From 0d9c2fc763dd766b868665a302ff4526748c4b36 Mon Sep 17 00:00:00 2001
From: Abseil Team <absl-team@google.com>
Date: Tue, 2 Jul 2024 08:40:43 -0700
Subject: Replace signed integer overflow, since that's undefined behavior,
 with unsigned integer overflow.

PiperOrigin-RevId: 648730502
Change-Id: I662c365c59be9e51f565fd215d284a96b7bd8743
---
 absl/synchronization/internal/graphcycles.cc      | 11 ++++++++++-
 absl/synchronization/internal/graphcycles.h       |  5 +++++
 absl/synchronization/internal/graphcycles_test.cc | 19 +++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/absl/synchronization/internal/graphcycles.cc b/absl/synchronization/internal/graphcycles.cc
index 61b4ec05..129067c1 100644
--- a/absl/synchronization/internal/graphcycles.cc
+++ b/absl/synchronization/internal/graphcycles.cc
@@ -211,7 +211,7 @@ class NodeSet {
   Vec<int32_t> table_;
   uint32_t occupied_;     // Count of non-empty slots (includes deleted slots)
 
-  static uint32_t Hash(int32_t a) { return static_cast<uint32_t>(a * 41); }
+  static uint32_t Hash(int32_t a) { return static_cast<uint32_t>(a) * 41; }
 
   // Return index for storing v.  May return an empty index or deleted index
   uint32_t FindIndex(int32_t v) const {
@@ -365,6 +365,14 @@ static Node* FindNode(GraphCycles::Rep* rep, GraphId id) {
   return (n->version == NodeVersion(id)) ? n : nullptr;
 }
 
+void GraphCycles::TestOnlyAddNodes(uint32_t n) {
+  uint32_t old_size = rep_->nodes_.size();
+  rep_->nodes_.resize(n);
+  for (auto i = old_size; i < n; ++i) {
+    rep_->nodes_[i] = nullptr;
+  }
+}
+
 GraphCycles::GraphCycles() {
   InitArenaIfNecessary();
   rep_ = new (base_internal::LowLevelAlloc::AllocWithArena(sizeof(Rep), arena))
@@ -373,6 +381,7 @@ GraphCycles::GraphCycles() {
 
 GraphCycles::~GraphCycles() {
   for (auto* node : rep_->nodes_) {
+    if (node == nullptr) { continue; }
     node->Node::~Node();
     base_internal::LowLevelAlloc::Free(node);
   }
diff --git a/absl/synchronization/internal/graphcycles.h b/absl/synchronization/internal/graphcycles.h
index ceba33e4..08f304bc 100644
--- a/absl/synchronization/internal/graphcycles.h
+++ b/absl/synchronization/internal/graphcycles.h
@@ -126,6 +126,11 @@ class GraphCycles {
   // Expensive: should only be called from graphcycles_test.cc.
   bool CheckInvariants() const;
 
+  // Test-only method to add more nodes. The nodes will not be valid, and this
+  // method should only be used to test the behavior of the graph when it is
+  // very full.
+  void TestOnlyAddNodes(uint32_t n);
+
   // ----------------------------------------------------
   struct Rep;
  private:
diff --git a/absl/synchronization/internal/graphcycles_test.cc b/absl/synchronization/internal/graphcycles_test.cc
index 3c6ef798..47410aad 100644
--- a/absl/synchronization/internal/graphcycles_test.cc
+++ b/absl/synchronization/internal/graphcycles_test.cc
@@ -14,6 +14,7 @@
 
 #include "absl/synchronization/internal/graphcycles.h"
 
+#include <climits>
 #include <map>
 #include <random>
 #include <unordered_set>
@@ -458,6 +459,24 @@ TEST_F(GraphCyclesTest, ManyEdges) {
   CheckInvariants(g_);
 }
 
+TEST(GraphCycles, IntegerOverflow) {
+  GraphCycles graph_cycles;
+  char *buf = (char *)nullptr;
+  GraphId prev_id = graph_cycles.GetId(buf);
+  buf += 1;
+  GraphId id = graph_cycles.GetId(buf);
+  ASSERT_TRUE(graph_cycles.InsertEdge(prev_id, id));
+
+  // INT_MAX / 40 is enough to cause an overflow when multiplied by 41.
+  graph_cycles.TestOnlyAddNodes(INT_MAX / 40);
+
+  buf += 1;
+  GraphId newid = graph_cycles.GetId(buf);
+  graph_cycles.HasEdge(prev_id, newid);
+
+  graph_cycles.RemoveNode(buf);
+}
+
 }  // namespace synchronization_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
-- 
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