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

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Date:	Fri, 6 Jul 2012 10:55:01 -0700
From:	Greg KH <greg@...ah.com>
To:	Jukka Ollila <jiiksteri@...il.com>
Cc:	linux-kernel@...r.kernel.org, kay@...y.org, jbeulich@...ell.com,
	alan@...rguk.ukuu.org.uk
Subject: Re: Bug 44211 - /proc/kmsg does not (always) block for 1-byte reads

On Fri, Jul 06, 2012 at 08:45:44PM +0300, Jukka Ollila wrote:
> Hello,
> 
> A few days ago I filed a kernel regression report concerning a change
> in /proc/kmsg behaviour with short reads:
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=44211
> 
> The comments suggest that this is probably intentional, but that it
> would be best make sure that the current semantics wrt short reads are
> as intended.
> 
> The problem appears on a Debian (unstable) system that drains
> /proc/kmsg into a separate fifo read by klogd(8):
> 
> /bin/dd bs=1 if=/proc/kmsg  of=/var/run/klogd/kmsg
> 
> With the recent kernel logging changes this /bin/dd exits immediately,
> as 1-byte reads are shorter than any log message could possibly be and
> read() returns 0. No dd feeding the fifo results in no logging and a
> rather unhappy klogd on the reading end of /var/run/klogd/kmsg.
> 
> I suppose a safe solution is to only do reads that are big enough for
> any single kernel message, but this is still a change that affects
> user space being shipped, so some might find it surprising.
> 
> I don't know what other distros do. Is it just Debian being the odd one out?

I think we just fixed this, what kernel version are you seeing this
problem on?

Kay did your other patches that I just accepted resolve this?

thanks,

greg k-h
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