vppinfra: fix sparse_vec_index() returning non-zero index for some nonexistent elements

If we are testing for an index whose membership bitmask happens to sit in LSB of uword,
and there is nothing else set in that uword in the member bitmask, the shortcut path
returning the valid index is taken even if the element with a given index doesn't
exist in the sparse vector. This happens because the count of leading zeroes on the
value of 0 is zero, which is equal to the value taken modulo bitsize uword.

Take care of that case.

Also add unittests showing the problem and verifying that sparse_vec_index2 does not
have the same issue.

Change-Id: I19117e13817c3e5de579b9250bb741de42491985
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>

3 files changed, 78 insertions, 0 deletions

diff --git a/src/plugins/unittest/CMakeLists.txt b/src/plugins/unittest/CMakeLists.txt
index 0af0dea250c..5c8825f4f3f 100644
--- a/src/plugins/unittest/CMakeLists.txt
+++ b/src/plugins/unittest/CMakeLists.txt
@@ -28,5 +28,6 @@ add_vpp_plugin(unittest
   session_test.c
   string_test.c
   tcp_test.c
+  sparse_vec_test.c
   unittest.c
 )
diff --git a/src/plugins/unittest/sparse_vec_test.c b/src/plugins/unittest/sparse_vec_test.c
new file mode 100644
index 00000000000..bb875452cdf
--- /dev/null
+++ b/src/plugins/unittest/sparse_vec_test.c
@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2018 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <vlib/vlib.h>
+#include <vppinfra/sparse_vec.h>
+
+static clib_error_t *
+test_sparse_vec_command_fn (vlib_main_t * vm,
+			    unformat_input_t * input,
+			    vlib_cli_command_t * cmd)
+{
+  /* A sparse vector ... */
+  int *spv = 0;
+  int i, c0, c1;
+  u32 i0, i1;
+
+  /* set one member */
+  sparse_vec_validate (spv, 42)[0] = 0x4242;
+  /* count how many times we can find it */
+  c0 = 0;
+  for (i = 0; i <= 0xffff; i++)
+    {
+      c0 += (sparse_vec_index (spv, i) != 0);
+    }
+
+  if (c0 != 1)
+    vlib_cli_output (vm, "sparse_vec_index failed: c0 is %d != 1", c0);
+
+  c0 = 0;
+  c1 = 0;
+  for (i = 0; i <= 0xffff; i++)
+    {
+      sparse_vec_index2 (spv, i, 0xffff ^ i, &i0, &i1);
+      c0 += (i0 != 0);
+      c1 += (i1 != 0);
+    }
+
+  if (c0 != 1)
+    vlib_cli_output (vm, "sparse_vec_index2 failed: c0 is %d != 1", c0);
+  if (c1 != 1)
+    vlib_cli_output (vm, "sparse_vec_index2 failed: c1 is %d != 1", c1);
+
+  return 0;
+}
+
+/* *INDENT-OFF* */
+VLIB_CLI_COMMAND (test_sparse_vec_command, static) =
+{
+  .path = "test sparse_vec",
+  .short_help = "test sparse_vec",
+  .function = test_sparse_vec_command_fn,
+};
+/* *INDENT-ON* */
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
diff --git a/src/vppinfra/sparse_vec.h b/src/vppinfra/sparse_vec.h
index cfa5778e290..54a92ce7a84 100644
--- a/src/vppinfra/sparse_vec.h
+++ b/src/vppinfra/sparse_vec.h
@@ -115,6 +115,10 @@ sparse_vec_index_internal (void *v,
 
   w = h->is_member_bitmap[i];
 
+  /* count_trailing_zeros(0) == 0, take care of that case */
+  if (PREDICT_FALSE (maybe_range == 0 && insert == 0 && w == 0))
+    return 0;
+
   if (PREDICT_TRUE (maybe_range == 0 && insert == 0 &&
 		    count_trailing_zeros (w) == b))
     return h->member_counts[i] + 1;