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SubjectRe: PATCH] firewire: add padding to some struct
JiSheng Zhang writes:
> On Fri, 18 Jul 2008 17:27:44 +0200
> Mikael Pettersson <mikpe@it.uu.se> wrote:
>
> > JiSheng Zhang writes:
> > > Hi,
> > > >From: Stefan Richter <stefanr@s5r6.in-berlin.de>
> > > >Reply-To:
> > > >To: JiSheng Zhang <jszhang3@mail.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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