Fix race condition updating local map data.

If the underlying local map changes, it's possible for multiple
threads to try and modify the map data associated with the UnwindLocalMap
object. Add a lock when generating the local map to avoid this problem.

In addition, add a read lock whenever any caller gets the maps iterator.
Updated all iterator callers to make this lock.

Bug: 29387050
Bug: 31067025

(cherry picked from commit 3a14004c7f)

Change-Id: Id00116f156a24b36085c0d5dfc3dde4d2ac55194

debuggerd/tombstone.cpp

@@ -368,6 +368,7 @@ static void dump_all_maps(Backtrace* backtrace, BacktraceMap* map, log_t* log, p
     ALOGE("Cannot get siginfo for %d: %s\n", tid, strerror(errno));
   }
 
+  ScopedBacktraceMapIteratorLock lock(map);
   _LOG(log, logtype::MAPS, "\n");
   if (!print_fault_address_marker) {
     _LOG(log, logtype::MAPS, "memory map:\n");

include/backtrace/BacktraceMap.h

@@ -71,6 +71,12 @@ public:
   bool IsWritable(uintptr_t pc) { return GetFlags(pc) & PROT_WRITE; }
   bool IsExecutable(uintptr_t pc) { return GetFlags(pc) & PROT_EXEC; }
 
+  // In order to use the iterators on this object, a caller must
+  // call the LockIterator and UnlockIterator function to guarantee
+  // that the data does not change while it's being used.
+  virtual void LockIterator() {}
+  virtual void UnlockIterator() {}
+
   typedef std::deque<backtrace_map_t>::iterator iterator;
   iterator begin() { return maps_.begin(); }
   iterator end() { return maps_.end(); }
@@ -102,4 +108,18 @@ protected:
   pid_t pid_;
 };
 
+class ScopedBacktraceMapIteratorLock {
+public:
+  explicit ScopedBacktraceMapIteratorLock(BacktraceMap* map) : map_(map) {
+    map->LockIterator();
+  }
+
+  ~ScopedBacktraceMapIteratorLock() {
+    map_->UnlockIterator();
+  }
+
+private:
+  BacktraceMap* map_;
+};
+
 #endif // _BACKTRACE_BACKTRACE_MAP_H

libbacktrace/BacktraceMap.cpp

@@ -35,8 +35,8 @@ BacktraceMap::~BacktraceMap() {
 }
 
 void BacktraceMap::FillIn(uintptr_t addr, backtrace_map_t* map) {
-  for (BacktraceMap::const_iterator it = begin();
-       it != end(); ++it) {
+  ScopedBacktraceMapIteratorLock lock(this);
+  for (BacktraceMap::const_iterator it = begin(); it != end(); ++it) {
     if (addr >= it->start && addr < it->end) {
       *map = *it;
       return;

libbacktrace/UnwindMap.cpp

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <pthread.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <sys/types.h>
@@ -72,6 +73,7 @@ bool UnwindMapRemote::Build() {
 }
 
 UnwindMapLocal::UnwindMapLocal() : UnwindMap(getpid()), map_created_(false) {
+  pthread_rwlock_init(&map_lock_, nullptr);
 }
 
 UnwindMapLocal::~UnwindMapLocal() {
@@ -82,9 +84,14 @@ UnwindMapLocal::~UnwindMapLocal() {
 }
 
 bool UnwindMapLocal::GenerateMap() {
+  // Lock so that multiple threads cannot modify the maps data at the
+  // same time.
+  pthread_rwlock_wrlock(&map_lock_);
+
   // It's possible for the map to be regenerated while this loop is occurring.
   // If that happens, get the map again, but only try at most three times
   // before giving up.
+  bool generated = false;
   for (int i = 0; i < 3; i++) {
     maps_.clear();
 
@@ -110,12 +117,17 @@ bool UnwindMapLocal::GenerateMap() {
     }
     // Check to see if the map changed while getting the data.
     if (ret != -UNW_EINVAL) {
-      return true;
+      generated = true;
+      break;
     }
   }
 
-  BACK_LOGW("Unable to generate the map.");
-  return false;
+  pthread_rwlock_unlock(&map_lock_);
+
+  if (!generated) {
+    BACK_LOGW("Unable to generate the map.");
+  }
+  return generated;
 }
 
 bool UnwindMapLocal::Build() {

libbacktrace/UnwindMap.h

@@ -17,6 +17,7 @@
 #ifndef _LIBBACKTRACE_UNWIND_MAP_H
 #define _LIBBACKTRACE_UNWIND_MAP_H
 
+#include <pthread.h>
 #include <stdint.h>
 #include <sys/types.h>
 
@@ -56,10 +57,15 @@ public:
 
   void FillIn(uintptr_t addr, backtrace_map_t* map) override;
 
+  void LockIterator() override { pthread_rwlock_rdlock(&map_lock_); }
+  void UnlockIterator() override { pthread_rwlock_unlock(&map_lock_); }
+
 private:
   bool GenerateMap();
 
   bool map_created_;
+
+  pthread_rwlock_t map_lock_;
 };
 
 #endif // _LIBBACKTRACE_UNWIND_MAP_H

libbacktrace/backtrace_test.cpp

@@ -896,6 +896,7 @@ void VerifyMap(pid_t pid) {
   std::unique_ptr<BacktraceMap> map(BacktraceMap::Create(pid));
 
   // Basic test that verifies that the map is in the expected order.
+  ScopedBacktraceMapIteratorLock lock(map.get());
   std::vector<map_test_t>::const_iterator test_it = test_maps.begin();
   for (BacktraceMap::const_iterator it = map->begin(); it != map->end(); ++it) {
     ASSERT_TRUE(test_it != test_maps.end());