Subject: [PATCH V2 2/5] arm64: Allow hw watchpoint at varied offset from base address

From: [email-masked] (Will Deacon)
Date: Tue, 8 Nov 2016 03:26:58 +0000

Hi Pratyush,

On Thu, Oct 20, 2016 at 11:18:14AM +0530, Pratyush Anand wrote:
> ARM64 hardware supports watchpoint at any double word aligned address.
> However, it can select any consecutive bytes from offset 0 to 7 from that
> base address. For example, if base address is programmed as 0x420030 and
> byte select is 0x1C, then access of 0x420032,0x420033 and 0x420034 will
> generate a watchpoint exception.
> 
> Currently, we do not have such modularity. We can only program byte,
> halfword, word and double word access exception from any base address.
> 
> This patch adds support to overcome above limitations.
> 
> Signed-off-by: Pratyush Anand <[email-masked]>
> ---
>  arch/arm64/include/asm/hw_breakpoint.h |  2 +-
>  arch/arm64/kernel/hw_breakpoint.c      | 45 ++++++++++++++++------------------
>  arch/arm64/kernel/ptrace.c             |  5 ++--
>  3 files changed, 25 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/hw_breakpoint.h b/arch/arm64/include/asm/hw_breakpoint.h
> index 115ea2a64520..4f4e58bee9bc 100644
> --- a/arch/arm64/include/asm/hw_breakpoint.h
> +++ b/arch/arm64/include/asm/hw_breakpoint.h
> @@ -118,7 +118,7 @@ struct perf_event;
>  struct pmu;
>  
>  extern int arch_bp_generic_fields(struct arch_hw_breakpoint_ctrl ctrl,
> -				  int *gen_len, int *gen_type);
> +				  int *gen_len, int *gen_type, int *offset);
>  extern int arch_check_bp_in_kernelspace(struct perf_event *bp);
>  extern int arch_validate_hwbkpt_settings(struct perf_event *bp);
>  extern int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index 26a6bf77d272..3c2b96803eba 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -349,7 +349,7 @@ int arch_check_bp_in_kernelspace(struct perf_event *bp)
>   * to generic breakpoint descriptions.
>   */
>  int arch_bp_generic_fields(struct arch_hw_breakpoint_ctrl ctrl,
> -			   int *gen_len, int *gen_type)
> +			   int *gen_len, int *gen_type, int *offset)
>  {
>  	/* Type */
>  	switch (ctrl.type) {
> @@ -369,8 +369,10 @@ int arch_bp_generic_fields(struct arch_hw_breakpoint_ctrl ctrl,
>  		return -EINVAL;
>  	}
>  
> +	*offset = ffs(ctrl.len) - 1;

Do we want to fail the length == 0 case early? If you do add that check,
then you can use __ffs here instead.

>  	/* Len */
> -	switch (ctrl.len) {
> +	switch (ctrl.len >> *offset) {
>  	case ARM_BREAKPOINT_LEN_1:
>  		*gen_len = HW_BREAKPOINT_LEN_1;
>  		break;
> @@ -517,18 +519,17 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
>  		default:
>  			return -EINVAL;
>  		}
> -
> -		info->address &= ~alignment_mask;
> -		info->ctrl.len <<= offset;
>  	} else {
>  		if (info->ctrl.type == ARM_BREAKPOINT_EXECUTE)
>  			alignment_mask = 0x3;
>  		else
>  			alignment_mask = 0x7;
> -		if (info->address & alignment_mask)
> -			return -EINVAL;
> +		offset = info->address & alignment_mask;
>  	}
>  
> +	info->address &= ~alignment_mask;
> +	info->ctrl.len <<= offset;

Urgh, I really hate all this converting to and fro to keep perf happy.
FWIW, I'm@the point where I would seriously consider ripping out the
hw_breakpoint code and replacing it with a ptrace-specific backend so we
just drop all this crap. The only people that seem to use the perf interface
are those running the (failing) selftests. Given that we have to have
a bloody thread switch notifier *anyway*, all perf seems to give us is
complexity and restrictions.

>  	/*
>  	 * Disallow per-task kernel breakpoints since these would
>  	 * complicate the stepping code.
> @@ -665,8 +666,8 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
>  			      struct pt_regs *regs)
>  {
>  	int i, step = 0, *kernel_step, access;
> -	u32 ctrl_reg;
> -	u64 val, alignment_mask;
> +	u32 ctrl_reg, lens, lene;
> +	u64 val;
>  	struct perf_event *wp, **slots;
>  	struct debug_info *debug_info;
>  	struct arch_hw_breakpoint *info;
> @@ -684,25 +685,21 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
>  			goto unlock;
>  
>  		info = counter_arch_bp(wp);
> -		/* AArch32 watchpoints are either 4 or 8 bytes aligned. */
> -		if (is_compat_task()) {
> -			if (info->ctrl.len == ARM_BREAKPOINT_LEN_8)
> -				alignment_mask = 0x7;
> -			else
> -				alignment_mask = 0x3;
> -		} else {
> -			alignment_mask = 0x7;
> -		}
>  
> -		/* Check if the watchpoint value matches. */
> +		/* Check if the watchpoint value and byte select match. */
>  		val = read_wb_reg(AARCH64_DBG_REG_WVR, i);
> -		if (val != (addr & ~alignment_mask))
> -			goto unlock;
> -
> -		/* Possible match, check the byte address select to confirm. */
>  		ctrl_reg = read_wb_reg(AARCH64_DBG_REG_WCR, i);
>  		decode_ctrl_reg(ctrl_reg, &ctrl);
> -		if (!((1 << (addr & alignment_mask)) & ctrl.len))
> +		lens = ffs(ctrl.len) - 1;
> +		lene = fls(ctrl.len) - 1;

Again, you can use the '__' variants to avoid the '- 1'.

> +		/*
> +		 * FIXME: reported address can be anywhere between "the
> +		 * lowest address accessed by the memory access that
> +		 * triggered the watchpoint" and "the highest watchpointed
> +		 * address accessed by the memory access". So, it may not
> +		 * lie in the interval of watchpoint address range.
> +		 */
> +		if (addr < val + lens || addr > val + lene)
>  			goto unlock;
>  
>  		/*
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index e0c81da60f76..0eb366a94382 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -327,13 +327,13 @@ static int ptrace_hbp_fill_attr_ctrl(unsigned int note_type,
>  				     struct arch_hw_breakpoint_ctrl ctrl,
>  				     struct perf_event_attr *attr)
>  {
> -	int err, len, type, disabled = !ctrl.enabled;
> +	int err, len, type, offset, disabled = !ctrl.enabled;
>  
>  	attr->disabled = disabled;
>  	if (disabled)
>  		return 0;
>  
> -	err = arch_bp_generic_fields(ctrl, &len, &type);
> +	err = arch_bp_generic_fields(ctrl, &len, &type, &offset);
>  	if (err)
>  		return err;
>  
> @@ -352,6 +352,7 @@ static int ptrace_hbp_fill_attr_ctrl(unsigned int note_type,
>  
>  	attr->bp_len	= len;
>  	attr->bp_type	= type;
> +	attr->bp_addr	+= offset;

That's going to look pretty bizarre from userspace if it decides to read
back the address registers to find that they've mysteriously changed.

Perhaps ptrace_hbp_get_addr needs to retrieve the address from the
arch_hw_breakpoint, like we do for the ctrl register. What do you think?

Will