ext/opcache/zend_shared_alloc: use memfd for locking if available

A memfd is a virtual file that has no reachable path, therefore does
not clobber any filesystem.  It is deleted automatically as soon as
the last handle gets closed.  The feature is available since Linux
kernel 3.17.

Author: MaxKellermann

ext/opcache/config.m4

@@ -105,7 +105,7 @@ if test "$PHP_OPCACHE" != "no"; then
 
   fi
 
-  AC_CHECK_FUNCS([mprotect])
+  AC_CHECK_FUNCS([mprotect memfd_create])
 
   AC_MSG_CHECKING(for sysvipc shared memory support)
   AC_RUN_IFELSE([AC_LANG_SOURCE([[

ext/opcache/zend_shared_alloc.c

@@ -19,6 +19,13 @@
    +----------------------------------------------------------------------+
 */
 
+#if defined(__linux__) && defined(HAVE_MEMFD_CREATE)
+# ifndef _GNU_SOURCE
+#  define _GNU_SOURCE
+# endif
+# include <sys/mman.h>
+#endif
+
 #include <errno.h>
 #include "ZendAccelerator.h"
 #include "zend_shared_alloc.h"
@@ -81,6 +88,18 @@ void zend_shared_alloc_create_lock(char *lockfile_path)
 	zts_lock = tsrm_mutex_alloc();
 #endif
 
+#if defined(__linux__) && defined(HAVE_MEMFD_CREATE)
+	/* on Linux, we can use a memfd instead of a "real" file, so
+	 * we can do this without a writable filesystem and without
+	 * needing to clean up */
+	/* note: FreeBSD has memfd_create(), too, but fcntl(F_SETLKW)
+	 * on it fails with EBADF, therefore we use this only on
+	 * Linux */
+	lock_file = memfd_create("opcache_lock", MFD_CLOEXEC);
+	if (lock_file >= 0)
+		return;
+#endif
+
 	snprintf(lockfile_name, sizeof(lockfile_name), "%s/%sXXXXXX", lockfile_path, SEM_FILENAME_PREFIX);
 	lock_file = mkstemp(lockfile_name);
 	if (lock_file == -1) {