From 65110210b75d38908cdd84cb202cf013ccf6ed0e Mon Sep 17 00:00:00 2001
From: Greg Hudson <ghudson@mit.edu>
Date: Sun, 14 Aug 2016 12:08:16 -0400
Subject: [PATCH] Work around glibc OFD lock bug on 32-bit Linux

A bug in Gnu libc causes OFD locking to fail unpredictably on 32-bit
Linux, typically leading to deadlocks.  Work around this bug by using
the fcntl64 system call and struct flock64.

See also: https://sourceware.org/bugzilla/show_bug.cgi?id=20251

ticket: 8474
target_version: 1.14-next
tags: pullup
---
 src/lib/krb5/os/lock_file.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/lib/krb5/os/lock_file.c b/src/lib/krb5/os/lock_file.c
index a2f247c..2360c96 100644
--- a/src/lib/krb5/os/lock_file.c
+++ b/src/lib/krb5/os/lock_file.c
@@ -43,7 +43,29 @@
 
 #if defined(HAVE_FCNTL_H) && defined(F_SETLKW) && defined(F_RDLCK)
 #define POSIX_FILE_LOCKS
+
+/*
+ * Gnu libc bug 20251, currently unfixed, breaks OFD lock support on
+ * 32-bit platforms.  Work around this bug by explicitly using the
+ * fcntl64 system call and struct flock64.
+ */
+#if defined(__linux__) && __WORDSIZE == 32
+#include <sys/syscall.h>
+#ifdef SYS_fcntl64
+#define USE_FCNTL64
 #endif
+#endif
+#ifdef USE_FCNTL64
+/* Use the fcntl64 system call and struct flock64.  (Gnu libc does not
+ * define a fcntl64() function, so we must use syscall().) */
+#define fcntl(fd, cmd, arg) syscall(SYS_fcntl64, fd, cmd, arg)
+typedef struct flock64 fcntl_lock_st;
+#else
+/* Use regular fcntl() and struct flock. */
+typedef struct flock fcntl_lock_st;
+#endif
+
+#endif /* defined(HAVE_FCNTL_H) && defined(F_SETLKW) && defined(F_RDLCK) */
 
 #ifdef HAVE_FLOCK
 #ifndef sysvimp
@@ -66,7 +88,7 @@
  * older kernel than we were built with.
  */
 static int
-ofdlock(int fd, int cmd, struct flock *lock_arg)
+ofdlock(int fd, int cmd, fcntl_lock_st *lock_arg)
 {
 #ifdef F_OFD_SETLKW
     int st, ofdcmd;
@@ -89,7 +111,7 @@ krb5_lock_file(krb5_context context, int fd, int mode)
     krb5_error_code     retval = 0;
 #ifdef POSIX_FILE_LOCKS
     int lock_cmd = F_SETLKW;
-    struct flock lock_arg = { 0 };
+    fcntl_lock_st lock_arg = { 0 };
 #endif
 
     switch (mode & ~KRB5_LOCKMODE_DONTBLOCK) {
-- 
2.8.1