commit 0fbb60811076e6fcfd576287b6e6a16ac1a69c44
author Martin Wallgren <martin.wallgren@sonymobile.com> Tue Aug 11 15:10:31 2015 +0200
committer Adam Lesinski <adamlesinski@google.com> Wed Oct 12 14:56:31 2016 +0000
tree 316e242fdf315b77ab518670996598a11a91ca1b
parent 52cd2948a622a7793f315b29239c3faee7d84ab3

RRO: Synchronize access to overlays.list

idmap --scan is executed as a part of the pre-loading in ZygoteInit.
The pre loading is executed in parallel for each supported architecture
(32/64 bit). This will cause a race condition in the creation of the
overlays.list file and the idmap files for the system overlays.

Apply flock on overlays.list to prevent the file from being thrown away
and recreated when it is in use by another Zygote.

Cherry-picked from AOSP.

Bug: 28032298
Test: manual
Change-Id: I51d39f121d207b11181340b68b164b60020f0c61

cmds/idmap/scan.cpp

diff --git a/cmds/idmap/scan.cpp b/cmds/idmap/scan.cpp
index 6d30f0d..ab6adfb 100644
--- a/cmds/idmap/scan.cpp
+++ b/cmds/idmap/scan.cpp
@@ -1,5 +1,6 @@
 #include <dirent.h>
 #include <inttypes.h>
+#include <sys/file.h>
 #include <sys/stat.h>
 
 #include "idmap.h"
@@ -35,16 +36,31 @@
 
     bool writePackagesList(const char *filename, const SortedVector<Overlay>& overlayVector)
     {
-        FILE* fout = fopen(filename, "w");
+        // the file is opened for appending so that it doesn't get truncated
+        // before we can guarantee mutual exclusion via the flock
+        FILE* fout = fopen(filename, "a");
         if (fout == NULL) {
             return false;
         }
 
+        if (TEMP_FAILURE_RETRY(flock(fileno(fout), LOCK_EX)) != 0) {
+            fclose(fout);
+            return false;
+        }
+
+        if (TEMP_FAILURE_RETRY(ftruncate(fileno(fout), 0)) != 0) {
+            TEMP_FAILURE_RETRY(flock(fileno(fout), LOCK_UN));
+            fclose(fout);
+            return false;
+        }
+
         for (size_t i = 0; i < overlayVector.size(); ++i) {
             const Overlay& overlay = overlayVector[i];
             fprintf(fout, "%s %s\n", overlay.apk_path.string(), overlay.idmap_path.string());
         }
 
+        TEMP_FAILURE_RETRY(fflush(fout));
+        TEMP_FAILURE_RETRY(flock(fileno(fout), LOCK_UN));
         fclose(fout);
 
         // Make file world readable since Zygote (running as root) will read
@@ -171,9 +187,6 @@
 {
     String8 filename = String8(idmap_dir);
     filename.appendPath("overlays.list");
-    if (unlink(filename.string()) != 0 && errno != ENOENT) {
-        return EXIT_FAILURE;
-    }
 
     SortedVector<Overlay> overlayVector;
     const size_t N = overlay_dirs->size();

libs/androidfw/AssetManager.cpp

diff --git a/libs/androidfw/AssetManager.cpp b/libs/androidfw/AssetManager.cpp
index 8ea25d6..07044d0 100644
--- a/libs/androidfw/AssetManager.cpp
+++ b/libs/androidfw/AssetManager.cpp
@@ -35,6 +35,9 @@
 #include <utils/threads.h>
 #include <utils/Timers.h>
 #include <utils/Trace.h>
+#ifndef _WIN32
+#include <sys/file.h>
+#endif
 
 #include <assert.h>
 #include <dirent.h>
@@ -767,6 +770,12 @@
         return;
     }
 
+#ifndef _WIN32
+    if (TEMP_FAILURE_RETRY(flock(fileno(fin), LOCK_SH)) != 0) {
+        fclose(fin);
+        return;
+    }
+#endif
     char buf[1024];
     while (fgets(buf, sizeof(buf), fin)) {
         // format of each line:
@@ -797,6 +806,10 @@
             const_cast<AssetManager*>(this)->mZipSet.addOverlay(targetPackagePath, oap);
         }
     }
+
+#ifndef _WIN32
+    TEMP_FAILURE_RETRY(flock(fileno(fin), LOCK_UN));
+#endif
     fclose(fin);
 }