commit c2cc46670135e14dfb3e7482d9d584862addf547
author Josh Gao <jmgao@google.com> Sun Jan 22 16:42:32 2017 -0800
committer Josh Gao <jmgao@google.com> Mon Jan 23 11:34:49 2017 -0800
tree 8b243bfb7519cbbad91c1673486b79ad1bd9f2a1
parent 86dddf9f05b26ee4365821bcffecfebf2b881678

crash_dump: switch to PTRACE_SEIZE.

ptrace(PTRACE_ATTACH) does not immediately put the traced process in a
stopped state. If we manage to call PTRACE_CONT on it before it
finishes, we'll fail. Switch to using PTRACE_SEIZE and PTRACE_INTERRUPT
to guarantee that the target stops immediately.

This also has the advantage of never putting the process in group-stop
state, which means if we exit prematurely (e.g. if we crash during
stack unwinding), the target process will be resumed without any
intervention needed.

Bug: http://b/34472671
Test: while true; do debuggerd -b `pidof audioserver`; done
Change-Id: I7549f67489646cf300b8c9aa8c735e5e897806ef

debuggerd/crash_dump.cpp

diff --git a/debuggerd/crash_dump.cpp b/debuggerd/crash_dump.cpp
index cd939ef..1af00b4 100644
--- a/debuggerd/crash_dump.cpp
+++ b/debuggerd/crash_dump.cpp
@@ -57,8 +57,8 @@
 }
 
 // Attach to a thread, and verify that it's still a member of the given process
-static bool ptrace_attach_thread(pid_t pid, pid_t tid, std::string* error) {
-  if (ptrace(PTRACE_ATTACH, tid, 0, 0) != 0) {
+static bool ptrace_seize_thread(pid_t pid, pid_t tid, std::string* error) {
+  if (ptrace(PTRACE_SEIZE, tid, 0, 0) != 0) {
     *error = StringPrintf("failed to attach to thread %d: %s", tid, strerror(errno));
     return false;
   }
@@ -71,6 +71,12 @@
     *error = StringPrintf("thread %d is not in process %d", tid, pid);
     return false;
   }
+
+  // Put the task into ptrace-stop state.
+  if (ptrace(PTRACE_INTERRUPT, tid, 0, 0) != 0) {
+    PLOG(FATAL) << "failed to interrupt thread " << tid;
+  }
+
   return true;
 }
 
@@ -254,7 +260,7 @@
   check_process(target_proc_fd, target);
 
   std::string attach_error;
-  if (!ptrace_attach_thread(target, main_tid, &attach_error)) {
+  if (!ptrace_seize_thread(target, main_tid, &attach_error)) {
     LOG(FATAL) << attach_error;
   }
 
@@ -279,9 +285,9 @@
 
   LOG(INFO) << "performing dump of process " << target << " (target tid = " << main_tid << ")";
 
-  // At this point, the thread that made the request has been PTRACE_ATTACHed
-  // and has the signal that triggered things queued. Send PTRACE_CONT, and
-  // then wait for the signal.
+  // At this point, the thread that made the request has been attached and is
+  // in ptrace-stopped state. After resumption, the triggering signal that has
+  // been queued will be delivered.
   if (ptrace(PTRACE_CONT, main_tid, 0, 0) != 0) {
     PLOG(ERROR) << "PTRACE_CONT(" << main_tid << ") failed";
     exit(1);
@@ -320,7 +326,7 @@
     siblings.erase(main_tid);
 
     for (pid_t sibling_tid : siblings) {
-      if (!ptrace_attach_thread(target, sibling_tid, &attach_error)) {
+      if (!ptrace_seize_thread(target, sibling_tid, &attach_error)) {
         LOG(WARNING) << attach_error;
       } else {
         attached_siblings.insert(sibling_tid);