linux-kernel - Re: [PATCH] tty: n_hdlc: Use flexible-array member

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Date:   Tue, 21 Jan 2020 09:24:28 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Mikael Magnusson <mikachu@...il.com>
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: n_hdlc: Use flexible-array member



On 1/21/20 09:14, Gustavo A. R. Silva wrote:
> 
> 
> On 1/21/20 09:00, Mikael Magnusson wrote:
>> Gustavo Silva wrote:
>>> On 1/20/20 23:54, Jiri Slaby wrote:
>>>> On 21. 01. 20, 0:45, Gustavo A. R. Silva wrote:
>>>>> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
>>>>> index 98361acd3053..b5499ca8757e 100644
>>>>> --- a/drivers/tty/n_hdlc.c
>>>>> +++ b/drivers/tty/n_hdlc.c
>>>>> @@ -115,7 +115,7 @@
>>>>>  struct n_hdlc_buf {
>>>>>    struct list_head  list_item;
>>>>>    int     count;
>>>>> -  char      buf[1];
>>>>> +  char      buf[];
>>>>>  };
>>>>>  
>>>>>  #define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe)
>>>>
>>>> Have you checked, that you don't have to "+ 1" here now?
>>>>
>>>
>>> Yep. That's not necessary.
>>>
>>> _In terms of memory allocation_, zero-length/one-element arrays and flexible-array
>>> members work exactly the same way.
>>
>> This is not true, but maybe it's still not necessary in this particular code, I didn't examine it.
>>
> 
> I should have said _in terms of dynamic memory allocation_.
> 
> Your example is correct:
> 
> "... a one-element array always occupies at least as much space as a single object of the type."[1]
> 
> But the above does not affect on the current code.
> 

Oh wait! Yeah; I see the issue in #define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe) now...

I need to double check this.

Thanks for the feedback!
--
Gustavo