linux-kernel - Re: PATCH] firewire: add padding to some struct

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Message-ID: <488080B1.8010207@s5r6.in-berlin.de>
Date:	Fri, 18 Jul 2008 13:38:25 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	JiSheng Zhang <jszhang3@...l.ustc.edu.cn>
CC:	linux-kernel@...r.kernel.org,
	linux1394-devel@...ts.sourceforge.net, krh@...hat.com
Subject: Re: PATCH] firewire: add padding to some struct

JiSheng Zhang wrote:
> If p is a pointer to struct fw_cdev_event_response), p->data will point to the
> padding data rather than the right place, it will cause problem under some
> platforms. For example, in the function handle_device_event of libraw1394(ported
> to juju stack):
> .....
> case FW_CDEV_EVENT_RESPONSE:
>     rc = u64_to_ptr(u->response.closure);
>     if (rc->data != NULL)
> 	memcpy(rc->data, u->response.data, rc->length);//here it will lost the last four
> bytes
>     errcode = juju_to_raw1394_errcode(u->response.rcode);
> .....
> 
> Although this problem can be solved by add the offset to the pointer, but the
> member:__u32 data[0] lost its original meaning.

I don't understand what the problem is.  As long as both kernel and
library use "response.data" or "&response + offsetof(typeof(response),
data)", they will write and read at the correct location.

There would be a problem if one of the two used "&response +
sizeof(response)" instead.  Does this happen anywhere?  If so, then
these places need to be fixed, not the struct definition.
-- 
Stefan Richter
-=====-==--- -=== =--=-
http://arcgraph.de/sr/
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