linux-kernel - Re: [PATCH][next] Bluetooth: a2mp: Use struct

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Date:   Thu, 7 Feb 2019 22:07:10 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Joe Perches <joe@...ches.com>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] Bluetooth: a2mp: Use struct_size() helper



On 2/7/19 10:00 PM, Joe Perches wrote:
> On Thu, 2019-02-07 at 18:28 -0600, Gustavo A. R. Silva wrote:
>> One of the more common cases of allocation size calculations is finding
>> the size of a structure that has a zero-sized array at the end, along
>> with memory for some number of elements for that array. For example:
>>
>> struct foo {
>>     int stuff;
>>     struct boo entry[];
>> };
>>
>> size = sizeof(struct foo) + count * sizeof(struct boo);
>> instance = alloc(size, GFP_KERNEL)
>>
>> Instead of leaving these open-coded and prone to type mistakes, we can
>> now use the new struct_size() helper:
>>
>> size = struct_size(instance, entry, count);
>> instance = alloc(size, GFP_KERNEL)
>>
>> This code was detected with the help of Coccinelle.
> []
>> diff --git a/net/bluetooth/a2mp.c b/net/bluetooth/a2mp.c
> []
>> @@ -174,7 +174,7 @@ static int a2mp_discover_req(struct amp_mgr *mgr, struct sk_buff *skb,
>>  			num_ctrl++;
>>  	}
>>  
>> -	len = num_ctrl * sizeof(struct a2mp_cl) + sizeof(*rsp);
>> +	len = struct_size(rsp, cl, num_ctrl);
>>  	rsp = kmalloc(len, GFP_ATOMIC);
>>  	if (!rsp) {
>>  		read_unlock(&hci_dev_list_lock);
> 
> At least a weakness in this code is len is u16
> and struct_size is size_t so there's a size
> truncation possible.
> 
> 

That's true.  I didn't change the type to size_t because of the call
to le16_to_cpu():

u16 len = le16_to_cpu(hdr->len);

I've been changing the type of the variable in other cases.

--
Gustavo