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, 6 insertions, 11 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index e64fa72..5a81b1e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1773,32 +1773,27 @@ static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *s
 	return 0;
 }
 
-static int shmem_follow_link_inline(struct dentry *dentry, struct nameidata *nd)
+static void *shmem_follow_link_inline(struct dentry *dentry, struct nameidata *nd)
 {
 	nd_set_link(nd, (char *)SHMEM_I(dentry->d_inode));
-	return 0;
+	return NULL;
 }
 
-static int shmem_follow_link(struct dentry *dentry, struct nameidata *nd)
+static void *shmem_follow_link(struct dentry *dentry, struct nameidata *nd)
 {
 	struct page *page = NULL;
 	int res = shmem_getpage(dentry->d_inode, 0, &page, SGP_READ, NULL);
 	nd_set_link(nd, res ? ERR_PTR(res) : kmap(page));
-	return 0;
+	return page;
 }
 
-static void shmem_put_link(struct dentry *dentry, struct nameidata *nd)
+static void shmem_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
 {
 	if (!IS_ERR(nd_get_link(nd))) {
-		struct page *page;
-
-		page = find_get_page(dentry->d_inode->i_mapping, 0);
-		if (!page)
-			BUG();
+		struct page *page = cookie;
 		kunmap(page);
 		mark_page_accessed(page);
 		page_cache_release(page);
-		page_cache_release(page);
 	}
 }