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: ff895103a84ab ("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>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6519b7afc4998c826035e7e80d5b85115c3b33c7..8d2b988126250da9078230ffe31d6b77c493f2dc 100644 (file)
--- a/kernel/trace/trace.c
+++ b/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);