debuggerd_handler: don't use clone(..., SIGCHLD, ...)

Processes that handle SIGCHLD can race with the crash handler to wait
on the crash_dump process. Use clone flags that cause the forked
child's death to not be reported via SIGCHLD, and don't bail out of
dumping when waitpid returns ECHILD (in case another thread is already
in a waitpid(..., __WALL))

Note that the use of waitid was switched to waitpid, because waitid
doesn't support __WCLONE until kernel version 4.7.

Bug: none
Test: "debuggerd -b `pidof zygote64`" a few times (failed roughly 50%
      of the time previously)
Change-Id: Ia41a26a61f13c6f9aa85c4c2f88aef8d279d35ad

1 files changed, 5 insertions, 3 deletions

diff --git a/debuggerd/handler/debuggerd_handler.cpp b/debuggerd/handler/debuggerd_handler.cpp
index 4cc077dcc..21cc0c771 100644
--- a/debuggerd/handler/debuggerd_handler.cpp
+++ b/debuggerd/handler/debuggerd_handler.cpp
@@ -197,7 +197,7 @@ static int debuggerd_dispatch_pseudothread(void* arg) {
   }
 
   // Don't use fork(2) to avoid calling pthread_atfork handlers.
-  int forkpid = clone(nullptr, nullptr, SIGCHLD, nullptr);
+  int forkpid = clone(nullptr, nullptr, 0, nullptr);
   if (forkpid == -1) {
     __libc_format_log(ANDROID_LOG_FATAL, "libc", "failed to fork in debuggerd signal handler: %s",
                       strerror(errno));
@@ -237,10 +237,12 @@ static int debuggerd_dispatch_pseudothread(void* arg) {
     close(pipefds[0]);
 
     // Don't leave a zombie child.
-    siginfo_t child_siginfo;
-    if (TEMP_FAILURE_RETRY(waitid(P_PID, forkpid, &child_siginfo, WEXITED)) != 0) {
+    int status;
+    if (TEMP_FAILURE_RETRY(waitpid(forkpid, &status, __WCLONE)) == -1 && errno != ECHILD) {
       __libc_format_log(ANDROID_LOG_FATAL, "libc", "failed to wait for crash_dump helper: %s",
                         strerror(errno));
+    } else if (WIFSTOPPED(status) || WIFSIGNALED(status)) {
+      __libc_format_log(ANDROID_LOG_FATAL, "libc", "crash_dump helper crashed or stopped");
       thread_info->crash_dump_started = false;
     }
   }