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>

fs/iomap/buffered-io.c

@@ -1077,24 +1077,16 @@ vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
 	struct page *page = vmf->page;
 	struct inode *inode = file_inode(vmf->vma->vm_file);
 	unsigned long length;
-	loff_t offset, size;
+	loff_t offset;
 	ssize_t ret;
 
 	lock_page(page);
-	size = i_size_read(inode);
-	offset = page_offset(page);
-	if (page->mapping != inode->i_mapping || offset > size) {
-		/* We overload EFAULT to mean page got truncated */
-		ret = -EFAULT;
+	ret = page_mkwrite_check_truncate(page, inode);
+	if (ret < 0)
 		goto out_unlock;
-	}
-
-	/* page is wholly or partially inside EOF */
-	if (offset > size - PAGE_SIZE)
-		length = offset_in_page(size);
-	else
-		length = PAGE_SIZE;
+	length = ret;
 
+	offset = page_offset(page);
 	while (length > 0) {
 		ret = iomap_apply(inode, offset, length,
 				IOMAP_WRITE | IOMAP_FAULT, ops, page,

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 */