tracing/events: Show real number in array fields

Currently we have in something like the sched_switch event:

  field:char prev_comm[TASK_COMM_LEN];	offset:12;	size:16;	signed:1;

When a userspace tool such as perf tries to parse this, the
TASK_COMM_LEN is meaningless. This is done because the TRACE_EVENT() macro
simply uses a #len to show the string of the length. When the length is
an enum, we get a string that means nothing for tools.

By adding a static buffer and a mutex to protect it, we can store the
string into that buffer with snprintf and show the actual number.
Now we get:

  field:char prev_comm[16];       offset:12;      size:16;        signed:1;

Something much more useful.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 16 insertions, 4 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 0725eeab1937..35fde09b81de 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -27,6 +27,12 @@
 
 DEFINE_MUTEX(event_mutex);
 
+DEFINE_MUTEX(event_storage_mutex);
+EXPORT_SYMBOL_GPL(event_storage_mutex);
+
+char event_storage[EVENT_STORAGE_SIZE];
+EXPORT_SYMBOL_GPL(event_storage);
+
 LIST_HEAD(ftrace_events);
 LIST_HEAD(ftrace_common_fields);
 
diff --git a/kernel/trace/trace_export.c b/kernel/trace/trace_export.c
index 4ba44deaac25..4b74d71705c0 100644
--- a/kernel/trace/trace_export.c
+++ b/kernel/trace/trace_export.c
@@ -83,13 +83,19 @@ static void __always_unused ____ftrace_check_##name(void)	\
 
 #undef __array
 #define __array(type, item, len)                                        \
-        BUILD_BUG_ON(len > MAX_FILTER_STR_VAL);                         \
-        ret = trace_define_field(event_call, #type "[" #len "]", #item, \
+        do {                                                            \
+                BUILD_BUG_ON(len > MAX_FILTER_STR_VAL);                 \
+                mutex_lock(&event_storage_mutex);                       \
+                snprintf(event_storage, sizeof(event_storage),          \
+                         "%s[%d]", #type, len);                         \
+                ret = trace_define_field(event_call, event_storage, #item, \
                                  offsetof(typeof(field), item),         \
                                  sizeof(field.item),                    \
                                  is_signed_type(type), FILTER_OTHER);   \
-        if (ret)                                                        \
-                return ret;
+                mutex_unlock(&event_storage_mutex);                     \
+                if (ret)                                                \
+                        return ret;                                     \
+        } while (0);
 
 #undef __array_desc
 #define __array_desc(type, container, item, len)                        \