From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EEAABC606B0 for ; Tue, 9 Jul 2019 08:11:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CDC06216C4 for ; Tue, 9 Jul 2019 08:11:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726043AbfGIILC (ORCPT ); Tue, 9 Jul 2019 04:11:02 -0400 Received: from bastet.se.axis.com ([195.60.68.11]:48295 "EHLO bastet.se.axis.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725895AbfGIILC (ORCPT ); Tue, 9 Jul 2019 04:11:02 -0400 Received: from localhost (localhost [127.0.0.1]) by bastet.se.axis.com (Postfix) with ESMTP id D33D9183C4; Tue, 9 Jul 2019 10:10:58 +0200 (CEST) X-Axis-User: NO X-Axis-NonUser: YES X-Virus-Scanned: Debian amavisd-new at bastet.se.axis.com Received: from bastet.se.axis.com ([IPv6:::ffff:127.0.0.1]) by localhost (bastet.se.axis.com [::ffff:127.0.0.1]) (amavisd-new, port 10024) with LMTP id dCYuBJE3n5uX; Tue, 9 Jul 2019 10:10:57 +0200 (CEST) Received: from boulder03.se.axis.com (boulder03.se.axis.com [10.0.8.17]) by bastet.se.axis.com (Postfix) with ESMTPS id A3C36183A0; Tue, 9 Jul 2019 10:10:56 +0200 (CEST) Received: from boulder03.se.axis.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7EB8A1E080; Tue, 9 Jul 2019 10:10:56 +0200 (CEST) Received: from boulder03.se.axis.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7304D1E05C; Tue, 9 Jul 2019 10:10:56 +0200 (CEST) Received: from thoth.se.axis.com (unknown [10.0.2.173]) by boulder03.se.axis.com (Postfix) with ESMTP; Tue, 9 Jul 2019 10:10:56 +0200 (CEST) Received: from lnxartpec.se.axis.com (lnxartpec.se.axis.com [10.88.4.9]) by thoth.se.axis.com (Postfix) with ESMTP id 66F562761; Tue, 9 Jul 2019 10:10:56 +0200 (CEST) Received: by lnxartpec.se.axis.com (Postfix, from userid 10564) id 60FE980211; Tue, 9 Jul 2019 10:10:56 +0200 (CEST) From: Vincent Whitchurch To: pmladek@suse.com, sergey.senozhatsky@gmail.com, rostedt@goodmis.org Cc: linux-kernel@vger.kernel.org, Vincent Whitchurch Subject: [PATCH] printk: Do not lose last line in kmsg dump Date: Tue, 9 Jul 2019 10:10:42 +0200 Message-Id: <20190709081042.31551-1-vincent.whitchurch@axis.com> X-Mailer: git-send-email 2.20.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org kmsg_dump_get_buffer() is supposed to select the youngest log messages which fit into the provided buffer. When that function determines the correct start index, by looping and calling msg_print_text() with a NULL buffer, msg_print_text() calculates a length which would allow the youngest log messages to completely fill the provided buffer. However, when doing the actual printing, an off-by-one error in msg_print_text() leads to that function allowing the provided buffer to only be filled to (size - 1). So if the lengths of the selected youngest log messages happen to precisely fill up the provided buffer, the last log message is not included. For example, with the following two final prints: [ 6.427502] AAAAAAAAAAAAA [ 6.427769] BBBBBBBB12345 A dump of a 64-byte buffer filled by kmsg_dump_get_buffer(), before this patch: 00000000: 3c 30 3e 5b 20 20 20 20 36 2e 35 32 32 31 39 37 <0>[ 6.522197 00000010: 5d 20 41 41 41 41 41 41 41 41 41 41 41 41 41 0a ] AAAAAAAAAAAAA. 00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ After this patch: 00000000: 3c 30 3e 5b 20 20 20 20 36 2e 34 32 37 35 30 32 <0>[ 6.427502 00000010: 5d 20 41 41 41 41 41 41 41 41 41 41 41 41 41 0a ] AAAAAAAAAAAAA. 00000020: 3c 30 3e 5b 20 20 20 20 36 2e 34 32 37 37 36 39 <0>[ 6.427769 00000030: 5d 20 42 42 42 42 42 42 42 42 31 32 33 34 35 0a ] BBBBBBBB12345. Note that this bug only affects the kmsg dump code. msg_print_text() is also used from the syslog code and console_unlock() but this bug does trigger there since the buffers used there are never filled up completely (since they are only used to print individual lines, and their size is always LOG_LINE_MAX + PREFIX_MAX, and PREFIX_MAX has a value which is larger than the largest possible prefix). Signed-off-by: Vincent Whitchurch --- I posted this patch two years ago and received no replies. This problem is still present in mainline. https://lore.kernel.org/patchwork/patch/781106/ kernel/printk/printk.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 1888f6a3b694..7679d779d5cc 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1318,7 +1318,7 @@ static size_t msg_print_text(const struct printk_log *msg, bool syslog, } if (buf) { - if (prefix_len + text_len + 1 >= size - len) + if (prefix_len + text_len + 1 > size - len) break; memcpy(buf + len, prefix, prefix_len); -- 2.20.0