[hwasan] Fix Thread reuse (try 2).

HwasanThreadList::DontNeedThread clobbers Thread::next_,
Breaking the freelist. As a result, only the top of the freelist ever
gets reused, and the rest of it is lost.

Since the Thread object with its associated ring buffer is only 8Kb, this is
typically only noticable in long running processes, such as fuzzers.

Fix the problem by switching from an intrusive linked list to a vector.

Differential Revision: https://reviews.llvm.org/D91392

compiler-rt/lib/hwasan/hwasan_thread.h

@@ -74,8 +74,6 @@ class Thread {
   HeapAllocationsRingBuffer *heap_allocations_;
   StackAllocationsRingBuffer *stack_allocations_;
 
-  Thread *next_;  // All live threads form a linked list.
-
   u64 unique_id_;  // counting from zero.
 
   u32 tagging_disabled_;  // if non-zero, malloc uses zero tag in this thread.

compiler-rt/lib/hwasan/hwasan_thread_list.h

@@ -66,40 +66,6 @@ static uptr RingBufferSize() {
   return 0;
 }
 
-struct ThreadListHead {
-  Thread *list_;
-
-  ThreadListHead() : list_(nullptr) {}
-
-  void Push(Thread *t) {
-    t->next_ = list_;
-    list_ = t;
-  }
-
-  Thread *Pop() {
-    Thread *t = list_;
-    if (t)
-      list_ = t->next_;
-    return t;
-  }
-
-  void Remove(Thread *t) {
-    Thread **cur = &list_;
-    while (*cur != t) cur = &(*cur)->next_;
-    CHECK(*cur && "thread not found");
-    *cur = (*cur)->next_;
-  }
-
-  template <class CB>
-  void ForEach(CB cb) {
-    Thread *t = list_;
-    while (t) {
-      cb(t);
-      t = t->next_;
-    }
-  }
-};
-
 struct ThreadStats {
   uptr n_live_threads;
   uptr total_stack_size;
@@ -123,14 +89,15 @@ class HwasanThreadList {
     Thread *t;
     {
       SpinMutexLock l(&list_mutex_);
-      t = free_list_.Pop();
-      if (t) {
+      if (!free_list_.empty()) {
+        t = free_list_.back();
+        free_list_.pop_back();
         uptr start = (uptr)t - ring_buffer_size_;
         internal_memset((void *)start, 0, ring_buffer_size_ + sizeof(Thread));
       } else {
         t = AllocThread();
       }
-      live_list_.Push(t);
+      live_list_.push_back(t);
     }
     t->Init((uptr)t - ring_buffer_size_, ring_buffer_size_);
     AddThreadStats(t);
@@ -142,12 +109,24 @@ class HwasanThreadList {
     ReleaseMemoryPagesToOS(start, start + thread_alloc_size_);
   }
 
+  void RemoveThreadFromLiveList(Thread *t) {
+    for (Thread *&t2 : live_list_)
+      if (t2 == t) {
+        // To remove t2, copy the last element of the list in t2's position, and
+        // pop_back(). This works even if t2 is itself the last element.
+        t2 = live_list_.back();
+        live_list_.pop_back();
+        return;
+      }
+    CHECK(0 && "thread not found in live list");
+  }
+
   void ReleaseThread(Thread *t) {
     RemoveThreadStats(t);
     t->Destroy();
     SpinMutexLock l(&list_mutex_);
-    live_list_.Remove(t);
-    free_list_.Push(t);
+    RemoveThreadFromLiveList(t);
+    free_list_.push_back(t);
     DontNeedThread(t);
   }
 
@@ -166,7 +145,7 @@ class HwasanThreadList {
   template <class CB>
   void VisitAllLiveThreads(CB cb) {
     SpinMutexLock l(&list_mutex_);
-    live_list_.ForEach(cb);
+    for (Thread *t : live_list_) cb(t);
   }
 
   void AddThreadStats(Thread *t) {
@@ -201,8 +180,8 @@ class HwasanThreadList {
   uptr ring_buffer_size_;
   uptr thread_alloc_size_;
 
-  ThreadListHead free_list_;
-  ThreadListHead live_list_;
+  InternalMmapVector<Thread *> free_list_;
+  InternalMmapVector<Thread *> live_list_;
   SpinMutex list_mutex_;
 
   ThreadStats stats_;

compiler-rt/test/hwasan/TestCases/Linux/reuse-threads.cpp

@@ -0,0 +1,55 @@
+// Test that Thread objects are reused.
+// RUN: %clangxx_hwasan -mllvm -hwasan-instrument-stack=0 %s -o %t && %env_hwasan_opts=verbose_threads=1 %run %t 2>&1 | FileCheck %s
+
+#include <assert.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include <sanitizer/hwasan_interface.h>
+
+#include "../utils.h"
+
+pthread_barrier_t bar;
+
+void *threadfn(void *) {
+  pthread_barrier_wait(UNTAG(&bar));
+  return nullptr;
+}
+
+void start_stop_threads() {
+  constexpr int N = 2;
+  pthread_t threads[N];
+
+  pthread_barrier_init(UNTAG(&bar), nullptr, N + 1);
+  for (auto &t : threads)
+    pthread_create(&t, nullptr, threadfn, nullptr);
+
+  pthread_barrier_wait(UNTAG(&bar));
+
+  for (auto &t : threads)
+    pthread_join(t, nullptr);
+  pthread_barrier_destroy(UNTAG(&bar));
+}
+
+int main() {
+  // Cut off initial threads.
+  // CHECK: === test start ===
+  untag_fprintf(stderr, "=== test start ===\n");
+
+  // CHECK: Creating  : T{{[0-9]+}} [[A:0x[0-9a-f]+]] stack:
+  // CHECK: Creating  : T{{[0-9]+}} [[B:0x[0-9a-f]+]] stack:
+  start_stop_threads();
+
+  // CHECK-DAG: Creating  : T{{[0-9]+}} [[A]] stack:
+  // CHECK-DAG: Creating  : T{{[0-9]+}} [[B]] stack:
+  start_stop_threads();
+
+  // CHECK-DAG: Creating  : T{{[0-9]+}} [[A]] stack:
+  // CHECK-DAG: Creating  : T{{[0-9]+}} [[B]] stack:
+  start_stop_threads();
+
+  return 0;
+}

compiler-rt/test/hwasan/TestCases/thread-uaf.c

@@ -34,8 +34,8 @@ void *Use(void *arg) {
   // CHECK: in Deallocate
   // CHECK: previously allocated here:
   // CHECK: in Allocate
-  // CHECK: Thread: T2 0x
-  // CHECK: Thread: T3 0x
+  // CHECK-DAG: Thread: T2 0x
+  // CHECK-DAG: Thread: T3 0x
   // CHECK-DAG: Thread: T0 0x
   // CHECK-DAG: Thread: T1 0x
   __sync_fetch_and_add(&state, 1);