From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757517AbaHZLXj (ORCPT ); Tue, 26 Aug 2014 07:23:39 -0400 Received: from mail-qa0-f45.google.com ([209.85.216.45]:53018 "EHLO mail-qa0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754426AbaHZLXh (ORCPT ); Tue, 26 Aug 2014 07:23:37 -0400 From: Patrick Palka To: linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, pmladek@suse.cz, jack@suse.cz, rostedt@goodmis.org, Patrick Palka Subject: [PATCH] Fix faulty logic in the case of recursive printk Date: Tue, 26 Aug 2014 07:23:22 -0400 Message-Id: <1409052202-9498-1-git-send-email-patrick@parcs.ath.cx> X-Mailer: git-send-email 2.1.0 In-Reply-To: <20140826084833.GA1581@quack.suse.cz> References: <20140826084833.GA1581@quack.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thanks for reviewing! I have put back the call to strlen() and adjusted the commit message accordingly. Patrick -- >8 -- We shouldn't set text_len in the code path that detects printk recursion because text_len corresponds to the length of the string inside textbuf. A few lines down from the line text_len = strlen(recursion_msg); is the line text_len += vscnprintf(text + text_len, ...); So if printk detects recursion, it sets text_len to 29 (the length of recursion_msg) and logs an error. Then the message supplied by the caller of printk is stored inside textbuf but offset by 29 bytes. This means that the output of the recursive call to printk will contain 29 bytes of garbage in front of it. This defect is caused by commit 458df9fd ("printk: remove separate printk_sched buffers and use printk buf instead") which turned the line text_len = vscnprintf(text, ...); into text_len += vscnprintf(text + text_len, ...); To fix this, this patch avoids setting text_len when logging the printk recursion error. This patch also marks unlikely() the branch leading up to this code. Signed-off-by: Patrick Palka --- kernel/printk/printk.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index e04c455..1ce7706 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1665,15 +1665,15 @@ asmlinkage int vprintk_emit(int facility, int level, raw_spin_lock(&logbuf_lock); logbuf_cpu = this_cpu; - if (recursion_bug) { + if (unlikely(recursion_bug)) { static const char recursion_msg[] = "BUG: recent printk recursion!"; recursion_bug = 0; - text_len = strlen(recursion_msg); /* emit KERN_CRIT message */ printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0, - NULL, 0, recursion_msg, text_len); + NULL, 0, recursion_msg, + strlen(recursion_msg)); } /* -- 2.1.0