From: Benoît Ganne <bganne@cisco.com>
Date: Fri, 26 Feb 2021 12:30:32 +0000 (+0100)
Subject: vlib: fix clear trace buffer race condition
X-Git-Tag: v21.10-rc0~452
X-Git-Url: https://gerrit.fd.io/r/gitweb?a=commitdiff_plain;ds=sidebyside;h=3ec024c2d259ad6423dcc83da01d782acf6b7046;p=vpp.git

vlib: fix clear trace buffer race condition

Type: fix

Change-Id: I2384e052bee91a275c3b97a00542819b1d646c88
Signed-off-by: Benoît Ganne <bganne@cisco.com>
---

diff --git a/src/vlib/trace_funcs.h b/src/vlib/trace_funcs.h
index 357079c4e3d..d9d8d7933ee 100644
--- a/src/vlib/trace_funcs.h
+++ b/src/vlib/trace_funcs.h
@@ -58,7 +58,7 @@ vlib_add_trace_inline (vlib_main_t * vm,
 {
   vlib_trace_main_t *tm = &vm->trace_main;
   vlib_trace_header_t *h;
-  u32 n_data_words;
+  u32 n_data_words, trace_index;
 
   ASSERT (vnet_trace_placeholder);
 
@@ -83,12 +83,23 @@ vlib_add_trace_inline (vlib_main_t * vm,
     if (PREDICT_FALSE (!vlib_add_handoff_trace (vm, b)))
       return vnet_trace_placeholder;
 
-  vlib_validate_trace (tm, b);
+  /*
+   * there is a small chance of a race condition with 'clear trace' here: if a
+   * buffer was set to be traced before the 'clear trace' and is still going
+   * through the graph after the 'clear trace', its trace_index is staled as
+   * the pool was destroyed.
+   * The pool may have been re-allocated because of a new traced buffer, and
+   * the trace_index might be valid by pure (bad) luck. In that case the trace
+   * will be a mix of both buffer traces, but this should be acceptable.
+   */
+  trace_index = vlib_buffer_get_trace_index (b);
+  if (PREDICT_FALSE (pool_is_free_index (tm->trace_buffer_pool, trace_index)))
+    return vnet_trace_placeholder;
 
   n_data_bytes = round_pow2 (n_data_bytes, sizeof (h[0]));
   n_data_words = n_data_bytes / sizeof (h[0]);
-  vec_add2_aligned (tm->trace_buffer_pool[vlib_buffer_get_trace_index (b)], h,
-		    1 + n_data_words, sizeof (h[0]));
+  vec_add2_aligned (tm->trace_buffer_pool[trace_index], h, 1 + n_data_words,
+		    sizeof (h[0]));
 
   h->time = vm->cpu_time_last_node_dispatch;
   h->n_data = n_data_words;