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    SubjectRe: [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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