Zygote : Block SIGCHLD during fork.

We close the android logging related sockets prior as late as possible
before every fork to avoid having to whitelist them. If one of the
zygote's children dies after this point (but prior to the fork), we can
end up reopening the logging sockets from the SIGCHLD signal handler.

To prevent this from happening, block SIGCHLD during this critical
section.

Bug: 32693692
Test: Manual

(cherry picked from commit e9a525829a354c92983a35455ccab16d1b0d3892)

Zygote: Unblock SIGCHLD in the parent after fork.

Follow up to change e9a525829a354c92983a. Allows the zygote to
receive SIGCHLD again and prevents the zygote from getting into a
zombie state if it's killed.

Contributed-By: rhed_jao <rhed_jao@htc.com>
Bug: 32693692
Test: manual

(cherry picked from commit c7161f756e86b98f2244a04d9207b47149965fd7)

Change-Id: If89903a29c84dfc9b056f9e19618046874bba689

1 files changed, 24 insertions, 0 deletions

diff --git a/core/jni/com_android_internal_os_Zygote.cpp b/core/jni/com_android_internal_os_Zygote.cpp
index a1fef4a4d963..4f9ea68ade5a 100644
--- a/core/jni/com_android_internal_os_Zygote.cpp
+++ b/core/jni/com_android_internal_os_Zygote.cpp
@@ -465,6 +465,20 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
   SetForkLoad(true);
 #endif
 
+  sigset_t sigchld;
+  sigemptyset(&sigchld);
+  sigaddset(&sigchld, SIGCHLD);
+
+  // Temporarily block SIGCHLD during forks. The SIGCHLD handler might
+  // log, which would result in the logging FDs we close being reopened.
+  // This would cause failures because the FDs are not whitelisted.
+  //
+  // Note that the zygote process is single threaded at this point.
+  if (sigprocmask(SIG_BLOCK, &sigchld, nullptr) == -1) {
+    ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+    RuntimeAbort(env, __LINE__, "Call to sigprocmask(SIG_BLOCK, { SIGCHLD }) failed.");
+  }
+
   // Close any logging related FDs before we start evaluating the list of
   // file descriptors.
   __android_log_close();
@@ -496,6 +510,11 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
       RuntimeAbort(env, __LINE__, "Unable to reopen whitelisted descriptors.");
     }
 
+    if (sigprocmask(SIG_UNBLOCK, &sigchld, nullptr) == -1) {
+      ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+      RuntimeAbort(env, __LINE__, "Call to sigprocmask(SIG_UNBLOCK, { SIGCHLD }) failed.");
+    }
+
     // Keep capabilities across UID change, unless we're staying root.
     if (uid != 0) {
       EnableKeepCapabilities(env);
@@ -633,6 +652,11 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
     SetForkLoad(false);
 #endif
 
+    // We blocked SIGCHLD prior to a fork, we unblock it here.
+    if (sigprocmask(SIG_UNBLOCK, &sigchld, nullptr) == -1) {
+      ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+      RuntimeAbort(env, __LINE__, "Call to sigprocmask(SIG_UNBLOCK, { SIGCHLD }) failed.");
+    }
   }
   return pid;
 }