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

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Date:   Tue, 21 Jan 2020 16:00:53 +0100
From:   Mikael Magnusson <mikachu@...il.com>
To:     gustavo@...eddedor.com
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: n_hdlc: Use flexible-array member

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.

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.

-- 
Mikael Magnusson