From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934269AbaHZJTt (ORCPT ); Tue, 26 Aug 2014 05:19:49 -0400 Received: from cantor2.suse.de ([195.135.220.15]:39280 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753960AbaHZJTs (ORCPT ); Tue, 26 Aug 2014 05:19:48 -0400 Date: Tue, 26 Aug 2014 11:19:41 +0200 From: Petr =?iso-8859-1?Q?Ml=E1dek?= To: Patrick Palka Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, jack@suse.cz, rostedt@goodmis.org Subject: Re: [PATCH] Fix faulty logic in the case of recursive printk Message-ID: <20140826091941.GA2360@dhcp128.suse.cz> References: <1408896096-16605-1-git-send-email-patrick@parcs.ath.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1408896096-16605-1-git-send-email-patrick@parcs.ath.cx> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 24-08-14 12:01:36, Patrick Palka wrote: > 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 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 > > text_len = vscnprintf(text + text_len, ...); > > 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 performs a couple of local > micro-optimizations (use unlikely() and ARRAY_SIZE()). Great catch! And I agree with Jack about that strlen. > 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..c101ec2 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, > + ARRAY_SIZE(recursion_msg) - 1); > } > > /* > -- > 2.1.0 >