[PATCH 4.3 190/200] FS-Cache: Handle a write to the page immediately beyond the EOF marker

[Greg Kroah-Hartman]

4.3-stable review patch.  If anyone has any objections, please let me know.

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

From: David Howells <dhowells@xxxxxxxxxx>

commit 102f4d900c9c8f5ed89ae4746d493fe3ebd7ba64 upstream.

Handle a write being requested to the page immediately beyond the EOF
marker on a cache object.  Currently this gets an assertion failure in
CacheFiles because the EOF marker is used there to encode information about
a partial page at the EOF - which could lead to an unknown blank spot in
the file if we extend the file over it.

The problem is actually in fscache where we check the index of the page
being written against store_limit.  store_limit is set to the number of
pages that we're allowed to store by fscache_set_store_limit() - which
means it's one more than the index of the last page we're allowed to store.
The problem is that we permit writing to a page with an index _equal_ to
the store limit - when we should reject that case.

Whilst we're at it, change the triggered assertion in CacheFiles to just
return -ENOBUFS instead.

The assertion failure looks something like this:

CacheFiles: Assertion failed
1000 < 7b1 is false
------------[ cut here ]------------
kernel BUG at fs/cachefiles/rdwr.c:962!
...
RIP: 0010:[<ffffffffa02c9e83>]  [<ffffffffa02c9e83>] cachefiles_write_page+0x273/0x2d0 [cachefiles]

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/cachefiles/rdwr.c |   67 ++++++++++++++++++++++++++++-----------------------
 fs/fscache/page.c    |    2 -
 2 files changed, 38 insertions(+), 31 deletions(-)

--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -905,6 +905,15 @@ int cachefiles_write_page(struct fscache
 	cache = container_of(object->fscache.cache,
 			     struct cachefiles_cache, cache);
 
+	pos = (loff_t)page->index << PAGE_SHIFT;
+
+	/* We mustn't write more data than we have, so we have to beware of a
+	 * partial page at EOF.
+	 */
+	eof = object->fscache.store_limit_l;
+	if (pos >= eof)
+		goto error;
+
 	/* write the page to the backing filesystem and let it store it in its
 	 * own time */
 	path.mnt = cache->mnt;
@@ -912,40 +921,38 @@ int cachefiles_write_page(struct fscache
 	file = dentry_open(&path, O_RDWR | O_LARGEFILE, cache->cache_cred);
 	if (IS_ERR(file)) {
 		ret = PTR_ERR(file);
-	} else {
-		pos = (loff_t) page->index << PAGE_SHIFT;
-
-		/* we mustn't write more data than we have, so we have
-		 * to beware of a partial page at EOF */
-		eof = object->fscache.store_limit_l;
-		len = PAGE_SIZE;
-		if (eof & ~PAGE_MASK) {
-			ASSERTCMP(pos, <, eof);
-			if (eof - pos < PAGE_SIZE) {
-				_debug("cut short %llx to %llx",
-				       pos, eof);
-				len = eof - pos;
-				ASSERTCMP(pos + len, ==, eof);
-			}
-		}
-
-		data = kmap(page);
-		ret = __kernel_write(file, data, len, &pos);
-		kunmap(page);
-		if (ret != len)
-			ret = -EIO;
-		fput(file);
+		goto error_2;
 	}
 
-	if (ret < 0) {
-		if (ret == -EIO)
-			cachefiles_io_error_obj(
-				object, "Write page to backing file failed");
-		ret = -ENOBUFS;
+	len = PAGE_SIZE;
+	if (eof & ~PAGE_MASK) {
+		if (eof - pos < PAGE_SIZE) {
+			_debug("cut short %llx to %llx",
+			       pos, eof);
+			len = eof - pos;
+			ASSERTCMP(pos + len, ==, eof);
+		}
 	}
 
-	_leave(" = %d", ret);
-	return ret;
+	data = kmap(page);
+	ret = __kernel_write(file, data, len, &pos);
+	kunmap(page);
+	fput(file);
+	if (ret != len)
+		goto error_eio;
+
+	_leave(" = 0");
+	return 0;
+
+error_eio:
+	ret = -EIO;
+error_2:
+	if (ret == -EIO)
+		cachefiles_io_error_obj(object,
+					"Write page to backing file failed");
+error:
+	_leave(" = -ENOBUFS [%d]", ret);
+	return -ENOBUFS;
 }
 
 /*
--- a/fs/fscache/page.c
+++ b/fs/fscache/page.c
@@ -816,7 +816,7 @@ static void fscache_write_op(struct fsca
 		goto superseded;
 	page = results[0];
 	_debug("gang %d [%lx]", n, page->index);
-	if (page->index > op->store_limit) {
+	if (page->index >= op->store_limit) {
 		fscache_stat(&fscache_n_store_pages_over_limit);
 		goto superseded;
 	}