d131501..dd7affb 100644
>>> --- a/arch/arm64/include/asm/traps.h
>>> +++ b/arch/arm64/include/asm/traps.h
>>> @@ -37,6 +37,8 @@ struct undef_hook {
>>>
>>>   void arm64_notify_segfault(struct pt_regs *regs, unsigned long addr);
>>>
>>> +void arm64_skip_trapped_instr(struct pt_regs *regs, unsigned long size);
>>> +
>>
>> Personally I think this is a poor name as it implies there is no effect
>> from the operation. I suspect arm64_update_regs is a little too generic
>> through. Naming things as ever is hard....
>
> What about arm64_step_trapped_instr? Don't know if it is much better
> but maybe less misleading than just "skip".
>
> Or arm64_setup_next_instr?

Yes, I like arm64_setup_next_instr() better.

>
>>
>>>   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 f0e6d71..1f38208 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_skip_trapped_instr(regs, 4);
>>>   	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_skip_trapped_instr(regs, 4);
>>>   	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_skip_trapped_instr(regs, 4);
>>>   	return rc;
>>>   }
>>
>> Why didn't we use AARCH64_INSN_SIZE for these here?
>
> Because these are AARCH32 instructions, although it is the same size,
> I thought it would be misleading to use AARCH64 macro.

Ahh I was thrown by the fact we are in armv8_deprecated.c but of course
AArch32 is part of armv8...

>
>>
>>>
>>>   static int t16_setend_handler(struct pt_regs *regs, u32 instr)
>>>   {
>>>   	int rc = compat_setend_handler(regs, (instr >> 3) & 1);
>>> -	regs->pc += 2;
>>> +	arm64_skip_trapped_instr(regs, 2);
>>>   	return rc;
>>>   }
>>
>> I guess we don't have an equivalent AARCH32_T16_INSN_SIZE?
>>
>
> Yes, I was a bit hesitant because thumb2 can have 16 and 32bits
> instructions, but I guess having "T16" in the macro name makes it
> clear it should be used only on 16bits instructions.
>
> I'll add this define, and might as well add AARCH32_INSN_SIZE.

Maybe AARCH32_T32_INSN_SIZE and AARCH32_T16_INSN_SIZE? Then it is clear
there are two encodings.

>
>>>
>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>>> index cd52d36..2956d5a 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_skip_trapped_instr(regs, AARCH64_INSN_SIZE);
>>>   	}
>>>
>>>   	return rc;
>>> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
>>> index 5ea4b85..ed9d856 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_skip_trapped_instr(struct pt_regs *regs, unsigned long size)
>>> +{
>>> +	regs->pc += size;
>>> +
>>> +	/*
>>> +	 * If we were single stepping, we want to get the step exception after
>>> +	 * we return from the skipped exception
>>> +	 */
>>> +	regs->pstate &= ~DBG_SPSR_SS;
>>> +}
>>> +
>>>   static LIST_HEAD(undef_hook);
>>>   static DEFINE_RAW_SPINLOCK(undef_lock);
>>>
>>> @@ -480,7 +491,7 @@ static void user_cache_maint_handler(unsigned int esr, struct pt_regs *regs)
>>>   	if (ret)
>>>   		arm64_notify_segfault(regs, address);
>>>   	else
>>> -		regs->pc += 4;
>>> +		arm64_skip_trapped_instr(regs, AARCH64_INSN_SIZE);
>>>   }
>>>
>>>   static void ctr_read_handler(unsigned int esr, struct pt_regs *regs)
>>> @@ -490,7 +501,7 @@ static void ctr_read_handler(unsigned int esr, struct pt_regs *regs)
>>>
>>>   	pt_regs_write_reg(regs, rt, val);
>>>
>>> -	regs->pc += 4;
>>> +	arm64_skip_trapped_instr(regs, AARCH64_INSN_SIZE);
>>>   }
>>>
>>>   static void cntvct_read_handler(unsigned int esr, struct pt_regs *regs)
>>> @@ -498,7 +509,7 @@ static void cntvct_read_handler(unsigned int esr, struct pt_regs *regs)
>>>   	int rt = (esr & ESR_ELx_SYS64_ISS_RT_MASK) >> ESR_ELx_SYS64_ISS_RT_SHIFT;
>>>
>>>   	pt_regs_write_reg(regs, rt, arch_counter_get_cntvct());
>>> -	regs->pc += 4;
>>> +	arm64_skip_trapped_instr(regs, AARCH64_INSN_SIZE);
>>>   }
>>>
>>>   static void cntfrq_read_handler(unsigned int esr, struct pt_regs *regs)
>>> @@ -506,7 +517,7 @@ static void cntfrq_read_handler(unsigned int esr, struct pt_regs *regs)
>>>   	int rt = (esr & ESR_ELx_SYS64_ISS_RT_MASK) >> ESR_ELx_SYS64_ISS_RT_SHIFT;
>>>
>>>   	pt_regs_write_reg(regs, rt, arch_timer_get_rate());
>>> -	regs->pc += 4;
>>> +	arm64_skip_trapped_instr(regs, AARCH64_INSN_SIZE);
>>>   }
>>>
>>>   struct sys64_hook {
>>> @@ -761,7 +772,7 @@ static int bug_handler(struct pt_regs *regs, unsigned int esr)
>>>   	}
>>>
>>>   	/* If thread survives, skip over the BUG instruction and continue: */
>>> -	regs->pc += AARCH64_INSN_SIZE;	/* skip BRK and resume */
>>> +	arm64_skip_trapped_instr(regs, AARCH64_INSN_SIZE);
>>>   	return DBG_HOOK_HANDLED;
>>>   }
>>
>> It would be nice to find a better name but otherwise:
>>
>> Reviewed-by: Alex Benn?e <[email-masked]>
>>
>
> Thanks,

--
Alex Benn?e