From mboxrd@z Thu Jan 1 00:00:00 1970 From: Crestez Dan Leonard Subject: __inet_hash_connect port_offset parameter Date: Wed, 27 May 2015 18:25:54 +0300 Message-ID: <5565E202.8010202@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit To: netdev@vger.kernel.org Return-path: Received: from mail-wi0-f173.google.com ([209.85.212.173]:37605 "EHLO mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751143AbbE0PZw (ORCPT ); Wed, 27 May 2015 11:25:52 -0400 Received: by wifw1 with SMTP id w1so27088827wif.0 for ; Wed, 27 May 2015 08:25:51 -0700 (PDT) Received: from [10.205.20.121] ([109.100.41.154]) by mx.google.com with ESMTPSA id bm9sm27406739wjc.21.2015.05.27.08.25.49 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 27 May 2015 08:25:50 -0700 (PDT) Sender: netdev-owner@vger.kernel.org List-ID: Hello, I'm confused about the port_offset parameter to __inet_hash_connect. When allocating the local port for an outgoing TCP connection the port search looks something like this: static u32 hint; u32 offset = hint + port_offset; inet_get_local_port_range(net, &low, &high); remaining = (high - low) + 1; for (i = 1; i <= remaining; i++) { port = low + (i + offset) % remaining; /* check port is free */ The port_offset is calculated for v4 and v6 based on a hash of src/dst addresses, presumably in order to improve security. I see a few issues with this: - The port_offset is calculated even if the local port was already assigned via bind. This wastes a few cycles. - Keeping the last searched port as a static variable is a bad idea on multicore cpus. Starting a lot of connections to the same target will result in lock contention in the bind hash. This is probably only visible in highly synthetic tests. - When doing a port search at bind() time the search starts from "prandom_32()". Is this "less secure" for port allocation? I bet most applications are not aware of this difference. Wouldn't it be better to use the same local port search mechanism at both bind (inet_csk_get_port) and connect (__inet_hash_connect) time, based on starting from a random point? It would also make connecting slightly faster. Regards, Leonard