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

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Date:	Fri, 6 Jul 2012 13:30:01 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Kay Sievers <kay@...y.org>, Greg Kroah-Hartman <greg@...ah.com>
Cc:	Jukka Ollila <jiiksteri@...il.com>, linux-kernel@...r.kernel.org,
	jbeulich@...ell.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: Regression - /proc/kmsg does not (always) block for 1-byte reads

Kay, this needs to be fixed.

Suggested fix: just use the 'seq_printf()' interfaces, which do the
proper buffering, and allow any size reads of various packetized data.

Of course, I'd also suggest that whoever was the genius who thought it
was a good idea to read things ONE F*CKING BYTE AT A TIME with system
calls for each byte should be retroactively aborted. Who the f*ck does
idiotic things like that? How did they noty die as babies, considering
that they were likely too stupid to find a tit to suck on?

                 Linus

On Fri, Jul 6, 2012 at 12:38 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> 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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