Test cached PID using getpid instead of pthread_getattr_np.

Previously, we tested that glibc's cached PID was correct by relying on
an assert inside of pthread_getattr_np. This relies on a fairly
implementation-specific detail of pthread_getattr_np and is also
problematic, as glibc's pthread_getattr_np relies on reading
/proc/self/maps when called from the main thread (but the test chroots).

This change inspects the cached PID more directly using getpid instead.

BUG=646191

Review-Url: https://codereview.chromium.org/2474703003
Cr-Commit-Position: refs/heads/master@{#429369}

sandbox/linux/services/credentials_unittest.cc

@@ -246,18 +246,19 @@ void SignalHandler(int sig) {
   signal_handler_called = 1;
 }
 
+// glibc (and some other libcs) caches the PID and TID in TLS. This test
+// verifies that these values are correct after DropFilesystemAccess.
 // Disabled on ASAN because of crbug.com/451603.
 SANDBOX_TEST(Credentials, DISABLE_ON_ASAN(DropFileSystemAccessPreservesTLS)) {
   // Probably missing kernel support.
   if (!Credentials::MoveToNewUserNS()) return;
   CHECK(Credentials::DropFileSystemAccess(ProcUtil::OpenProc().get()));
 
-  // In glibc, pthread_getattr_np makes an assertion about the cached PID/TID in
+  // The libc getpid implementation may return a cached PID. Ensure that
-  // TLS.
+  // it matches the PID returned from the getpid system call.
-  pthread_attr_t attr;
+  CHECK_EQ(sys_getpid(), getpid());
-  EXPECT_EQ(0, pthread_getattr_np(pthread_self(), &attr));
 
-  // raise also uses the cached TID in glibc.
+  // raise uses the cached TID in glibc.
   struct sigaction action = {};
   action.sa_handler = &SignalHandler;
   PCHECK(sigaction(SIGUSR1, &action, nullptr) == 0);