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

Gustavo A. R. Silva
2020-07-22T23:20:37Z

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@kernel.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;
>>>>>
>>>>> /**
>>>>>