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

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Date:   Wed, 22 Jul 2020 18:01:54 -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



On 7/22/20 17:40, Winkler, Tomas wrote:
>>
>> 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.
> 
> 
> Frankly I will post other version of this that cleans the whole file. 
> 

Sounds good. :)

>>>
>>>>> 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.
> 
> I was going over https://github.com/KSPP/linux/issues/79, I'm not sure this all related  to flexible arrays,

I've opened a new issue for this:

https://github.com/KSPP/linux/issues/86

And I'm already including the link above in these sorts
of patches; please, see:

https://lore.kernel.org/lkml/20200717215500.GA13910@embeddedor/

> those are just reserved struct members. So because it's hard to identify a legitimate usage of single element arrays
> we are going to kill them all? It's more esthetic / readability issue here but there might be some legit use case for one element array, no?
> 

The code is continuously evolving and, if a one-element array is not intended
to be used as a variable-length array at all, I frankly cannot think of any another
use that is not merely esthetic/readability. :)

Thanks
--
Gustavo

> 
>>
>> 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;
>>>>>
>>>>>  /**
>>>>>