Use OFD locks where available

Linux 3.15 has added OFD locks, which contend with POSIX file locks
but are owned by the open file description instead of the process.
Use these in krb5_lock_file where available, for safer concurrency
behavior.

ticket: 8023 (new)

diff --git a/src/lib/krb5/os/lock_file.c b/src/lib/krb5/os/lock_file.c
index 224e58fbf5b106cbad2dc5f3b7e013801b54d6cf..a2f247cd994d62c1430fb3fb491de5918becbb5c 100644 (file)
--- a/src/lib/krb5/os/lock_file.c
+++ b/src/lib/krb5/os/lock_file.c
@@ -57,6 +57,30 @@
 #endif
 #endif
 
+#ifdef POSIX_FILE_LOCKS
+/*
+ * Try to use OFD locks where available (e.g. Linux 3.15 and later).  OFD locks
+ * contend with regular POSIX file locks, but are owned by the open file
+ * description instead of the process, which is much better for thread safety.
+ * Fall back to regular POSIX locks on EINVAL in case we are running with an
+ * older kernel than we were built with.
+ */
+static int
+ofdlock(int fd, int cmd, struct flock *lock_arg)
+{
+#ifdef F_OFD_SETLKW
+    int st, ofdcmd;
+
+    assert(cmd == F_SETLKW || cmd == F_SETLK);
+    ofdcmd = (cmd == F_SETLKW) ? F_OFD_SETLKW : F_OFD_SETLK;
+    st = fcntl(fd, ofdcmd, lock_arg);
+    if (st == 0 || errno != EINVAL)
+        return st;
+#endif
+    return fcntl(fd, cmd, lock_arg);
+}
+#endif /* POSIX_FILE_LOCKS */
+
 /*ARGSUSED*/
 krb5_error_code
 krb5_lock_file(krb5_context context, int fd, int mode)
@@ -105,7 +129,7 @@ krb5_lock_file(krb5_context context, int fd, int mode)
     lock_arg.l_whence = 0;
     lock_arg.l_start = 0;
     lock_arg.l_len = 0;
-    if (fcntl(fd, lock_cmd, &lock_arg) == -1) {
+    if (ofdlock(fd, lock_cmd, &lock_arg) == -1) {
         if (errno == EACCES || errno == EAGAIN) /* see POSIX/IEEE 1003.1-1988,
                                                    6.5.2.4 */
             return(EAGAIN);