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

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Message-Id: <20080719154115.21334197.jszhang3@mail.ustc.edu.cn>
Date:	Sat, 19 Jul 2008 15:41:15 +0800
From:	JiSheng Zhang <jszhang3@...l.ustc.edu.cn>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	stefanr@...6.in-berlin.de, linux-kernel@...r.kernel.org,
	linux1394-devel@...ts.sourceforge.net, krh@...hat.com
Subject: Re: PATCH] firewire: add padding to some struct

On Fri, 18 Jul 2008 17:27:44 +0200
Mikael Pettersson <mikpe@...uu.se> wrote:

> JiSheng Zhang writes:
>  > Hi,
>  > >From: Stefan Richter <stefanr@...6.in-berlin.de>
>  > >Reply-To: 
>  > >To: JiSheng Zhang <jszhang3@...l.ustc.edu.cn>
>  > >Subject: Re: PATCH] firewire: add padding to some struct
>  > >Date:Fri, 18 Jul 2008 13:38:25 +0200
>  > >
>  > >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
> 
> Define "the right place". If p->data[] isn't the place for the data,
> then something's seriously wrong with either the producer or the
> consumer of that data -- or the data type definition if either is HW.
> 
>  > > > 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.
>  > >
>  > This patch can fix the problem while not changing the struct definition.
>  > 
>  > 
>  > Thanks in advance,
>  > JiSheng
>  > 
>  > --- old/drivers/firewire/fw-cdev.c	2008-07-14 05:51:29.000000000 +0800
>  > +++ new/drivers/firewire/fw-cdev.c	2008-07-18 20:20:45.841328585 +0800
>  > @@ -382,9 +382,9 @@
>  >  
>  >  	response->response.type   = FW_CDEV_EVENT_RESPONSE;
>  >  	response->response.rcode  = rcode;
>  > -	queue_event(client, &response->event,
>  > -		    &response->response, sizeof(response->response),
>  > -		    response->response.data, response->response.length);
>  > +	queue_event(client, &response->event, &response->response, 
>  > +		    sizeof(response->response) + response->response.length,
>  > +		    NULL, 0);
>  >  }
> 
> Neither of these look correct.
> If sizeof(struct ...) != offsetof(struct ..., data) as you claim is possible,
> then the old code will copy too much to/from ->response but the correct amount
> to/from ->response.data, and the new code will copy too much to/from ->response.data.
The old code will copy 4 extra bytes totally on some platforms, the new code
is correct. The old one queue like this:
struct ...(excluding the padding bytes)|4 padding bytes|4 padding bytes|data
> 
> That's why C has offsetof():
> 
> 	queue_event(client, &response->event,
> 		   &response->response,
> 		   offsetof(typeof(*response->responce), data), // I don't know the struct name
> 		   response->response.data, response->response.length);

Bye,
JiSheng
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