am f84a5918: Merge "Use pread() in ZipFileRO for Linux" into gingerbread

Merge commit 'f84a5918f5e0572c0280fa705970ef7459b45393' into gingerbread-plus-aosp

* commit 'f84a5918f5e0572c0280fa705970ef7459b45393':
  Use pread() in ZipFileRO for Linux

include/utils/ZipFileRO.h

@@ -14,13 +14,19 @@
  * limitations under the License.
  */
 
-//
+/*
-// Read-only access to Zip archives, with minimal heap allocation.
+ * Read-only access to Zip archives, with minimal heap allocation.
-//
+ *
-// This is similar to the more-complete ZipFile class, but no attempt
+ * This is similar to the more-complete ZipFile class, but no attempt
-// has been made to make them interchangeable.  This class operates under
+ * has been made to make them interchangeable.  This class operates under
-// a very different set of assumptions and constraints.
+ * a very different set of assumptions and constraints.
-//
+ *
+ * One such assumption is that if you're getting file descriptors for
+ * use with this class as a child of a fork() operation, you must be on
+ * a pread() to guarantee correct operation. This is because pread() can
+ * atomically read at a file offset without worrying about a lock around an
+ * lseek() + read() pair.
+ */
 #ifndef __LIBS_ZIPFILERO_H
 #define __LIBS_ZIPFILERO_H
 
@@ -55,6 +61,10 @@ typedef void* ZipEntryRO;
  * the record structure.  However, this requires a private mapping of
  * every page that the Central Directory touches.  Easier to tuck a copy
  * of the string length into the hash table entry.
+ *
+ * NOTE: If this is used on file descriptors inherited from a fork() operation,
+ * you must be on a platform that implements pread() to guarantee correctness
+ * on the shared file descriptors.
  */
 class ZipFileRO {
 public:

libs/utils/ZipFileRO.cpp

@@ -508,6 +508,36 @@ bool ZipFileRO::getEntryInfo(ZipEntryRO entry, int* pMethod, size_t* pUncompLen,
 
         unsigned char lfhBuf[kLFHLen];
 
+#ifdef HAVE_PREAD
+        /*
+         * This file descriptor might be from zygote's preloaded assets,
+         * so we need to do an pread() instead of a lseek() + read() to
+         * guarantee atomicity across the processes with the shared file
+         * descriptors.
+         */
+        ssize_t actual =
+                TEMP_FAILURE_RETRY(pread(mFd, lfhBuf, sizeof(lfhBuf), localHdrOffset));
+
+        if (actual != sizeof(lfhBuf)) {
+            LOGW("failed reading lfh from offset %ld\n", localHdrOffset);
+            return false;
+        }
+
+        if (get4LE(lfhBuf) != kLFHSignature) {
+            LOGW("didn't find signature at start of lfh; wanted: offset=%ld data=0x%08x; "
+                    "got: data=0x%08lx\n",
+                    localHdrOffset, kLFHSignature, get4LE(lfhBuf));
+            return false;
+        }
+#else /* HAVE_PREAD */
+        /*
+         * For hosts don't have pread() we cannot guarantee atomic reads from
+         * an offset in a file. Android should never run on those platforms.
+         * File descriptors inherited from a fork() share file offsets and
+         * there would be nothing to protect from two different processes
+         * calling lseek() concurrently.
+         */
+
         {
             AutoMutex _l(mFdLock);
 
@@ -531,6 +561,7 @@ bool ZipFileRO::getEntryInfo(ZipEntryRO entry, int* pMethod, size_t* pUncompLen,
                 return false;
             }
         }
+#endif /* HAVE_PREAD */
 
         off_t dataOffset = localHdrOffset + kLFHLen
             + get2LE(lfhBuf + kLFHNameLen) + get2LE(lfhBuf + kLFHExtraLen);