tracing: Avoid NULL return from hist_field_name() on truncation

hist_field_name() returns "" everywhere except the fully-qualified
VAR_REF/EXPR case, where snprintf() truncation returns NULL early
and bypasses the bottom NULL->"" guard. Callers don't expect NULL:
strcat(expr, hist_field_name(field, 0)) at trace_events_hist.c:1758
and the strcmp() in the sort-key match loop at :4804 both deref it.

system and event_name are bounded by MAX_EVENT_NAME_LEN, but the
field name on a VAR_REF is kstrdup'd from a histogram variable
name parsed out of the trigger string and has no length cap, so
a long enough var name in a fully qualified reference can reach
the truncation path.

Keep the length check but leave field_name as "" on overflow.

Link: https://patch.msgid.link/20260508195747.25492-1-devnexen@gmail.com
Fixes: 5ec1d1e97de1 ("tracing: Rebuild full_name on each hist_field_name() call")
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 0dbbf6cca9bc947efc68faa1ca5798f36c51021c..eb2c2bc8bc3d52e6d35fc72c1d0a5878f7bf04fc 100644 (file)
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -1369,10 +1369,8 @@ static const char *hist_field_name(struct hist_field *field,
                        len = snprintf(full_name, sizeof(full_name), fmt,
                                       field->system, field->event_name,
                                       field->name);
-                       if (len >= sizeof(full_name))
-                               return NULL;
-
-                       field_name = full_name;
+                       if (len < sizeof(full_name))
+                               field_name = full_name;
                } else
                        field_name = field->name;
        } else if (field->flags & HIST_FIELD_FL_TIMESTAMP)