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

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Date:   Tue, 21 Jan 2020 09:14:15 -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 09:00, 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.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Thanks
--
Gustavo

> Consider the following:
> #include <stdio.h>
> 
> struct flex {
>   int count;
>   char buf[PRE];
>   char flex[];
> };
> 
> struct one {
>   int count;
>   char buf[PRE];
>   char one[1];
> };
> 
> void main() {
>   printf("%ld %ld\n", sizeof(struct flex), sizeof(struct one));
> }
> 
> --snip--
> 
> % gcc -o siz siz.c -std=c99 -DPRE=7 && ./siz
> 12 12
> % gcc -o siz siz.c -std=c99 -DPRE=8 && ./siz
> 12 16
> 
> Since all the preceding stuff in the struct in the patch is aligned, then the [1] will definitely add something to the sizeof count.
>