[PATCH] eCryptfs: fix possible fault in ecryptfs_sync_page

[Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>]

This will avoid a possible fault in ecryptfs_sync_page().

In the function, eCryptfs calls sync_page() method of a lower 
filesystem without checking its existence.  However, there 
are many filesystems that don't have this method including
network filesystems such as NFS, AFS, and so forth.
They may fail when an eCryptfs page is waiting for lock.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
---
 fs/ecryptfs/mmap.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/ecryptfs/mmap.c b/fs/ecryptfs/mmap.c
index e4ab7bc..fd3f94d 100644
--- a/fs/ecryptfs/mmap.c
+++ b/fs/ecryptfs/mmap.c
@@ -834,7 +834,8 @@ static void ecryptfs_sync_page(struct page *page)
 		ecryptfs_printk(KERN_DEBUG, "find_lock_page failed\n");
 		return;
 	}
-	lower_page->mapping->a_ops->sync_page(lower_page);
+	if (lower_page->mapping->a_ops->sync_page)
+		lower_page->mapping->a_ops->sync_page(lower_page);
 	ecryptfs_printk(KERN_DEBUG, "Unlocking page with index = [0x%.16x]\n",
 			lower_page->index);
 	unlock_page(lower_page);