[PATCH] r/o bind mounts: get write access for vfs_rename() callers

This also uses the little helper in the NFS code to make an if() a little bit
less ugly.  We introduced the helper at the beginning of the series.
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
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>

fs/namei.c

@@ -2718,8 +2718,12 @@ static int do_rename(int olddfd, const char *oldname,
 	if (new_dentry == trap)
 		goto exit5;
 
+	error = mnt_want_write(oldnd.path.mnt);
+	if (error)
+		goto exit5;
 	error = vfs_rename(old_dir->d_inode, old_dentry,
 				   new_dir->d_inode, new_dentry);
+	mnt_drop_write(oldnd.path.mnt);
 exit5:
 	dput(new_dentry);
 exit4:

fs/nfsd/vfs.c

@@ -1710,13 +1710,20 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 	if (ndentry == trap)
 		goto out_dput_new;
 
-#ifdef MSNFS
-	if ((ffhp->fh_export->ex_flags & NFSEXP_MSNFS) &&
+	if (svc_msnfs(ffhp) &&
 		((atomic_read(&odentry->d_count) > 1)
 		 || (atomic_read(&ndentry->d_count) > 1))) {
 			host_err = -EPERM;
-	} else
-#endif
+			goto out_dput_new;
+	}
+
+	host_err = -EXDEV;
+	if (ffhp->fh_export->ex_path.mnt != tfhp->fh_export->ex_path.mnt)
+		goto out_dput_new;
+	host_err = mnt_want_write(ffhp->fh_export->ex_path.mnt);
+	if (host_err)
+		goto out_dput_new;
+
 	host_err = vfs_rename(fdir, odentry, tdir, ndentry);
 	if (!host_err && EX_ISSYNC(tfhp->fh_export)) {
 		host_err = nfsd_sync_dir(tdentry);
@@ -1724,6 +1731,8 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 			host_err = nfsd_sync_dir(fdentry);
 	}
 
+	mnt_drop_write(ffhp->fh_export->ex_path.mnt);
+
  out_dput_new:
 	dput(ndentry);
  out_dput_old: