Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753012AbdIFUs0 (ORCPT ); Wed, 6 Sep 2017 16:48:26 -0400 Received: from mail-oi0-f66.google.com ([209.85.218.66]:36181 "EHLO mail-oi0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752501AbdIFUsY (ORCPT ); Wed, 6 Sep 2017 16:48:24 -0400 X-Google-Smtp-Source: ADKCNb7NRibRrVcDxQm1r29qelXC6omRmalnRPZ2Z/r2uuwYJxQHlPADG389eWB1ZbMlNJe/f39svNgvbRMKkSHTC9A= MIME-Version: 1.0 In-Reply-To: <7791dee1-64a9-dcaa-c5d9-0b04a44bc526@akamai.com> References: <20170906195825.3715290-1-arnd@arndb.de> <7791dee1-64a9-dcaa-c5d9-0b04a44bc526@akamai.com> From: Arnd Bergmann Date: Wed, 6 Sep 2017 22:48:22 +0200 X-Google-Sender-Auth: IGsF-oW8p2YwFyhd3o9zRL6BE9c Message-ID: Subject: Re: [PATCH] netfilter: xt_hashlimit: avoid 64-bit division To: Vishwanath Pai Cc: Pablo Neira Ayuso , Jozsef Kadlecsik , Florian Westphal , "David S. Miller" , Josh Hunt , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, Networking , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1819 Lines: 49 On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai wrote: > On 09/06/2017 03:57 PM, Arnd Bergmann wrote: >> 64-bit division is expensive on 32-bit architectures, and >> requires a special function call to avoid a link error like: >> >> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common': >> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod' >> >> In the case of hashlimit_mt_common, we don't actually need a >> 64-bit operation, we can simply rewrite the function slightly >> to make that clear to the compiler. >> >> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode") >> Signed-off-by: Arnd Bergmann >> --- >> net/netfilter/xt_hashlimit.c | 5 ++++- >> 1 file changed, 4 insertions(+), 1 deletion(-) >> >> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c >> index 10d48234f5f4..50b53d86eef5 100644 >> --- a/net/netfilter/xt_hashlimit.c >> +++ b/net/netfilter/xt_hashlimit.c >> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user) >> { >> u64 r; >> >> - r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL; >> + if (user > 0xFFFFFFFFULL) >> + return 0; >> + >> + r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL; >> r = (r - 1) << 4; >> return r; >> } >> > > I have submitted another patch to fix this: > https://patchwork.ozlabs.org/patch/809881/ > > We have seen this problem before, I was careful not to introduce this > again in the new patch but clearly I overlooked this particular line :( > > In the other cases we fixed it by replacing division with div64_u64(). div64_u64() seems needlessly expensive here since the dividend is known to be a 32-bit number. I guess the function is not called frequently though, so it doesn't matter much. Arnd