linux-kernel - Re: [PATCH v2] xdp: Sample xdp program implementing ip forward

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Date:   Tue, 10 Oct 2017 14:37:51 -0700
From:   David Daney <ddaney@...iumnetworks.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        Christina Jacob <christina.jacob.koikara@...il.com>
Cc:     Christina Jacob <Christina.Jacob@...ium.com>, daniel@...earbox.net,
        Sunil.Goutham@...ium.com, netdev@...r.kernel.org,
        dsahern@...il.com, linux-kernel@...r.kernel.org, brouer@...hat.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] xdp: Sample xdp program implementing ip forward

On 10/10/2017 10:19 AM, Stephen Hemminger wrote:
> On Tue, 10 Oct 2017 12:58:52 +0530
> Christina Jacob <christina.jacob.koikara@...il.com> wrote:
> 
>> +/* Get the mac address of the interface given interface name */
>> +static long *getmac(char *iface)
>> +{
>> +	int fd;
>> +	struct ifreq ifr;
>> +	long *mac = NULL;
>> +
>> +	fd = socket(AF_INET, SOCK_DGRAM, 0);
>> +	ifr.ifr_addr.sa_family = AF_INET;
>> +	strncpy(ifr.ifr_name, iface, IFNAMSIZ - 1);
>> +	ioctl(fd, SIOCGIFHWADDR, &ifr);
>> +	mac = (long *)ifr.ifr_hwaddr.sa_data;
>> +	close(fd);
>> +	return mac;
> 
> Always check return value of ioctl.
> You are assuming sizeof(long) > 6 bytes.
> Also the byte order.

Also:

Returning the address of a local variable (ifr.ifr_hwaddr.sa_data), and 
then dereferencing it outside of the function is not correct.

The casting of the char sa_data[] to a long * may cause alignment faults 
on some architectures.  The may also be endinaness issues depending on 
how the data are manipulated if you pack all those chars into a long.

If we think that a MAC address is char[6], then it may be best to define 
the data structures as such and manipulate it as an array instead of 
trying to pack it into a long.

Keep working on this though, this program will surely be useful.

David Daney