fix(ndk): Switch unwinders based on required async signal-safety

When capturing a stacktrace from a signal handler or C++ terminate 
handler different unwinders deadlock on different architectures and/or
Android API levels. This change uses different unwinders based on
whether async signal safety is required or not, by supported API levels.

It introduces libunwindstack as the signal-safe unwinder for API 15+ on
ARM64 and x86/x64 devices. For notify()/C++ exceptions, use libunwind,
nongnu libunwind, or libcorkscrew depending on the API level.

* libcorkscrew hangs when unwinding from a C++ exception on ARM 32-bit.
* libunwind hangs on ARM64 from a signal handler
* nongnu libunwind hangs on ARM32 from a signal handler if raise() is
  called directly

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "ndk/src/main/jni/external/libunwindstack"]
+	path = ndk/src/main/jni/external/libunwindstack
+	url = https://github.com/ivanarh/libunwindstack-ndk

CONTRIBUTING.md

@@ -75,6 +75,12 @@ installed to `~/.android-sdk`.
 
 ## Building the Library
 
+Be sure to clone the repository submodules:
+
+```shell
+git submodule update --init --recursive
+```
+
 You can build new `.aar` files as follows:
 
 ```shell

LICENSE.txt

@@ -18,3 +18,62 @@ NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+====
+
+The libunwindstack module located in ndk/src/main/jni/external/libunwindstack
+is licensed under the Apache Software License, Version 2.0.
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+https://android.googlesource.com/platform/system/core.git
+
+https://source.android.com/setup/start/licenses
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
+
+"License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document.
+
+"Licensor" shall mean the copyright owner or entity authorized by the copyright owner that is granting the License.
+
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+
+"You" (or "Your") shall mean an individual or Legal Entity exercising permissions granted by this License.
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+"Contribution" shall mean any work of authorship, including the original version of the Work and any modifications or additions to that Work or Derivative Works thereof, that is intentionally submitted to Licensor for inclusion in the Work by the copyright owner or by an individual or Legal Entity authorized to submit on behalf of the copyright owner. For the purposes of this definition, "submitted" means any form of electronic, verbal, or written communication sent to the Licensor or its representatives, including but not limited to communication on electronic mailing lists, source code control systems, and issue tracking systems that are managed by, or on behalf of, the Licensor for the purpose of discussing and improving the Work, but excluding communication that is conspicuously marked or otherwise designated in writing by the copyright owner as "Not a Contribution."
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+
+4. Redistribution. You may reproduce and distribute copies of the Work or Derivative Works thereof in any medium, with or without modifications, and in Source or Object form, provided that You meet the following conditions:
+
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+If the Work includes a "NOTICE" text file as part of its distribution, then any Derivative Works that You distribute must include a readable copy of the attribution notices contained within such NOTICE file, excluding those notices that do not pertain to any part of the Derivative Works, in at least one of the following places: within a NOTICE text file distributed as part of the Derivative Works; within the Source form or documentation, if provided along with the Derivative Works; or, within a display generated by the Derivative Works, if and wherever such third-party notices normally appear. The contents of the NOTICE file are for informational purposes only and do not modify the License. You may add Your own attribution notices within Derivative Works that You distribute, alongside or as an addendum to the NOTICE text from the Work, provided that such additional attribution notices cannot be construed as modifying the License.
+
+You may add Your own copyright statement to Your modifications and may provide additional or different license terms and conditions for use, reproduction, or distribution of Your modifications, or for any such Derivative Works as a whole, provided Your use, reproduction, and distribution of the Work otherwise complies with the conditions stated in this License.
+5. Submission of Contributions. Unless You explicitly state otherwise, any Contribution intentionally submitted for inclusion in the Work by You to the Licensor shall be under the terms and conditions of this License, without any additional terms or conditions. Notwithstanding the above, nothing herein shall supersede or modify the terms of any separate license agreement you may have executed with Licensor regarding such Contributions.
+
+6. Trademarks. This License does not grant permission to use the trade names, trademarks, service marks, or product names of the Licensor, except as required for reasonable and customary use in describing the origin of the Work and reproducing the content of the NOTICE file.
+
+7. Disclaimer of Warranty. Unless required by applicable law or agreed to in writing, Licensor provides the Work (and each Contributor provides its Contributions) on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. You are solely responsible for determining the appropriateness of using or redistributing the Work and assume any risks associated with Your exercise of permissions under this License.
+
+8. Limitation of Liability. In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall any Contributor be liable to You for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising as a result of this License or out of the use or inability to use the Work (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if such Contributor has been advised of the possibility of such damages.
+
+9. Accepting Warranty or Additional Liability. While redistributing the Work or Derivative Works thereof, You may choose to offer, and charge a fee for, acceptance of support, warranty, indemnity, or other liability obligations and/or rights consistent with this License. However, in accepting such obligations, You may act only on Your own behalf and on Your sole responsibility, not on behalf of any other Contributor, and only if You agree to indemnify, defend, and hold each Contributor harmless for any liability incurred by, or claims asserted against, such Contributor by reason of your accepting any such warranty or additional liability.
+
+END OF TERMS AND CONDITIONS

checkstyle.gradle

@@ -8,5 +8,6 @@ task("checkstyle", type: Checkstyle) {
     configFile rootProject.file("config/checkstyle/checkstyle.xml")
     source  "src/"
     include "**/*.java"
+    exclude "**/external/**/*.java"
     classpath = files()
 }

ndk/src/main/CMakeLists.txt

@@ -14,6 +14,7 @@ add_library( # Specifies the name of the library.
              jni/handlers/cpp_handler.cpp
              jni/utils/crash_info.c
              jni/utils/stack_unwinder.c
+             jni/utils/stack_unwinder_libunwindstack.cpp
              jni/utils/stack_unwinder_libcorkscrew.c
              jni/utils/stack_unwinder_libunwind.c
              jni/utils/stack_unwinder_simple.c
@@ -26,6 +27,7 @@ include_directories(
              jni/
              jni/deps
              jni/external/libunwind/include
+             jni/external/libunwindstack/include
              )
 
 target_include_directories(bugsnag-ndk PRIVATE ${BUGSNAG_DIR}/assets/include)
@@ -49,6 +51,8 @@ set_target_properties(bugsnag-ndk
                       COMPILE_OPTIONS
                       -Werror -Wall -pedantic)
 
+add_subdirectory(jni/external/libunwindstack/cmake)
+target_link_libraries(bugsnag-ndk unwindstack)
 if(${ANDROID_ABI} STREQUAL "armeabi" OR ${ANDROID_ABI} STREQUAL "armeabi-v7a")
     add_library(libunwind STATIC IMPORTED)
     set_target_properties(libunwind PROPERTIES IMPORTED_LOCATION

ndk/src/main/jni/bugsnag_ndk.c

@@ -46,7 +46,9 @@ JNIEXPORT void JNICALL Java_com_bugsnag_android_ndk_NativeBridge_install(
     JNIEnv *env, jobject _this, jstring _report_path, jboolean auto_notify,
     jint _api_level, jboolean is32bit) {
   bsg_environment *bugsnag_env = calloc(1, sizeof(bsg_environment));
-  bugsnag_env->unwind_style = bsg_get_unwind_type((int)_api_level, (bool)is32bit);
+  bsg_set_unwind_types((int)_api_level, (bool)is32bit,
+                       &bugsnag_env->signal_unwind_style,
+                       &bugsnag_env->unwind_style);
   bugsnag_env->report_header.big_endian =
       htonl(47) == 47; // potentially too clever, see man 3 htonl
   bugsnag_env->report_header.version = BUGSNAG_REPORT_VERSION;

ndk/src/main/jni/bugsnag_ndk.h

@@ -20,6 +20,13 @@ extern "C" {
 #endif
 
 typedef struct {
+    /**
+     * Unwinding style used for signal-safe handling
+     */
+    bsg_unwinder signal_unwind_style;
+    /**
+     * Preferred unwinding style
+     */
     bsg_unwinder unwind_style;
     bsg_report_header report_header;
     /**

ndk/src/main/jni/handlers/signal_handler.c

@@ -175,7 +175,7 @@ void bsg_handle_signal(int signum, siginfo_t *info,
   bsg_global_env->handling_crash = true;
   bsg_populate_report_as(bsg_global_env);
   bsg_global_env->next_report.exception.frame_count = bsg_unwind_stack(
-      bsg_global_env->unwind_style,
+      bsg_global_env->signal_unwind_style,
       bsg_global_env->next_report.exception.stacktrace, info, user_context);
 
   for (int i = 0; i < BSG_HANDLED_SIGNAL_COUNT; i++) {

ndk/src/main/jni/utils/stack_unwinder.c

@@ -1,6 +1,7 @@
 #include "stack_unwinder.h"
 #include "stack_unwinder_libcorkscrew.h"
 #include "stack_unwinder_libunwind.h"
+#include "stack_unwinder_libunwindstack.h"
 #include "stack_unwinder_simple.h"
 #include "string.h"
 #include <asm/siginfo.h>
@@ -9,18 +10,35 @@
 #include <ucontext.h>
 
 #define BSG_LIBUNWIND_LEVEL 21
+#define BSG_LIBUNWINDSTACK_LEVEL 15
+#define BSG_LIBUNWIND_LEVEL_ARM32 16
 #define BSG_LIBCORKSCREW_MIN_LEVEL 16
 #define BSG_LIBCORKSCREW_MAX_LEVEL 19
 
-bsg_unwinder bsg_get_unwind_type(int apiLevel, bool is32bit) {
-  if (apiLevel >= BSG_LIBUNWIND_LEVEL && bsg_configure_libunwind(is32bit)) {
-    return BSG_LIBUNWIND;
-  } else if (apiLevel <= BSG_LIBCORKSCREW_MAX_LEVEL &&
-             apiLevel >= BSG_LIBCORKSCREW_MIN_LEVEL &&
-             bsg_configure_libcorkscrew()) {
-    return BSG_LIBCORKSCREW;
+void bsg_set_unwind_types(int apiLevel, bool is32bit, bsg_unwinder *signal_type,
+                          bsg_unwinder *other_type) {
+#if defined(__arm__)
+  if (apiLevel >= BSG_LIBUNWIND_LEVEL_ARM32 && is32bit &&
+      bsg_configure_libunwind(is32bit)) {
+    if (apiLevel >= BSG_LIBUNWIND_LEVEL) {
+      *signal_type = BSG_LIBUNWIND;
+    } else if (bsg_configure_libcorkscrew()) {
+      *signal_type = BSG_LIBCORKSCREW;
+    } else {
+      *signal_type = BSG_CUSTOM_UNWIND;
+    }
+    *other_type = BSG_LIBUNWIND;
+    return;
+  }
+#endif
+  if (apiLevel >= BSG_LIBUNWINDSTACK_LEVEL) {
+    bsg_configure_libunwind(is32bit);
+    *signal_type = BSG_LIBUNWINDSTACK;
+    *other_type = BSG_LIBUNWIND;
+  } else {
+    *signal_type = BSG_CUSTOM_UNWIND;
+    *other_type = BSG_CUSTOM_UNWIND;
   }
-  return BSG_CUSTOM_UNWIND;
 }
 
 void bsg_insert_fileinfo(ssize_t frame_count,
@@ -46,7 +64,10 @@ ssize_t bsg_unwind_stack(bsg_unwinder unwind_style,
                          bsg_stackframe stacktrace[BUGSNAG_FRAMES_MAX],
                          siginfo_t *info, void *user_context) {
   ssize_t frame_count = 0;
-  if (unwind_style == BSG_LIBUNWIND) {
+  if (unwind_style == BSG_LIBUNWINDSTACK) {
+    frame_count =
+        bsg_unwind_stack_libunwindstack(stacktrace, info, user_context);
+  } else if (unwind_style == BSG_LIBUNWIND) {
     frame_count = bsg_unwind_stack_libunwind(stacktrace, info, user_context);
   } else if (unwind_style == BSG_LIBCORKSCREW) {
     frame_count = bsg_unwind_stack_libcorkscrew(stacktrace, info, user_context);

ndk/src/main/jni/utils/stack_unwinder.h

@@ -11,19 +11,23 @@ extern "C" {
 
 typedef enum {
   BSG_LIBUNWIND,
+  BSG_LIBUNWINDSTACK,
   BSG_LIBCORKSCREW,
   BSG_CUSTOM_UNWIND,
 } bsg_unwinder;
 
+
 /**
  * Based on the current environment, determine what unwinding library to use.
  *
  * Android API level 21+: libunwind
- * Android API level 16-19: libcorkscrew
+ * Android API level 16-19: libunwind, unless in a signal handler. Then
+ * libcorkscrew.
  * Everything else: custom unwinding logic
- * @return the preferred unwind type
  */
-bsg_unwinder bsg_get_unwind_type(int apiLevel, bool is32bit);
+void bsg_set_unwind_types(int apiLevel, bool is32bit,
+                          bsg_unwinder *signal_type,
+                          bsg_unwinder *other_type);
 
 /**
  * Unwind the stack using the preferred tool/style. If info and a user

ndk/src/main/jni/utils/stack_unwinder_libunwind.c

@@ -102,6 +102,9 @@ bsg_unwind_stack_libunwind(bsg_stackframe stacktrace[BUGSNAG_FRAMES_MAX],
     return bsg_unwind_stack_libunwind_arm32(stacktrace, info, user_context);
   }
 #endif
+  if (bsg_global_libunwind_state == NULL) {
+    return 0;
+  }
   bsg_global_libunwind_state->frame_count = 0;
   // The return value of _Unwind_Backtrace sits on a throne of lies
   _Unwind_Backtrace(bsg_libunwind_callback, bsg_global_libunwind_state);

ndk/src/main/jni/utils/stack_unwinder_libunwindstack.cpp

@@ -0,0 +1,68 @@
+#include "../utils/string.h"
+#include "stack_unwinder_libunwindstack.h"
+#include <stdlib.h>
+#include <ucontext.h>
+#include <unwindstack/Elf.h>
+#include <unwindstack/MapInfo.h>
+#include <unwindstack/Maps.h>
+#include <unwindstack/Memory.h>
+#include <unwindstack/Regs.h>
+
+ssize_t
+bsg_unwind_stack_libunwindstack(bsg_stackframe stacktrace[BUGSNAG_FRAMES_MAX],
+                                siginfo_t *info, void *user_context) {
+  if (user_context == NULL) {
+    return 0; // only handle unwinding from signals
+  }
+
+  const std::unique_ptr<unwindstack::Regs> regs(
+      unwindstack::Regs::CreateFromUcontext(unwindstack::Regs::CurrentArch(),
+                                            user_context));
+
+  // if (user_context != NULL) {
+  // regs = unwindstack::Regs::CreateFromUcontext(
+  // unwindstack::Regs::CurrentArch(), user_context);
+  //} else {
+  // regs = unwindstack::Regs::CreateFromLocal();
+  //}
+
+  std::string unw_function_name;
+  unwindstack::LocalMaps maps;
+
+  if (!maps.Parse()) {
+    stacktrace[0].frame_address = regs->pc(); // only known frame
+    return 1;
+  }
+
+  const std::shared_ptr<unwindstack::Memory> memory(
+      new unwindstack::MemoryLocal);
+
+  int frame_count = 0;
+  for (int i = 0; i < BUGSNAG_FRAMES_MAX; i++) {
+    unwindstack::MapInfo *const map_info = maps.Find(regs->pc());
+    unwindstack::Elf *const elf = map_info->GetElf(memory, false);
+    if (!map_info || !elf) {
+      stacktrace[frame_count++].frame_address = regs->pc();
+      break;
+    }
+
+    // Getting value of program counter relative module where a function is
+    // located.
+    const uint64_t rel_pc = elf->GetRelPc(regs->pc(), map_info);
+    uint64_t adjusted_rel_pc = rel_pc;
+    if (frame_count != 0) {
+      // If it's not a first frame we need to rewind program counter value to
+      // previous instruction. For the first frame pc from ucontext points
+      // exactly to a failed instruction, for other frames rel_pc will contain
+      // return address after function call instruction.
+      adjusted_rel_pc -= regs->GetPcAdjustment(rel_pc, elf);
+    }
+    stacktrace[frame_count++].frame_address = regs->pc();
+    bool finished = false;
+    if (!elf->Step(rel_pc, adjusted_rel_pc, map_info->elf_offset, regs.get(),
+                   memory.get(), &finished)) {
+      break;
+    }
+  }
+  return frame_count;
+}

ndk/src/main/jni/utils/stack_unwinder_libunwindstack.h

@@ -0,0 +1,13 @@
+#ifndef BUGSNAG_UTILS_STACK_UNWINDER_LIBUNWINDSTACK_H
+#define BUGSNAG_UTILS_STACK_UNWINDER_LIBUNWINDSTACK_H
+
+#include "../report.h"
+#include <signal.h>
+
+#ifdef __cplusplus
+extern "C"
+#endif
+ssize_t
+bsg_unwind_stack_libunwindstack(bsg_stackframe stacktrace[BUGSNAG_FRAMES_MAX],
+                                siginfo_t *info, void *user_context);
+#endif