gfs2: Fix underflow in gfs2_page_mkwrite

On filesystems with a block size smaller than PAGE_SIZE and non-empty
files smaller then PAGE_SIZE, gfs2_page_mkwrite could end up allocating
excess blocks beyond the end of the file, similar to fallocate.  This
doesn't make sense; fix it.

Reported-by: Bob Peterson <rpeterso@redhat.com>
Fixes: 184b4e60853d ("gfs2: Fix end-of-file handling in gfs2_page_mkwrite")
Cc: stable@vger.kernel.org # v5.5+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
index 7b75719..3704187 100644 (file)
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -450,8 +450,8 @@ static vm_fault_t gfs2_page_mkwrite(struct vm_fault *vmf)
        file_update_time(vmf->vma->vm_file);
 
        /* page is wholly or partially inside EOF */
-       if (offset > size - PAGE_SIZE)
-               length = offset_in_page(size);
+       if (size - offset < PAGE_SIZE)
+               length = size - offset;
        else
                length = PAGE_SIZE;