Linux 4.7 compat: fix zpl_get_acl returns invalid acl pointer

Starting from Linux 4.7, get_acl will set acl cache pointer to temporary
sentinel value before calling i_op->get_acl. Therefore we can't compare
against ACL_NOT_CACHED and return.

Since from Linux 3.14, get_acl already check the cache for us, so we
disable this in zpl_get_acl.

Linux 4.7 also does set_cached_acl for us so we disable it in zpl_get_acl.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Nikolay Borisov <n.borisov.lkml@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#4944
Closes openzfs#4946

config/kernel-acl.m4

@@ -224,3 +224,23 @@ AC_DEFUN([ZFS_AC_KERNEL_INODE_OPERATIONS_GET_ACL], [
 		AC_MSG_RESULT(no)
 	])
 ])
+
+dnl #
+dnl # 4.7 API change,
+dnl # The kernel get_acl will now check cache before calling i_op->get_acl and
+dnl # do set_cached_acl after that, so i_op->get_acl don't need to do that
+dnl # anymore.
+dnl #
+AC_DEFUN([ZFS_AC_KERNE_GET_ACL_HANDLE_CACHE], [
+	AC_MSG_CHECKING([whether uncached_acl_sentinel() exists])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+	],[
+		void *sentinel __attribute__ ((unused)) = uncached_acl_sentinel(NULL);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_KERNEL_GET_ACL_HANDLE_CACHE, 1, [uncached_acl_sentinel() exists])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -46,6 +46,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_INODE_OPERATIONS_CHECK_ACL
 	ZFS_AC_KERNEL_INODE_OPERATIONS_CHECK_ACL_WITH_FLAGS
 	ZFS_AC_KERNEL_INODE_OPERATIONS_GET_ACL
+	ZFS_AC_KERNE_GET_ACL_HANDLE_CACHE
 	ZFS_AC_KERNEL_SHOW_OPTIONS
 	ZFS_AC_KERNEL_FILE_INODE
 	ZFS_AC_KERNEL_FSYNC

module/zfs/zpl_xattr.c

@@ -1017,9 +1017,16 @@ zpl_get_acl(struct inode *ip, int type)
 	char *name;
 	int size;
 
+	/*
+	 * As of Linux 3.14, the kernel get_acl will check this for us.
+	 * Also as of Linux 4.7, comparing against ACL_NOT_CACHED is wrong
+	 * as the kernel get_acl will set it to temporary sentinel value.
+	 */
+#ifndef HAVE_KERNEL_GET_ACL_HANDLE_CACHE
 	acl = get_cached_acl(ip, type);
 	if (acl != ACL_NOT_CACHED)
 		return (acl);
+#endif
 
 	switch (type) {
 	case ACL_TYPE_ACCESS:
@@ -1049,8 +1056,11 @@ zpl_get_acl(struct inode *ip, int type)
 	if (size > 0)
 		kmem_free(value, size);
 
+	/* As of Linux 4.7, the kernel get_acl will set this for us */
+#ifndef HAVE_KERNEL_GET_ACL_HANDLE_CACHE
 	if (!IS_ERR(acl))
 		zpl_set_cached_acl(ip, type, acl);
+#endif
 
 	return (acl);
 }