linux-kernel - Regression - /proc/kmsg does not (always) block for 1-byte reads

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Message-ID: <20120706203825.20ce3e47@pyramind.ukuu.org.uk>
Date:	Fri, 6 Jul 2012 20:38:25 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Jukka Ollila <jiiksteri@...il.com>
Cc:	linux-kernel@...r.kernel.org, kay@...y.org, jbeulich@...ell.com,
	greg@...ah.com, torvalds@...ux-foundation.org
Subject: Regression - /proc/kmsg does not (always) block for 1-byte reads

On Fri, 6 Jul 2012 20:45:44 +0300
Jukka Ollila <jiiksteri@...il.com> 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?

If this is observed on an actual standard distro userspace and breaks it
then its a regression and it needs fixing or reverting.

Cc'ing Linus
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