Handle segfaults while walking native heap

Vendor blobs on ryu mprotect heap pages, causing segfaults when dumping
unreachable memory.  Handle segfaults within HeapWalker by mapping a
zero page over any unreadable pages.  HeapWalker runs in the forked
process, so the mapping will not affect the original process.

Bug: 28269332
Change-Id: I16245af722123f2ad467cbc6f245a70666c55544
(cherry picked from commit ba5d9ff6d9674a0f1e985b49f53863045aff558d)

1 files changed, 84 insertions, 0 deletions

diff --git a/libmemunreachable/ScopedSignalHandler.h b/libmemunreachable/ScopedSignalHandler.h
new file mode 100644
index 000000000..e006d435e
--- /dev/null
+++ b/libmemunreachable/ScopedSignalHandler.h
@@ -0,0 +1,84 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#ifndef LIBMEMUNREACHABLE_SCOPED_SIGNAL_HANDLER_H_
+#define LIBMEMUNREACHABLE_SCOPED_SIGNAL_HANDLER_H_
+
+#include <errno.h>
+#include <signal.h>
+
+#include <functional>
+
+#include "android-base/macros.h"
+
+#include "log.h"
+
+class ScopedSignalHandler {
+ public:
+  using Fn = std::function<void(ScopedSignalHandler&, int, siginfo_t*, void*)>;
+
+  ScopedSignalHandler(Allocator<Fn> allocator) : allocator_(allocator), signal_(-1) {}
+  ~ScopedSignalHandler() {
+    reset();
+  }
+
+  template <class F>
+  void install(int signal, F&& f) {
+    LOG_ALWAYS_FATAL_IF(signal_ != -1, "ScopedSignalHandler already installed");
+
+    handler_ = SignalFn(std::allocator_arg, allocator_,
+        [=](int signal, siginfo_t* si, void* uctx) {
+          f(*this, signal, si, uctx);
+        });
+
+    struct sigaction act{};
+    act.sa_sigaction = [](int signal, siginfo_t* si, void* uctx) {
+      handler_(signal, si, uctx);
+    };
+    act.sa_flags = SA_SIGINFO;
+
+    int ret = sigaction(signal, &act, &old_act_);
+    if (ret < 0) {
+      LOG_ALWAYS_FATAL("failed to install segfault handler: %s", strerror(errno));
+    }
+
+    signal_ = signal;
+  }
+
+  void reset() {
+    if (signal_ != -1) {
+      int ret = sigaction(signal_, &old_act_, NULL);
+      if (ret < 0) {
+        ALOGE("failed to uninstall segfault handler");
+      }
+      handler_ = SignalFn{};
+      signal_ = -1;
+    }
+  }
+
+
+ private:
+  using SignalFn = std::function<void(int, siginfo_t*, void*)>;
+  DISALLOW_COPY_AND_ASSIGN(ScopedSignalHandler);
+  Allocator<Fn> allocator_;
+  int signal_;
+  struct sigaction old_act_;
+  // TODO(ccross): to support multiple ScopedSignalHandlers handler_ would need
+  // to be a static map of signals to handlers, but allocated with Allocator.
+  static SignalFn handler_;
+};
+
+#endif // LIBMEMUNREACHABLE_SCOPED_SIGNAL_HANDLER_H_