kdb: Add message about CONFIG_DEBUG_RODATA on failure to install breakpoint

On x86, if CONFIG_DEBUG_RODATA is set, one cannot set breakpoints
via KDB.  Apparently this is a well-known problem, as at least one distribution
now ships with both KDB enabled and CONFIG_DEBUG_RODATA=y for security reasons.

This patch adds an printk message to the breakpoint failure case,
in order to provide suggestions about how to use the debugger.
Reported-by: NTim Bird <tim.bird@am.sony.com>
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>
Acked-by: NTim Bird <tim.bird@am.sony.com>

kernel/debug/kdb/kdb_bp.c

@@ -153,6 +153,13 @@ static int _kdb_bp_install(struct pt_regs *regs, kdb_bp_t *bp)
 	} else {
 		kdb_printf("%s: failed to set breakpoint at 0x%lx\n",
 			   __func__, bp->bp_addr);
+#ifdef CONFIG_DEBUG_RODATA
+		if (!bp->bp_type) {
+			kdb_printf("Software breakpoints are unavailable.\n"
+				   "  Change the kernel CONFIG_DEBUG_RODATA=n\n"
+				   "  OR use hw breaks: help bph\n");
+		}
+#endif
 		return 1;
 	}
 	return 0;