linux-kernel - Re: [PATCH v2] mei: Avoid the use of one-element arrays

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Date:   Wed, 22 Jul 2020 14:29:26 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "Winkler, Tomas" <tomas.winkler@...el.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] mei: Avoid the use of one-element arrays

Hi Tomas,

Please, see my comments below...

On 7/22/20 14:04, Winkler, Tomas wrote:
> 
>>
>> Hi all,
>>
>> Friendly ping: who can take this? :)
>>
>> Thanks
>> --
>> Gustavo
>>
>> On 7/14/20 16:45, Gustavo A. R. Silva wrote:
>>> One-element arrays are being deprecated[1]. Replace the one-element
>>> arrays with a simple value type u8 reserved, once this is just a
>>> placeholder for alignment.
>>>
>>> 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://github.com/KSPP/linux/issues/79
>>>
>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>> ---
>>> Changes in v2:
>>>  - Use a more concise changelog text.
>>>
>>>  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;
> 
> This should be probably in a different patch it's not related to the second part.
> 
>>> 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
> I have second thoughts of this part as all reserved fields in this file are of form u8 reserved[X], 
> so we will lose that uniformity with this change, you have to look at the file as whole
> not just at the patch.  So I prefer we drop that part of the patch. 
> 

This is actually the main point of this patch: the removal of one-element arrays.
And yeah, every place in the kernel that uses the form that you mention will see
it's uniformity slightly modified, and that's for a good cause: the removal of
one-element arrays, so we can enable bounds checking.

Thanks
--
Gustavo

>>> @@ -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;
>>>
>>>  /**
>>>