tracing/synthetic: Skip first entry for stack traces

While debugging another issue I noticed that the stack trace output
contains the number of entries on top:

         <idle>-0       [000] d..4.   203.322502: wake_lat: pid=0 delta=2268270616 stack=STACK:
=> 0x10
=> __schedule+0xac6/0x1a98
=> schedule+0x126/0x2c0
=> schedule_timeout+0x242/0x2c0
=> __wait_for_common+0x434/0x680
=> __wait_rcu_gp+0x198/0x3e0
=> synchronize_rcu+0x112/0x138
=> ring_buffer_reset_online_cpus+0x140/0x2e0
=> tracing_reset_online_cpus+0x15c/0x1d0
=> tracing_set_clock+0x180/0x1d8
=> hist_register_trigger+0x486/0x670
=> event_hist_trigger_parse+0x494/0x1318
=> trigger_process_regex+0x1d4/0x258
=> event_trigger_write+0xb4/0x170
=> vfs_write+0x210/0xad0
=> ksys_write+0x122/0x208

Fix this by skipping the first element. Also replace the pointer
logic with an index variable which is easier to read.

Link: https://lkml.kernel.org/r/20230816154928.4171614-3-svens@linux.ibm.com

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 00cf3d6 ("tracing: Allow synthetic events to pass around stacktraces")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: svens-s390

kernel/trace/trace_events_synth.c

@@ -350,7 +350,7 @@ static enum print_line_t print_synth_event(struct trace_iterator *iter,
 	struct trace_seq *s = &iter->seq;
 	struct synth_trace_event *entry;
 	struct synth_event *se;
-	unsigned int i, n_u64;
+	unsigned int i, j, n_u64;
 	char print_fmt[32];
 	const char *fmt;
 
@@ -389,18 +389,13 @@ static enum print_line_t print_synth_event(struct trace_iterator *iter,
 				n_u64 += STR_VAR_LEN_MAX / sizeof(u64);
 			}
 		} else if (se->fields[i]->is_stack) {
-			unsigned long *p, *end;
 			union trace_synth_field *data = &entry->fields[n_u64];
-
-			p = (void *)entry + data->as_dynamic.offset;
-			end = (void *)p + data->as_dynamic.len - (sizeof(long) - 1);
+			unsigned long *p = (void *)entry + data->as_dynamic.offset;
 
 			trace_seq_printf(s, "%s=STACK:\n", se->fields[i]->name);
-
-			for (; *p && p < end; p++)
-				trace_seq_printf(s, "=> %pS\n", (void *)*p);
+			for (j = 1; j < data->as_dynamic.len / sizeof(long); j++)
+				trace_seq_printf(s, "=> %pS\n", (void *)p[j]);
 			n_u64++;
-
 		} else {
 			struct trace_print_flags __flags[] = {
 			    __def_gfpflag_names, {-1, NULL} };
@@ -490,10 +485,6 @@ static unsigned int trace_stack(struct synth_trace_event *entry,
 			break;
 	}
 
-	/* Include the zero'd element if it fits */
-	if (len < HIST_STACKTRACE_DEPTH)
-		len++;
-
 	len *= sizeof(long);
 
 	/* Find the dynamic section to copy the stack into. */