tree 1478b45629f2cd2a499a4b06f9938c2ad3ed0942
parent 95201a40604791bc4a2e8d066429be89fb82b46d
author David Howells <dhowells@redhat.com> 1446650442 +0000
committer Al Viro <viro@zeniv.linux.org.uk> 1447225862 -0500

FS-Cache: Handle a write to the page immediately beyond the EOF marker

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]

Cc: stable@vger.kernel.org # v2.6.31+; earlier - that + backport of a17754f (at least)
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>