From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=5.0 tests=DKIM_INVALID,DKIM_SIGNED, MAILING_LIST_MULTI,RCVD_IN_MSPIKE_H2 autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 28211 invoked from network); 7 Oct 2022 23:07:08 -0000 Received: from second.openwall.net (193.110.157.125) by inbox.vuxu.org with ESMTPUTF8; 7 Oct 2022 23:07:08 -0000 Received: (qmail 11789 invoked by uid 550); 7 Oct 2022 23:07:05 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 10129 invoked from network); 7 Oct 2022 23:04:39 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kleine-koenig.org; s=2022; t=1665183868; bh=Ftoy9l3kLn2571s1jB0Uzfvpsm8v81I9ChvlkMc7l2k=; h=Date:To:From:Subject:From; b=P1xtbiFNo0tN64RtKANLGCtB6cxlfxcez/HiwQsPIUhBOqLK2R4Ex1bAymxukQkM6 Djw4Y2veE+gfgYMDg9imNt7Vi50ogVWaV+utTtEwLoCUrujig5+R3EzB8QoKiAAxEg d/G43/0BWmWCDVkxa/xhyb87GDf0iLQjMEsuDRKAGMTLHQGizZShdFp2mFdRHprAqJ bvfeCIhto2AeeCmszJzkMeth1nnxNB6Hmb0ywAVzJBNBi/H3L0x8wLLqRPKuqXHOUh M0JBduiLnM1WacpRnP5KhLtlnui19ivNg2XW9yDBAUGgMkr0Jhi2u6hnWKpelYPQfr mLHxawsJbonUA== Message-ID: <0175978a-476c-e5f3-1da0-12bb78de7f54@kleine-koenig.org> Date: Sat, 8 Oct 2022 01:04:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.2 To: openwrt-devel@lists.openwrt.org, musl@lists.openwall.com Content-Language: en-US From: =?UTF-8?Q?Uwe_Kleine-K=c3=b6nig?= Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Subject: [musl] nslookup failures with coarse CLOCK_MONOTONIC Hello, on a TP-Link RE200 v1 (platform = ramips/mt7620) I experience often: root@ares:~# nslookup www.openwrt.org Server: 127.0.0.1 Address: 127.0.0.1:53 Non-authoritative answer: www.openwrt.org canonical name = wiki-01.infra.openwrt.org Name: wiki-01.infra.openwrt.org Address: 2a03:b0c0:3:d0::1af1:1 *** Can't find www.openwrt.org: No answer I narrowed the problem down to the following: nslookup creates and sends two querys (for A and AAAA) using res_mkquery(). Each query has a more or less random ID and nslookup matches the received responses using these IDs to the sent querys. Looking at the sent queries using tcpdump, I saw that in the above scenario the two IDs are identical. Then nslookup matches the first received answer to the first query and discards the second reply, as it's matched to the already handled first query, too. In a few cases where both lookups succeed, I saw the following pairs of IDs: 17372 37373 40961 60961 45955 419 47302 1766 Musl does the following to create the 16 bit ID: /* Make a reasonably unpredictable id */ clock_gettime(CLOCK_REALTIME, &ts); id = ts.tv_nsec + ts.tv_nsec/65536UL & 0xffff; q[0] = id/256; q[1] = id; (from musl's src/network/res_mkquery.c) My hypothesis now is that the monotonic clock has a resolution of 20 µs only. So if the two res_mkquery() calls are called within the same 20 µs tick, the IDs end up being identical. If they happen in two consecutive ticks, the IDs have a delta of 20000 or 20001 which matches the four cases observed above. To improve the situation I suggest something like: diff --git a/src/network/res_mkquery.c b/src/network/res_mkquery.c index 614bf7864b48..78b3095fe959 100644 --- a/src/network/res_mkquery.c +++ b/src/network/res_mkquery.c @@ -11,6 +11,7 @@ int __res_mkquery(int op, const char *dname, int class, int type, struct timespec ts; size_t l = strnlen(dname, 255); int n; + static unsigned int querycnt; if (l && dname[l-1]=='.') l--; if (l && dname[l-1]=='.') return -1; @@ -34,6 +35,8 @@ int __res_mkquery(int op, const char *dname, int class, int type, /* Make a reasonably unpredictable id */ clock_gettime(CLOCK_REALTIME, &ts); + /* force a different ID if mkquery was called twice during the same tick */ + ts.tv_nsec += querycnt++; id = ts.tv_nsec + ts.tv_nsec/65536UL & 0xffff; q[0] = id/256; q[1] = id; Would that make sense? Note I'm not subscribed to the musl mailing list, so please Cc: me on replies. Best regards Uwe