linux-kernel - Re: [PATCH 1/2] printk: use logbuf_mutex_lock to stop syslog

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Date:	Sat, 16 Jun 2012 20:59:22 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Kay Sievers <kay@...y.org>
Cc:	linux-kernel@...r.kernel.org, wfg@...ux.intel.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 1/2] printk: use logbuf_mutex_lock to stop syslog_seq
 from going wild

On Sat, Jun 16, 2012 at 02:42:38PM +0200, Kay Sievers wrote:
> On Sat, 2012-06-16 at 12:40 +0800, Yuanhan Liu wrote:
> > Although syslog_seq and log_next_seq stuff are protected by logbuf_lock
> > spin log, it's not enough. Say we have two processes A and B, and let
> > syslog_seq = N, while log_next_seq = N + 1, and the two processes both
> > come to syslog_print at almost the same time. And No matter which
> > process get the spin lock first, it will increase syslog_seq by one,
> > then release spin lock; thus later, another process increase syslog_seq
> > by one again. In this case, syslog_seq is bigger than syslog_next_seq.
> > And latter, it would make:
> >    wait_event_interruptiable(log_wait, syslog != log_next_seq)
> > don't wait any more even there is no new write comes. Thus it introduce
> > a infinite loop reading.
> 
> Oh, multiple readers on the same shared file descriptor are not useful,
> but sure, that needs fixing. Thanks for tracking that down!
> 
> Looks like the same issue existed in the original code already, it's
> just that it was granular at a single character level, and not a line,
> and the seqnum which icreases one-by-one, so the issue was hard to
> trigger.

Yes,  I think so, too.

> 
> We better make the mutexes interruptible, right?

Yes, you are right.

> Something like this?

BTW, since you already made a patch, should I write a version 2 based
on your comments?

Thanks,
Yuanhan Liu
> 
> Thanks,
> Kay
> 
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index 32462d2..5a01420 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -414,7 +414,10 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  	if (!user)
>  		return -EBADF;
>  
> -	mutex_lock(&user->lock);
> +	ret = mutex_lock_interruptible(&user->lock);
> +	if (ret)
> +		return ret;
> +
>  	raw_spin_lock(&logbuf_lock);
>  	while (user->seq == log_next_seq) {
>  		if (file->f_flags & O_NONBLOCK) {
> @@ -974,6 +977,7 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
>  {
>  	bool clear = false;
>  	static int saved_console_loglevel = -1;
> +	static DEFINE_MUTEX(syslog_mutex);
>  	int error;
>  
>  	error = check_syslog_permissions(type, from_file);
> @@ -1000,11 +1004,17 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
>  			error = -EFAULT;
>  			goto out;
>  		}
> +		error = mutex_lock_interruptible(&syslog_mutex);
> +		if (error)
> +			goto out;
>  		error = wait_event_interruptible(log_wait,
>  						 syslog_seq != log_next_seq);
> -		if (error)
> +		if (error) {
> +			mutex_unlock(&syslog_mutex);
>  			goto out;
> +		}
>  		error = syslog_print(buf, len);
> +		mutex_unlock(&syslog_mutex);
>  		break;
>  	/* Read/clear last kernel messages */
>  	case SYSLOG_ACTION_READ_CLEAR:
--
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