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

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Date:	Tue, 3 Mar 2009 19:35:01 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Theodore Tso <tytso@....edu>,
	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, 03 Mar 2009 21:49:26 -0500 Steven Rostedt <rostedt@...dmis.org> wrote:

> +static ssize_t
> +tracing_buffers_read(struct file *filp, char __user *ubuf,
> +		     size_t count, loff_t *ppos)
> +{
> +	struct ftrace_buffer_info *info = filp->private_data;
> +	unsigned int pos;
> +	ssize_t ret;
> +	size_t size;
> +
> +	/* Do we have previous read data to read? */
> +	if (info->read < PAGE_SIZE)
> +		goto read;
> +
> +	info->read = 0;
> +
> +	ret = ring_buffer_read_page(info->tr->buffer,
> +				    &info->spare,
> +				    count,
> +				    info->cpu, 0);
> +	if (ret < 0)
> +		return 0;
> +
> +	pos = ring_buffer_page_len(info->spare);
> +
> +	if (pos < PAGE_SIZE)
> +		memset(info->spare + pos, 0, PAGE_SIZE - pos);
> +
> +read:
> +	size = PAGE_SIZE - info->read;
> +	if (size > count)
> +		size = count;
> +
> +	ret = copy_to_user(ubuf, info->spare + info->read, size);
> +	if (ret)
> +		return -EFAULT;

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.

> +	*ppos += size;
> +	info->read += size;
> +
> +	return size;
> +}
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