Subject: [EDT] [PATCH 1/1] Fix: hw watchpoint continually triggers callback

From: [email-masked] (Will Deacon)
Date: Tue, 12 May 2015 13:45:23 +0100

On Tue, May 12, 2015 at 12:48:13PM +0100, Maninder Singh wrote:
> EP-2DAD0AFA905A4ACB804C4F82A001242F
> 
> On ARM, when a watchpoint is registered using register_wide_hw_breakpoint, 
> the callback handler endlessly runs until the watchpoint is unregistered.
> The reason for this issue is debug interrupts gets raised before executing the instruction,
> and after interrupt handling ARM tries to execute the same instruction again , which results
> in interrupt getting raised again.
> 
> This patch fixes this issue by using KPROBES (getting the instruction executed and incrementing PC
> to next instruction).
> 
> Signed-off-by: Vaneet Narang <[email-masked]>
> Signed-off-by: Maninder Singh <[email-masked]>
> Reviewed-by: Amit Arora <[email-masked]>
> Reviewed-by: Ajeet Yadav <[email-masked]>
> ---
>  arch/arm/kernel/hw_breakpoint.c |   18 ++++++++++++++++++
>  1 files changed, 18 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
> index dc7d0a9..ec72f86 100644
> --- a/arch/arm/kernel/hw_breakpoint.c
> +++ b/arch/arm/kernel/hw_breakpoint.c
> @@ -37,6 +37,9 @@
>  #include <asm/hw_breakpoint.h>
>  #include <asm/kdebug.h>
>  #include <asm/traps.h>
> +#ifdef CONFIG_KPROBES
> +#include <linux/kprobes.h>
> +#endif
>  
>  /* Breakpoint currently in use for each BRP. */
>  static DEFINE_PER_CPU(struct perf_event *, bp_on_reg[ARM_MAX_BRP]);
> @@ -757,6 +760,21 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
>  		 */
>  		if (!wp->overflow_handler)
>  			enable_single_step(wp, instruction_pointer(regs));
> +#ifdef CONFIG_KPROBES
> +		else {
> +			struct kprobe kp;
> +			unsigned long flags;
> +
> +			arch_uninstall_hw_breakpoint(wp);
> +			kp.addr = (kprobe_opcode_t *)instruction_pointer(regs);
> +			if (!arch_prepare_kprobe(&kp)) {
> +				local_irq_save(flags);
> +				kp.ainsn.insn_singlestep(&kp, regs);
> +				local_irq_restore(flags);
> +			}
> +			arch_install_hw_breakpoint(wp);
> +		}
> +#endif

I don't think this is the right thing to do at all; the kernel already
handles step exceptions using mismatched breakpoints when there is no
overflow handler specified (e.g. using perf mem events). If you register a
handler (e.g. gdb via ptrace) then you have to handle the step yourself.

Will