Fix nasty ncpfs symlink handling bug.

This bug could cause oopses and page state corruption, because ncpfs
used the generic page-cache symlink handlign functions.  But those
functions only work if the page cache is guaranteed to be "stable", ie a
page that was installed when the symlink walk was started has to still
be installed in the page cache at the end of the walk.

We could have fixed ncpfs to not use the generic helper routines, but it
is in many ways much cleaner to instead improve on the symlink walking
helper routines so that they don't require that absolute stability.

We do this by allowing "follow_link()" to return a error-pointer as a
cookie, which is fed back to the cleanup "put_link()" routine.  This
also simplifies NFS symlink handling.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 29 deletions

diff --git a/fs/nfs/symlink.c b/fs/nfs/symlink.c
index 35f10659914..18dc95b0b64 100644
--- a/fs/nfs/symlink.c
+++ b/fs/nfs/symlink.c
@@ -27,26 +27,14 @@
 
 /* Symlink caching in the page cache is even more simplistic
  * and straight-forward than readdir caching.
- *
- * At the beginning of the page we store pointer to struct page in question,
- * simplifying nfs_put_link() (if inode got invalidated we can't find the page
- * to be freed via pagecache lookup).
- * The NUL-terminated string follows immediately thereafter.
  */
 
-struct nfs_symlink {
-	struct page *page;
-	char body[0];
-};
-
 static int nfs_symlink_filler(struct inode *inode, struct page *page)
 {
-	const unsigned int pgbase = offsetof(struct nfs_symlink, body);
-	const unsigned int pglen = PAGE_SIZE - pgbase;
 	int error;
 
 	lock_kernel();
-	error = NFS_PROTO(inode)->readlink(inode, page, pgbase, pglen);
+	error = NFS_PROTO(inode)->readlink(inode, page, 0, PAGE_SIZE);
 	unlock_kernel();
 	if (error < 0)
 		goto error;
@@ -60,11 +48,10 @@ error:
 	return -EIO;
 }
 
-static int nfs_follow_link(struct dentry *dentry, struct nameidata *nd)
+static void *nfs_follow_link(struct dentry *dentry, struct nameidata *nd)
 {
 	struct inode *inode = dentry->d_inode;
 	struct page *page;
-	struct nfs_symlink *p;
 	void *err = ERR_PTR(nfs_revalidate_inode(NFS_SERVER(inode), inode));
 	if (err)
 		goto read_failed;
@@ -78,28 +65,20 @@ static int nfs_follow_link(struct dentry *dentry, struct nameidata *nd)
 		err = ERR_PTR(-EIO);
 		goto getlink_read_error;
 	}
-	p = kmap(page);
-	p->page = page;
-	nd_set_link(nd, p->body);
-	return 0;
+	nd_set_link(nd, kmap(page));
+	return page;
 
 getlink_read_error:
 	page_cache_release(page);
 read_failed:
 	nd_set_link(nd, err);
-	return 0;
+	return NULL;
 }
 
-static void nfs_put_link(struct dentry *dentry, struct nameidata *nd)
+static void nfs_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
 {
-	char *s = nd_get_link(nd);
-	if (!IS_ERR(s)) {
-		struct nfs_symlink *p;
-		struct page *page;
-
-		p = container_of(s, struct nfs_symlink, body[0]);
-		page = p->page;
-
+	if (cookie) {
+		struct page *page = cookie;
 		kunmap(page);
 		page_cache_release(page);
 	}