Disable return PAC in __pthread_start.

This function doesn't return, but it does appear in stack traces. Avoid
using return PAC in this function because we may end up resetting IA,
which may confuse unwinders due to mismatching keys.

Bug: 189808795
Change-Id: I953da9078acd1d43eb7a47fb11f75caa0099fa12

2 files changed, 24 insertions, 4 deletions

diff --git a/libc/bionic/pthread_create.cpp b/libc/bionic/pthread_create.cpp
index 46d9e8672..121b26f82 100644
--- a/libc/bionic/pthread_create.cpp
+++ b/libc/bionic/pthread_create.cpp
@@ -331,6 +331,11 @@ void __set_stack_and_tls_vma_name(bool is_main_thread) {
 extern "C" int __rt_sigprocmask(int, const sigset64_t*, sigset64_t*, size_t);
 
 __attribute__((no_sanitize("hwaddress")))
+#ifdef __aarch64__
+// This function doesn't return, but it does appear in stack traces. Avoid using return PAC in this
+// function because we may end up resetting IA, which may confuse unwinders due to mismatching keys.
+__attribute__((target("branch-protection=bti")))
+#endif
 static int __pthread_start(void* arg) {
   pthread_internal_t* thread = reinterpret_cast<pthread_internal_t*>(arg);
 
diff --git a/tests/stack_unwinding_test.cpp b/tests/stack_unwinding_test.cpp
index 0ff6f30a6..2f891a6e1 100644
--- a/tests/stack_unwinding_test.cpp
+++ b/tests/stack_unwinding_test.cpp
@@ -66,13 +66,28 @@ static int noinline unwind_one_frame_deeper() {
   return count;
 }
 
-TEST(stack_unwinding, easy) {
+static void UnwindTest() {
   int count = 0;
   _Unwind_Backtrace(FrameCounter, &count);
   int deeper_count = unwind_one_frame_deeper();
   ASSERT_EQ(count + 1, deeper_count);
 }
 
+TEST(stack_unwinding, easy) {
+  UnwindTest();
+}
+
+TEST(stack_unwinding, thread) {
+  pthread_t thread;
+  ASSERT_EQ(0, pthread_create(&thread, nullptr, [](void*) -> void* {
+    UnwindTest();
+    return nullptr;
+  }, nullptr));
+  void *retval;
+  ASSERT_EQ(0, pthread_join(thread, &retval));
+  EXPECT_EQ(nullptr, retval);
+}
+
 struct UnwindData {
   volatile bool signal_handler_complete = false;
   int expected_frame_count = 0;
@@ -98,7 +113,7 @@ static void verify_unwind_data(const UnwindData& unwind_data) {
   EXPECT_EQ(unwind_data.handler_frame_count + 1, unwind_data.handler_one_deeper_frame_count);
 }
 
-static void noinline UnwindTest() {
+static void noinline SignalUnwindTest() {
   g_unwind_data = {};
 
   _Unwind_Backtrace(FrameCounter, &g_unwind_data.expected_frame_count);
@@ -114,12 +129,12 @@ static void noinline UnwindTest() {
 TEST(stack_unwinding, unwind_through_signal_frame) {
   ScopedSignalHandler ssh(SIGUSR1, UnwindSignalHandler);
 
-  UnwindTest();
+  SignalUnwindTest();
 }
 
 // On LP32, the SA_SIGINFO flag gets you __restore_rt instead of __restore.
 TEST(stack_unwinding, unwind_through_signal_frame_SA_SIGINFO) {
   ScopedSignalHandler ssh(SIGUSR1, UnwindSignalHandler, SA_SIGINFO);
 
-  UnwindTest();
+  SignalUnwindTest();
 }