[PATCH] r/o bind mounts: make access() use new r/o helper

It is OK to let access() go without using a mnt_want/drop_write() pair because
it doesn't actually do writes to the filesystem, and it is inherently racy
anyway.  This is a rare case when it is OK to use __mnt_is_readonly()
directly.

Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 11 insertions, 2 deletions

diff --git a/fs/open.c b/fs/open.c
index 4d690e33446f..e58382d57e72 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -459,8 +459,17 @@ asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode)
         if(res || !(mode & S_IWOTH) ||
            special_file(nd.path.dentry->d_inode->i_mode))
                 goto out_path_release;
-
-        if(IS_RDONLY(nd.path.dentry->d_inode))
+        /*
+         * This is a rare case where using __mnt_is_readonly()
+         * is OK without a mnt_want/drop_write() pair.  Since
+         * no actual write to the fs is performed here, we do
+         * not need to telegraph to that to anyone.
+         *
+         * By doing this, we accept that this access is
+         * inherently racy and know that the fs may change
+         * state before we even see this result.
+         */
+        if (__mnt_is_readonly(nd.path.mnt))
                 res = -EROFS;
 
 out_path_release: