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

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Date:	Sat, 19 Jul 2008 12:09:18 +0200
From:	Mikael Pettersson <mikpe@...uu.se>
To:	JiSheng Zhang <jszhang3@...l.ustc.edu.cn>
Cc:	Mikael Pettersson <mikpe@...uu.se>, 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

JiSheng Zhang writes:
 > 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 new code is only correct if the variable length payload starts
after the struct, i.e. (void*)(&response->response + 1), and not at
the data field, i.e. (void*)response->response.data.
Which is it? That's why I asked:

 > > 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.
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