Messages in this thread | | | Date | Sat, 16 Jun 2012 20:59:22 +0800 | From | Yuanhan Liu <> | 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:
| |