tracing: Use EVENT_NULL_STR macro instead of open coding "(null)"

The TRACE_EVENT macros has some dependency if a __string() field is NULL,
where it will save "(null)" as the string. This string is also used by
__assign_str(). It's better to create a single macro instead of having
something that will not be caught by the compiler if there is an
unfortunate typo.

Link: https://lore.kernel.org/linux-trace-kernel/20240222211443.106216915@goodmis.org

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chuck Lever <chuck.lever@oracle.com>
Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/linux/trace_events.h

@@ -17,6 +17,9 @@ struct dentry;
 struct bpf_prog;
 union bpf_attr;
 
+/* Used for event string fields when they are NULL */
+#define EVENT_NULL_STR		"(null)"
+
 const char *trace_print_flags_seq(struct trace_seq *p, const char *delim,
 				  unsigned long flags,
 				  const struct trace_print_flags *flag_array);

include/trace/events/sunrpc.h

@@ -1327,18 +1327,18 @@ TRACE_EVENT(xs_stream_read_data,
 		__field(ssize_t, err)
 		__field(size_t, total)
 		__string(addr, xprt ? xprt->address_strings[RPC_DISPLAY_ADDR] :
-				"(null)")
+				EVENT_NULL_STR)
 		__string(port, xprt ? xprt->address_strings[RPC_DISPLAY_PORT] :
-				"(null)")
+				EVENT_NULL_STR)
 	),
 
 	TP_fast_assign(
 		__entry->err = err;
 		__entry->total = total;
 		__assign_str(addr, xprt ?
-			xprt->address_strings[RPC_DISPLAY_ADDR] : "(null)");
+			xprt->address_strings[RPC_DISPLAY_ADDR] : EVENT_NULL_STR);
 		__assign_str(port, xprt ?
-			xprt->address_strings[RPC_DISPLAY_PORT] : "(null)");
+			xprt->address_strings[RPC_DISPLAY_PORT] : EVENT_NULL_STR);
 	),
 
 	TP_printk("peer=[%s]:%s err=%zd total=%zu", __get_str(addr),
@@ -1783,7 +1783,7 @@ TRACE_EVENT(svc_process,
 		__string(service, name)
 		__string(procedure, svc_proc_name(rqst))
 		__string(addr, rqst->rq_xprt ?
-			 rqst->rq_xprt->xpt_remotebuf : "(null)")
+			 rqst->rq_xprt->xpt_remotebuf : EVENT_NULL_STR)
 	),
 
 	TP_fast_assign(
@@ -1793,7 +1793,7 @@ TRACE_EVENT(svc_process,
 		__assign_str(service, name);
 		__assign_str(procedure, svc_proc_name(rqst));
 		__assign_str(addr, rqst->rq_xprt ?
-			     rqst->rq_xprt->xpt_remotebuf : "(null)");
+			     rqst->rq_xprt->xpt_remotebuf : EVENT_NULL_STR);
 	),
 
 	TP_printk("addr=%s xid=0x%08x service=%s vers=%u proc=%s",

include/trace/stages/stage5_get_offsets.h

@@ -47,7 +47,7 @@
 
 #undef __string
 #define __string(item, src) __dynamic_array(char, item,			\
-		    strlen((const char *)(src) ? : "(null)") + 1)	\
+		    strlen((const char *)(src) ? : EVENT_NULL_STR) + 1)	\
 	__data_offsets->item##_ptr_ = src;
 
 #undef __string_len
@@ -70,7 +70,7 @@
 
 #undef __rel_string
 #define __rel_string(item, src) __rel_dynamic_array(char, item,		\
-		    strlen((const char *)(src) ? : "(null)") + 1)	\
+		    strlen((const char *)(src) ? : EVENT_NULL_STR) + 1)	\
 	__data_offsets->item##_ptr_ = src;
 
 #undef __rel_string_len

include/trace/stages/stage6_event_callback.h

@@ -32,14 +32,14 @@
 
 #undef __assign_str
 #define __assign_str(dst, src)						\
-	memcpy(__get_str(dst), __data_offsets.dst##_ptr_ ? : "(null)",	\
+	memcpy(__get_str(dst), __data_offsets.dst##_ptr_ ? : EVENT_NULL_STR, \
 	       __get_dynamic_array_len(dst))
 
 #undef __assign_str_len
 #define __assign_str_len(dst, src, len)					\
 	do {								\
 		memcpy(__get_str(dst),					\
-		       __data_offsets.dst##_ptr_ ? : "(null)", len);	\
+		       __data_offsets.dst##_ptr_ ? : EVENT_NULL_STR, len); \
 		__get_str(dst)[len] = '\0';				\
 	} while(0)
 
@@ -94,14 +94,14 @@
 
 #undef __assign_rel_str
 #define __assign_rel_str(dst, src)					\
-	memcpy(__get_rel_str(dst), __data_offsets.dst##_ptr_ ? : "(null)", \
+	memcpy(__get_rel_str(dst), __data_offsets.dst##_ptr_ ? : EVENT_NULL_STR, \
 	       __get_rel_dynamic_array_len(dst))
 
 #undef __assign_rel_str_len
 #define __assign_rel_str_len(dst, src, len)				\
 	do {								\
 		memcpy(__get_rel_str(dst),				\
-		       __data_offsets.dst##_ptr_ ? : "(null)", len);	\
+		       __data_offsets.dst##_ptr_ ? : EVENT_NULL_STR, len); \
 		__get_rel_str(dst)[len] = '\0';				\
 	} while (0)