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)

Also contains a port of commit c7161f756e86b98f2244a04d9207b.

Change-Id: I27dc83218df592ace8bff2942e8d94dcd5d61931

Author: narayank

vm/native/dalvik_system_Zygote.cpp

@@ -600,6 +600,23 @@ static pid_t forkAndSpecializeCommon(const u4* args, bool isSystemServer, bool l
 
     dvmDumpLoaderStats("zygote");
 
+    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, NULL) == -1) {
+        ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed:  %s", strerror(errno));
+        dvmAbort();
+    }
+
+
     // Close any logging related FDs before we start evaluating the list of
     // file descriptors.
     __android_log_close();
@@ -665,6 +682,11 @@ static pid_t forkAndSpecializeCommon(const u4* args, bool isSystemServer, bool l
             ALOGE("Unable to reopen whitelisted descriptors.");
             dvmAbort();
         }
+
+        if (sigprocmask(SIG_UNBLOCK, &sigchld, NULL) == -1) {
+            ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+            dvmAbort();
+        }
 #endif /* HAVE_ANDROID_OS */
 
         if (mountMode != MOUNT_EXTERNAL_NONE) {
@@ -759,6 +781,13 @@ static pid_t forkAndSpecializeCommon(const u4* args, bool isSystemServer, bool l
         /* the parent process */
         free(seInfo);
         free(niceName);
+
+        // We blocked SIGCHLD prior to a fork, we unblock it here.
+        if (sigprocmask(SIG_UNBLOCK, &sigchld, NULL) == -1) {
+            ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+            dvmAbort();
+        }
+
     }
 
     return pid;