linux-kernel - Re: [PATCH 2/2] tracing: Use trace_seq_used() and seq_buf

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Date:	Wed, 19 Nov 2014 11:00:37 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH 2/2] tracing: Use trace_seq_used() and seq_buf_used()
 instead of len

On Wed, 19 Nov 2014 15:40:05 +0100
Petr Mladek <pmladek@...e.cz> wrote:

> > Regardless of overflow or not (or even if trace_seq is full), that if
> > statement will prevent this from doing any buffer overflows.
> > 
> > s->seq.len will never be more than s->seq.size after the test against
> > TRACE_MAX_PRINT. So I see no harm here.
> 
> Ah, I see. Well, I would feel more comfortable if it uses
> trace_seq_used() or if there is some explanation in a comment.
> But you are right, it is safe as it is. Feel free to leave it.
> 

OK, I added this just for you:

BTW, using trace_seq_used() would not be good enough because it could
return s->seq.size. Which would overflow the buffer on the

	s->buffer[s->seq.len] = 0;

-- Steve

>From 1922acc9987d23d0b9c1ad28dc3eaafc1cf2d6b7 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
Date: Wed, 19 Nov 2014 10:56:41 -0500
Subject: [PATCH] tracing: Add paranoid size check in trace_printk_seq()

To be really paranoid about writing out of bound data in
trace_printk_seq(), add another check of len compared to size.

Link: http://lkml.kernel.org/r/20141119144004.GB2332@dhcp128.suse.cz

Suggested-by: Petr Mladek <pmladek@...e.cz>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 kernel/trace/trace.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9023446b2c2b..26facec4625e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6656,6 +6656,14 @@ trace_printk_seq(struct trace_seq *s)
 	if (s->seq.len >= TRACE_MAX_PRINT)
 		s->seq.len = TRACE_MAX_PRINT;
 
+	/*
+	 * More paranoid code. Although the buffer size is set to
+	 * PAGE_SIZE, and TRACE_MAX_PRINT is 1000, this is just
+	 * an extra layer of protection.
+	 */
+	if (WARN_ON_ONCE(s->seq.len >= s->seq.size))
+		s->seq.len = s->seq.size - 1;
+
 	/* should be zero ended, but we are paranoid. */
 	s->buffer[s->seq.len] = 0;
 
-- 
1.8.1.4

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