linux-kernel - Re: [PATCH 5/5] tracing: add binary buffer files for use with splice

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Message-ID: <20090304044614.GL32284@mit.edu>
Date:	Tue, 3 Mar 2009 23:46:14 -0500
From:	Theodore Tso <tytso@....edu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Pekka Paalanen <pq@....fi>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Martin Bligh <mbligh@...gle.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Baron <jbaron@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jiaying Zhang <jiayingz@...gle.com>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	mrubin@...gle.com, md@...gle.com,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 5/5] tracing: add binary buffer files for use with
	splice

On Tue, Mar 03, 2009 at 10:43:16PM -0500, Steven Rostedt wrote:
> > Conventionally a read() system call will return the number of bytes
> > copied, and will only return -EFOO if the number of bytes copied was
> > zero.
> > 
> > Lots of parts of the kernel break this, but it's usually device drivers
> > and scruffy pseudo files, in which case a partial file read doesn't
> > make much sense.  This doesn't make the broken behaviour right, but at
> > least we have a bit of a weaselly excuse in that case.
> > 
> 
> I just went by the read man page:
> 
>        EFAULT buf is outside your accessible address space.
> 

What is normally done in this case if buf is within the addressable
space, but buf+x is not, where x < len, then read(fd, buf, len) will
fill copy as many characters it can into buf until it runs out of
valid addres space, and then return that count as a partial read.

      	     	    	     	    	 -  Ted
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