Query: arm64: hwbreakpoint: single stepping in case of custom overflow_handler

[panand@redhat.com (Pratyush Anand)]

Hi Will,

When we run test from samples/hw_breakpoint/data_breakpoint.c, it triggers 
continuous watchpoint exception handler.

Reproducer:
# insmod data_breakpoint.ko ksym=__sysrq_enabled
# cat /proc/sys/kernel/sysrq

So, wanted to understand that why do we not allow single stepping in case 
overflow_handler is a customized one and not from perf infrastructure?

Patch [1] allows to work with a custom overflow_handler, but I am not sure if 
that could be an acceptable choice.

There are issues with samples/hw_breakpoint/data_breakpoint.c, even when using 
patch [1],because overflow_handler  of test calls dump_stack(). I am not yet 
sure,what happened here..my guess is that dump_stack() triggered a SW BRK 
exception somewhere. Anyway,thats a secondary problem,I can look into if patch 
[1] makes sense.


~Pratyush

[1]
diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
index 749f81779420..ea8ab0656dd0 100644
--- a/arch/arm64/kernel/hw_breakpoint.c
+++ b/arch/arm64/kernel/hw_breakpoint.c
@@ -661,7 +661,7 @@ static int breakpoint_handler(unsigned long unused, 
unsigned int esr,
                 perf_bp_event(bp, regs);

                 /* Do we need to handle the stepping? */
-               if (is_default_overflow_handler(bp))
+               if (bp->overflow_handler)
                         step = 1;
  unlock:
                 rcu_read_unlock();
@@ -789,7 +789,7 @@ static int watchpoint_handler(unsigned long addr, unsigned 
int esr,
                 perf_bp_event(wp, regs);

                 /* Do we need to handle the stepping? */
-               if (is_default_overflow_handler(wp))
+               if (wp->overflow_handler)
                         step = 1;
         }
         if (min_dist > 0 && min_dist != -1) {
@@ -800,7 +800,7 @@ static int watchpoint_handler(unsigned long addr, unsigned 
int esr,
                 perf_bp_event(wp, regs);

                 /* Do we need to handle the stepping? */
-               if (is_default_overflow_handler(wp))
+               if (wp->overflow_handler)
                         step = 1;
         }
         rcu_read_unlock();