Re: [PATCH V2 3/5] arm64: hw_breakpoint: Handle inexact watchpoint addresses

[Will Deacon]

On Thu, Oct 20, 2016 at 11:18:15AM +0530, Pratyush Anand wrote:
> From: Pavel Labath <test.tberghammer@xxxxxxxxx>
> 
> Arm64 hardware does not always report a watchpoint hit address that
> matches one of the watchpoints set. It can also report an address
> "near" the watchpoint if a single instruction access both watched and
> unwatched addresses. There is no straight-forward way, short of
> disassembling the offending instruction, to map that address back to
> the watchpoint.
> 
> Previously, when the hardware reported a watchpoint hit on an address
> that did not match our watchpoint (this happens in case of instructions
> which access large chunks of memory such as "stp") the process would
> enter a loop where we would be continually resuming it (because we did
> not recognise that watchpoint hit) and it would keep hitting the
> watchpoint again and again. The tracing process would never get
> notified of the watchpoint hit.
> 
> This commit fixes the problem by looking at the watchpoints near the
> address reported by the hardware. If the address does not exactly match
> one of the watchpoints we have set, it attributes the hit to the
> nearest watchpoint we have.  This heuristic is a bit dodgy, but I don't
> think we can do much more, given the hardware limitations.
> 
> [panand: reworked to rebase on his patches]
> 
> Signed-off-by: Pavel Labath <labath@xxxxxxxxxx>
> Signed-off-by: Pratyush Anand <panand@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/hw_breakpoint.c | 94 +++++++++++++++++++++++++++------------
>  1 file changed, 66 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index 3c2b96803eba..c57bc90b8286 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -662,11 +662,46 @@ unlock:
>  }
>  NOKPROBE_SYMBOL(breakpoint_handler);
>  
> +/*
> + * Arm64 hardware does not always report a watchpoint hit address that matches
> + * one of the watchpoints set. It can also report an address "near" the
> + * watchpoint if a single instruction access both watched and unwatched
> + * addresses. There is no straight-forward way, short of disassembling the
> + * offending instruction, to map that address back to the watchpoint. This
> + * function computes the distance of the memory access from the watchpoint as a
> + * heuristic for the likelyhood that a given access triggered the watchpoint.
> + *
> + * See Section D2.10.5 "Determining the memory location that caused a Watchpoint
> + * exception" of ARMv8 Architecture Reference Manual for details.
> + *
> + * The function returns the distance of the address from the bytes watched by
> + * the watchpoint. In case of an exact match, it returns 0.
> + */
> +static u64 get_distance_from_watchpoint(unsigned long addr, u64 val,
> +					struct arch_hw_breakpoint_ctrl *ctrl)
> +{
> +	u64 wp_low, wp_high;
> +	u32 lens, lene;
> +
> +	lens = ffs(ctrl->len) - 1;
> +	lene = fls(ctrl->len) - 1;
> +
> +	wp_low = val + lens;
> +	wp_high = val + lene;
> +	if (addr < wp_low)
> +		return wp_low - addr;
> +	else if (addr > wp_high)
> +		return addr - wp_high;
> +	else
> +		return 0;
> +}
> +
>  static int watchpoint_handler(unsigned long addr, unsigned int esr,
>  			      struct pt_regs *regs)
>  {
> -	int i, step = 0, *kernel_step, access;
> -	u32 ctrl_reg, lens, lene;
> +	int i, step = 0, *kernel_step, access, closest_match = 0;
> +	u64 min_dist = -1, dist;
> +	u32 ctrl_reg;
>  	u64 val;
>  	struct perf_event *wp, **slots;
>  	struct debug_info *debug_info;
> @@ -676,31 +711,15 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
>  	slots = this_cpu_ptr(wp_on_reg);
>  	debug_info = &current->thread.debug;
>  
> +	/*
> +	 * Find all watchpoints that match the reported address. If no exact
> +	 * match is found. Attribute the hit to the closest watchpoint.
> +	 */
> +	rcu_read_lock();
>  	for (i = 0; i < core_num_wrps; ++i) {
> -		rcu_read_lock();
> -
>  		wp = slots[i];
> -
>  		if (wp == NULL)
> -			goto unlock;
> -
> -		info = counter_arch_bp(wp);
> -
> -		/* Check if the watchpoint value and byte select match. */
> -		val = read_wb_reg(AARCH64_DBG_REG_WVR, i);
> -		ctrl_reg = read_wb_reg(AARCH64_DBG_REG_WCR, i);
> -		decode_ctrl_reg(ctrl_reg, &ctrl);
> -		lens = ffs(ctrl.len) - 1;
> -		lene = fls(ctrl.len) - 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;
> +			continue;
>  
>  		/*
>  		 * Check that the access type matches.
> @@ -709,18 +728,37 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
>  		access = (esr & AARCH64_ESR_ACCESS_MASK) ? HW_BREAKPOINT_W :
>  			 HW_BREAKPOINT_R;
>  		if (!(access & hw_breakpoint_type(wp)))
> -			goto unlock;
> +			continue;
> +
> +		/* Check if the watchpoint value and byte select match. */
> +		val = read_wb_reg(AARCH64_DBG_REG_WVR, i);
> +		ctrl_reg = read_wb_reg(AARCH64_DBG_REG_WCR, i);
> +		decode_ctrl_reg(ctrl_reg, &ctrl);
> +		dist = get_distance_from_watchpoint(addr, val, &ctrl);
> +		if (dist < min_dist) {
> +			min_dist = dist;
> +			closest_match = i;
> +		}
> +		/* Is this an exact match? */
> +		if (dist != 0)
> +			continue;
>  
> +		info = counter_arch_bp(wp);
>  		info->trigger = addr;
>  		perf_bp_event(wp, regs);
>  
>  		/* Do we need to handle the stepping? */
>  		if (is_default_overflow_handler(wp))
>  			step = 1;
> -
> -unlock:
> -		rcu_read_unlock();
>  	}
> +	if (min_dist > 0 && min_dist != -1) {
> +		/* No exact match found. */
> +		wp = slots[closest_match];
> +		info = counter_arch_bp(wp);
> +		info->trigger = addr;
> +		perf_bp_event(wp, regs);
> +	}

Why don't we need to bother with the stepping in the case of a non-exact
match?

Will