[PATCH] uml: hostfs - fix possible PAGE_CACHE_SHIFT overflows

Prevent page->index << PAGE_CACHE_SHIFT from overflowing.

There is a casting there, but was added without care, so it's at the wrong
place. Note the extra parens around the shift - "+" is higher precedence than
"<<", leading to a GCC warning which saved all us.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/hostfs/hostfs_kern.c

@@ -501,11 +501,16 @@ int hostfs_commit_write(struct file *file, struct page *page, unsigned from,
 	long long start;
 	int err = 0;
 
-	start = (long long) (page->index << PAGE_CACHE_SHIFT) + from;
+	start = (((long long) page->index) << PAGE_CACHE_SHIFT) + from;
 	buffer = kmap(page);
 	err = write_file(FILE_HOSTFS_I(file)->fd, &start, buffer + from,
 			 to - from);
 	if(err > 0) err = 0;
+
+	/* Actually, if !err, write_file has added to-from to start, so, despite
+	 * the appearance, we are comparing i_size against the _last_ written
+	 * location, as we should. */
+
 	if(!err && (start > inode->i_size))
 		inode->i_size = start;