arm64: ptrace: avoid using HW_BREAKPOINT_EMPTY for disabled events

Commit 8f34a1d ("arm64: ptrace: use HW_BREAKPOINT_EMPTY type for
disabled breakpoints") fixed an issue with GDB trying to zero breakpoint
control registers. The problem there is that the arch hw_breakpoint code
will attempt to create a (disabled), execute breakpoint of length 0.

This will fail validation and report unexpected failure to GDB. To avoid
this, we treated disabled breakpoints as HW_BREAKPOINT_EMPTY, but that
seems to have broken with recent kernels, causing watchpoints to be
treated as TYPE_INST in the core code and returning ENOSPC for any
further breakpoints.

This patch fixes the problem by prioritising the `enable' field of the
breakpoint: if it is cleared, we simply update the perf_event_attr to
indicate that the thing is disabled and don't bother changing either the
type or the length. This reinforces the behaviour that the breakpoint
control register is essentially read-only apart from the enable bit
when disabling a breakpoint.

Cc: <stable@vger.kernel.org>
Reported-by: Aaron Liu <liucy214@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/ptrace.c

@@ -214,31 +214,29 @@ static int ptrace_hbp_fill_attr_ctrl(unsigned int note_type,
 {
 	int err, len, type, disabled = !ctrl.enabled;
 
-	if (disabled) {
-		len = 0;
-		type = HW_BREAKPOINT_EMPTY;
-	} else {
-		err = arch_bp_generic_fields(ctrl, &len, &type);
-		if (err)
-			return err;
-
-		switch (note_type) {
-		case NT_ARM_HW_BREAK:
-			if ((type & HW_BREAKPOINT_X) != type)
-				return -EINVAL;
-			break;
-		case NT_ARM_HW_WATCH:
-			if ((type & HW_BREAKPOINT_RW) != type)
-				return -EINVAL;
-			break;
-		default:
+	attr->disabled = disabled;
+	if (disabled)
+		return 0;
+
+	err = arch_bp_generic_fields(ctrl, &len, &type);
+	if (err)
+		return err;
+
+	switch (note_type) {
+	case NT_ARM_HW_BREAK:
+		if ((type & HW_BREAKPOINT_X) != type)
 			return -EINVAL;
-		}
+		break;
+	case NT_ARM_HW_WATCH:
+		if ((type & HW_BREAKPOINT_RW) != type)
+			return -EINVAL;
+		break;
+	default:
+		return -EINVAL;
 	}
 
 	attr->bp_len	= len;
 	attr->bp_type	= type;
-	attr->disabled	= disabled;
 
 	return 0;
 }