linux-kernel - Re: Bug 44211 - /proc/kmsg does not (always) block for 1-byte reads

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Date:	Sun, 8 Jul 2012 15:06:20 +0200
From:	Kay Sievers <kay@...y.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Greg KH <greg@...ah.com>, Jukka Ollila <jiiksteri@...il.com>,
	linux-kernel@...r.kernel.org, jbeulich@...ell.com
Subject: Re: Bug 44211 - /proc/kmsg does not (always) block for 1-byte reads

On Sun, Jul 8, 2012 at 2:59 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> The patch will not fix the underlying problem, but just make it behave
>> more like it was and allow partial message reads. This is a years old
>> problem, the net is full of bugreports of stuff going wrong with
>> running dd bs=1 on /proc/kmsg. It is a really stupid idea, and can not
>> work for many other reasons too. The interface can not safely be used
>> that way, it does not have the usual semantics, it always returned 0
>> for read() whenever it needed to.
>
> If you are breaking the semantics perhaps that should also get fixed ?

I hopefully just restored the old semantics now.

Fixing it properly would be a bigger code change, and it can't use the
far-too-simple tunneling through the syslog() syscall to feed
/proc/kmsg.

If the seq_file interface could be used, that would probably be the
best option, but I have no good idea how to make blocking reads, and
concurrent non-blocks work with the seq_file stuff.

This is how read() in /proc/kmsg works and it is not protected by a
lock or anything, and there is a not too small window between the
check and the action. Things just go wrong if there is more than a
single reader, but that was the case since forever.

static ssize_t kmsg_read(struct file *file, char __user *buf,
                         size_t count, loff_t *ppos)
{
        if ((file->f_flags & O_NONBLOCK) &&
            !do_syslog(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_FILE))
                return -EAGAIN;
        return do_syslog(SYSLOG_ACTION_READ, buf, count, SYSLOG_FROM_FILE);
}

Thanks,
Kay
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