[patch 06/23] NFS: Fix a potential file corruption issue when writing

Greg KH
2008-02-22T21:46:12Z

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2.6.22-stable review patch. If anyone has any objections, please let us
know.

------------------

From: Trond Myklebust <Trond.Myklebust@netapp.com>

patch 5d47a35600270e7115061cb1320ee60ae9bcb6b8 in mainline.

If the inode is flagged as having an invalid mapping, then we can't rely on
the PageUptodate() flag. Ensure that we don't use the "anti-fragmentation"
write optimisation in nfs_updatepage(), since that will cause NFS to write
out areas of the page that are no longer guaranteed to be up to date.

A potential corruption could occur in the following scenario:

client 1 client 2
=============== ===============
fd=open("f",O_CREAT|O_WRONLY,0644);
write(fd,"fubar\n",6); // cache last page
close(fd);
fd=open("f",O_WRONLY|O_APPEND);
write(fd,"foo\n",4);
close(fd);

fd=open("f",O_WRONLY|O_APPEND);
write(fd,"bar\n",4);
close(fd);
-----
The bug may lead to the file "f" reading 'fubar\n\0\0\0\nbar\n' because
client 2 does not update the cached page after re-opening the file for
write. Instead it keeps it marked as PageUptodate() until someone calls
invalidate_inode_pages2() (typically by calling read()).

The bug was introduced by commit 44b11874ff583b6e766a05856b04f3c492c32b84
"NFS: Separate metadata and page cache revalidation mechanisms"

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/nfs/write.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)

--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -710,6 +710,17 @@ int nfs_flush_incompatible(struct file *
}

/*
+ * If the page cache is marked as unsafe or invalid, then we can't rely on
+ * the PageUptodate() flag. In this case, we will need to turn off
+ * write optimisations that depend on the page contents being correct.
+ */
+static int nfs_write_pageuptodate(struct page *page, struct inode *inode)
+{
+ return PageUptodate(page) &&
+ !(NFS_I(inode)->cache_validity & (NFS_INO_REVAL_PAGECACHE|NFS_INO_INVALID_DATA));
+}
+
+/*
* Update and possibly write a cached page of an NFS file.
*
* XXX: Keep an eye on generic_file_read to make sure it doesn't do bad
@@ -730,10 +741,13 @@ int nfs_updatepage(struct file *file, st
(long long)(page_offset(page) +offset));

/* If we're not using byte range locks, and we know the page
- * is entirely in cache, it may be more efficient to avoid
- * fragmenting write requests.
+ * is up to date, it may be more efficient to extend the write
+ * to cover the entire page in order to avoid fragmentation
+ * inefficiencies.
*/
- if (PageUptodate(page) && inode->i_flock == NULL && !(file->f_mode & O_SYNC)) {
+ if (nfs_write_pageuptodate(page, inode) &&
+ inode->i_flock == NULL &&
+ !(file->f_mode & O_SYNC)) {
count = max(count + offset, nfs_page_length(page));
offset = 0;
}

--