tracing: Do not allocate buffer in trace_find_next_entry() in atomic

When dumping out the trace data in latency format, a check is made to peek
at the next event to compare its timestamp to the current one, and if the
delta is of a greater size, it will add a marker showing so. But to do this,
it needs to save the current event otherwise peeking at the next event will
remove the current event. To save the event, a temp buffer is used, and if
the event is bigger than the temp buffer, the temp buffer is freed and a
bigger buffer is allocated.

This allocation is a problem when called in atomic context. The only way
this gets called via atomic context is via ftrace_dump(). Thus, use a static
buffer of 128 bytes (which covers most events), and if the event is bigger
than that, simply return NULL. The callers of trace_find_next_entry() need
to handle a NULL case, as that's what would happen if the allocation failed.

Link: https://lore.kernel.org/r/20200326091256.GR11705@shao2-debian

Fixes: ff895103 ("tracing: Save off entry when peeking at next entry")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -3472,6 +3472,9 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu,
 	return next;
 }
 
+#define STATIC_TEMP_BUF_SIZE	128
+static char static_temp_buf[STATIC_TEMP_BUF_SIZE];
+
 /* Find the next real entry, without updating the iterator itself */
 struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
 					  int *ent_cpu, u64 *ent_ts)
@@ -3480,13 +3483,26 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
 	int ent_size = iter->ent_size;
 	struct trace_entry *entry;
 
+	/*
+	 * If called from ftrace_dump(), then the iter->temp buffer
+	 * will be the static_temp_buf and not created from kmalloc.
+	 * If the entry size is greater than the buffer, we can
+	 * not save it. Just return NULL in that case. This is only
+	 * used to add markers when two consecutive events' time
+	 * stamps have a large delta. See trace_print_lat_context()
+	 */
+	if (iter->temp == static_temp_buf &&
+	    STATIC_TEMP_BUF_SIZE < ent_size)
+		return NULL;
+
 	/*
 	 * The __find_next_entry() may call peek_next_entry(), which may
 	 * call ring_buffer_peek() that may make the contents of iter->ent
 	 * undefined. Need to copy iter->ent now.
 	 */
 	if (iter->ent && iter->ent != iter->temp) {
-		if (!iter->temp || iter->temp_size < iter->ent_size) {
+		if ((!iter->temp || iter->temp_size < iter->ent_size) &&
+		    !WARN_ON_ONCE(iter->temp == static_temp_buf)) {
 			kfree(iter->temp);
 			iter->temp = kmalloc(iter->ent_size, GFP_KERNEL);
 			if (!iter->temp)
@@ -9203,6 +9219,9 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 
 	/* Simulate the iterator */
 	trace_init_global_iter(&iter);
+	/* Can not use kmalloc for iter.temp */
+	iter.temp = static_temp_buf;
+	iter.temp_size = STATIC_TEMP_BUF_SIZE;
 
 	for_each_tracing_cpu(cpu) {
 		atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);