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

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Message-ID: <957960eb-118f-21c7-8901-50f54d65d7cb@embeddedor.com>
Date:   Tue, 21 Jan 2020 08:27:41 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Jiri Slaby <jslaby@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: n_hdlc: Use flexible-array member



On 1/20/20 23:54, Jiri Slaby wrote:
> On 21. 01. 20, 0:45, Gustavo A. R. Silva wrote:
>> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
>> presence of a "variable length array":
>>
>> struct something {
>>     int length;
>>     u8 data[1];
>> };
>>
>> struct something *instance;
>>
>> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
>> instance->length = size;
>> memcpy(instance->data, source, size);
>>
>> There is also 0-byte arrays. Both cases pose confusion for things like
>> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
>> to declare variable-length types such as the one above is a flexible array
>> member[2] which need to be the last member of a structure and empty-sized:
>>
>> struct something {
>>         int stuff;
>>         u8 data[];
>> };
>>
>> Also, by making use of the mechanism above, we will get a compiler warning
>> in case the flexible array does not occur last in the structure, which
>> will help us prevent some kind of undefined behavior bugs from being
>> unadvertenly introduced[3] to the codebase from now on.
>>
>> [1] https://github.com/KSPP/linux/issues/21
>> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>> ---
>>  drivers/tty/n_hdlc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> 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.

> Other than that:
> Acked-by: Jiri Slaby <jslaby@...e.cz>
> 

Thanks!
--
Gustavo