[tor-commits] [tor/master] Move the ctime part of choose_array_element_by_weight into di_ops

[nickm at torproject.org]

commit e40cfc4425d1e173b5d122e38ab07dd798b11925
Author: Nick Mathewson <nickm at torproject.org>
Date:   Wed Apr 27 10:33:46 2016 -0400

    Move the ctime part of choose_array_element_by_weight into di_ops
    
    This way it gets the ctime options.
---
 src/common/di_ops.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 src/common/di_ops.h |  2 ++
 src/or/routerlist.c | 35 +++--------------------------------
 3 files changed, 50 insertions(+), 32 deletions(-)

diff --git a/src/common/di_ops.c b/src/common/di_ops.c
index 5dfe828..e671af6 100644
--- a/src/common/di_ops.c
+++ b/src/common/di_ops.c
@@ -226,3 +226,48 @@ safe_mem_is_zero(const void *mem, size_t sz)
   return 1 & ((total - 1) >> 8);
 }
 
+/** Time-invariant 64-bit greater-than; works on two integers in the range
+ * (0,INT64_MAX). */
+#if SIZEOF_VOID_P == 8
+#define gt_i64_timei(a,b) ((a) > (b))
+#else
+static inline int
+gt_i64_timei(uint64_t a, uint64_t b)
+{
+  int64_t diff = (int64_t) (b - a);
+  int res = diff >> 63;
+  return res & 1;
+}
+#endif
+
+/**
+ * Given an array of list of <b>n_entries</b> uint64_t values, whose sum is
+ * <b>total</b>, find the first i such that the total of all elements 0...i is
+ * greater than rand_val.
+ *
+ * Try to perform this operation in a constant-time way.
+ */
+int
+select_array_member_cumulative_timei(const uint64_t *entries, int n_entries,
+                                     uint64_t total, uint64_t rand_val)
+{
+  int i, i_chosen=-1, n_chosen=0;
+  uint64_t total_so_far = 0;
+
+  for (i = 0; i < n_entries; ++i) {
+    total_so_far += entries[i];
+    if (gt_i64_timei(total_so_far, rand_val)) {
+      i_chosen = i;
+      n_chosen++;
+      /* Set rand_val to INT64_MAX rather than stopping the loop. This way,
+       * the time we spend in the loop does not leak which element we chose. */
+      rand_val = INT64_MAX;
+    }
+  }
+  tor_assert(total_so_far == total);
+  tor_assert(n_chosen == 1);
+  tor_assert(i_chosen >= 0);
+  tor_assert(i_chosen < n_entries);
+
+  return i_chosen;
+}
diff --git a/src/common/di_ops.h b/src/common/di_ops.h
index 6e77b5c..f1050a0 100644
--- a/src/common/di_ops.h
+++ b/src/common/di_ops.h
@@ -42,6 +42,8 @@ void dimap_add_entry(di_digest256_map_t **map,
                      const uint8_t *key, void *val);
 void *dimap_search(const di_digest256_map_t *map, const uint8_t *key,
                    void *dflt_val);
+int select_array_member_cumulative_timei(const uint64_t *entries, int n_entries,
+                                         uint64_t total, uint64_t rand_val);
 
 #endif
 
diff --git a/src/or/routerlist.c b/src/or/routerlist.c
index 620b614..b6247a1 100644
--- a/src/or/routerlist.c
+++ b/src/or/routerlist.c
@@ -2103,20 +2103,6 @@ scale_array_elements_to_u64(uint64_t *entries_out, const double *entries_in,
 #undef SCALE_TO_U64_MAX
 }
 
-/** Time-invariant 64-bit greater-than; works on two integers in the range
- * (0,INT64_MAX). */
-#if SIZEOF_VOID_P == 8
-#define gt_i64_timei(a,b) ((a) > (b))
-#else
-static inline int
-gt_i64_timei(uint64_t a, uint64_t b)
-{
-  int64_t diff = (int64_t) (b - a);
-  int res = diff >> 63;
-  return res & 1;
-}
-#endif
-
 /** Pick a random element of <b>n_entries</b>-element array <b>entries</b>,
  * choosing each element with a probability proportional to its (uint64_t)
  * value, and return the index of that element.  If all elements are 0, choose
@@ -2125,8 +2111,7 @@ gt_i64_timei(uint64_t a, uint64_t b)
 STATIC int
 choose_array_element_by_weight(const uint64_t *entries, int n_entries)
 {
-  int i, i_chosen=-1, n_chosen=0;
-  uint64_t total_so_far = 0;
+  int i;
   uint64_t rand_val;
   uint64_t total = 0;
 
@@ -2143,22 +2128,8 @@ choose_array_element_by_weight(const uint64_t *entries, int n_entries)
 
   rand_val = crypto_rand_uint64(total);
 
-  for (i = 0; i < n_entries; ++i) {
-    total_so_far += entries[i];
-    if (gt_i64_timei(total_so_far, rand_val)) {
-      i_chosen = i;
-      n_chosen++;
-      /* Set rand_val to INT64_MAX rather than stopping the loop. This way,
-       * the time we spend in the loop does not leak which element we chose. */
-      rand_val = INT64_MAX;
-    }
-  }
-  tor_assert(total_so_far == total);
-  tor_assert(n_chosen == 1);
-  tor_assert(i_chosen >= 0);
-  tor_assert(i_chosen < n_entries);
-
-  return i_chosen;
+  return select_array_member_cumulative_timei(
+                           entries, n_entries, total, rand_val);
 }
 
 /** When weighting bridges, enforce these values as lower and upper