fs: Fix page_mkwrite off-by-one errors

The check in block_page_mkwrite that is meant to determine whether an
offset is within the inode size is off by one.  This bug has been copied
into iomap_page_mkwrite and several filesystems (ubifs, ext4, f2fs,
ceph).

Fix that by introducing a new page_mkwrite_check_truncate helper that
checks for truncate and computes the bytes in the page up to EOF.  Use
the helper in iomap.

NOTE from Darrick: The original patch fixed a number of filesystems, but
then there were merge conflicts with the f2fs for-next tree; a
subsequent re-submission of the patch had different btrfs changes with
no explanation; and Christoph complained that each per-fs fix should be
a separate patch.  In my view that's too much risk to take on, so I
decided to drop all the hunks except for iomap, since I've actually QA'd
XFS.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: drop everything but the iomap parts]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 37a4d9e32cd3..ccb14b6a16b5 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -636,4 +636,32 @@ static inline unsigned long dir_pages(struct inode *inode)
 			       PAGE_SHIFT;
 }
 
+/**
+ * page_mkwrite_check_truncate - check if page was truncated
+ * @page: the page to check
+ * @inode: the inode to check the page against
+ *
+ * Returns the number of bytes in the page up to EOF,
+ * or -EFAULT if the page was truncated.
+ */
+static inline int page_mkwrite_check_truncate(struct page *page,
+					      struct inode *inode)
+{
+	loff_t size = i_size_read(inode);
+	pgoff_t index = size >> PAGE_SHIFT;
+	int offset = offset_in_page(size);
+
+	if (page->mapping != inode->i_mapping)
+		return -EFAULT;
+
+	/* page is wholly inside EOF */
+	if (page->index < index)
+		return PAGE_SIZE;
+	/* page is wholly past EOF */
+	if (page->index > index || !offset)
+		return -EFAULT;
+	/* page is partially inside EOF */
+	return offset;
+}
+
 #endif /* _LINUX_PAGEMAP_H */