linux-kernel - Re: [PATCH][next] mei: Avoid the use of one-element arrays

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Message-ID: <20200714180855.GA31158@embeddedor>
Date:   Tue, 14 Jul 2020 13:08:55 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     "Winkler, Tomas" <tomas.winkler@...el.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] mei: Avoid the use of one-element arrays

On Tue, Jul 14, 2020 at 05:54:32PM +0000, Winkler, Tomas wrote:
> > 
> > There is a regular need in the kernel to provide a way to declare having a
> > dynamically sized set of trailing elements in a structure.
> > Kernel code should always use “flexible array members”[1] for these cases
> > or, as in this particular case, replace the one-element array with a simple
> > value type u8 reserved once this is just a placeholder for alignment. The older
> > style of one-element or zero-length arrays should no longer be used[2].
> > 
> > Also, while there, use the preferred form for passing a size of a struct.
> > The alternative form where struct name is spelled out hurts readability and
> > introduces an opportunity for a bug when the variable type is changed but
> > the corresponding sizeof that is passed as argument is not.
> > 
> > [1] https://en.wikipedia.org/wiki/Flexible_array_member
> > [2] https://github.com/KSPP/linux/issues/79
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> I'm okay with the patch but in this case the description is a bit off. 
> In this case there was no intention for a flexible arrays its just a reserved field. 
> 

The reserved field is actually mentioned in the description:

"... or, as in this particular case, replace the one-element array with a simple
value type u8 reserved once this is just a placeholder for alignment."

Thanks
--
Gustavo

> > ---
> >  drivers/misc/mei/hbm.c | 4 ++--
> >  drivers/misc/mei/hw.h  | 6 +++---
> >  2 files changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/misc/mei/hbm.c b/drivers/misc/mei/hbm.c index
> > a44094cdbc36..f020d5594154 100644
> > --- a/drivers/misc/mei/hbm.c
> > +++ b/drivers/misc/mei/hbm.c
> > @@ -408,14 +408,14 @@ static int mei_hbm_add_cl_resp(struct mei_device
> > *dev, u8 addr, u8 status)  {
> >  	struct mei_msg_hdr mei_hdr;
> >  	struct hbm_add_client_response resp;
> > -	const size_t len = sizeof(struct hbm_add_client_response);
> > +	const size_t len = sizeof(resp);
> >  	int ret;
> > 
> >  	dev_dbg(dev->dev, "adding client response\n");
> > 
> >  	mei_hbm_hdr(&mei_hdr, len);
> > 
> > -	memset(&resp, 0, sizeof(struct hbm_add_client_response));
> > +	memset(&resp, 0, len);
> >  	resp.hbm_cmd = MEI_HBM_ADD_CLIENT_RES_CMD;
> >  	resp.me_addr = addr;
> >  	resp.status  = status;
> > diff --git a/drivers/misc/mei/hw.h b/drivers/misc/mei/hw.h index
> > b1a8d5ec88b3..8c0297f0e7f3 100644
> > --- a/drivers/misc/mei/hw.h
> > +++ b/drivers/misc/mei/hw.h
> > @@ -346,13 +346,13 @@ struct hbm_add_client_request {
> >   * @hbm_cmd: bus message command header
> >   * @me_addr: address of the client in ME
> >   * @status: if HBMS_SUCCESS then the client can now accept connections.
> > - * @reserved: reserved
> > + * @reserved: reserved for alignment.
> >   */
> >  struct hbm_add_client_response {
> >  	u8 hbm_cmd;
> >  	u8 me_addr;
> >  	u8 status;
> > -	u8 reserved[1];
> > +	u8 reserved;
> >  } __packed;
> > 
> >  /**
> > @@ -461,7 +461,7 @@ struct hbm_notification {
> >  	u8 hbm_cmd;
> >  	u8 me_addr;
> >  	u8 host_addr;
> > -	u8 reserved[1];
> > +	u8 reserved;
> >  } __packed;
> > 
> >  /**
> > --
> > 2.27.0
>