Write to /dev/port as non-root user - linux-kernel

Question

I am working on a rather complex C++ code which performs the following operation as standard user
fd = open("/dev/port",O_WRONLY);
...
lseek(fd, 0x2E,SEEK_SET);
...
write(fd,&buf,1);
I get a 'Operation not permitted' error on opening the file, despite having chmodded the file.
crwxrwxrwx 1 root kmem 1, 4 Sep 12 14:32 /dev/port
I know of security issues with chmodding /dev/port, but as far as we're concerned the system will run on a closed LAN.
To make it easier, this:
using namespace std;
int main(int argc, char *argv[])
{
int fd=-1;
// fd1=open("/dev/port",O_RDWR|O_NDELAY);
vector<string> fnames;
fnames.push_back("/dev/port");
fnames.push_back("/dev/tty0");
string fname;
for(int i=0;i<fnames.size();i++)
{
fname = fnames[i];
fd=open(fname.c_str(),O_RDWR | O_NDELAY);
if(fd<0)
{
cout << fname << " " << fd << endl;
cout << fname << " " << strerror(errno) << endl;
}
else
{
cout << "Open ok: " << fname << endl;
}
}
return 0;
}
returns this:
me#myPC:~/test$ ./main
/dev/port -1
/dev/port Operation not permitted
Open ok: /dev/tty0
with these permission rights
me#myPC:~/test$ ll /dev/tty /dev/port
crw-rw-rw- 1 root kmem 1, 4 Sep 12 14:32 /dev/port
crw-rw-rw- 1 root tty 5, 0 Sep 12 15:51 /dev/tty

Answer 1

To open /dev/port you need the capability CAP_SYS_RAWIO, in addition to permission to open the file.
drivers/char/mem.c:730
static int open_port(struct inode * inode, struct file * filp)
{
return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
}
You can either gain this by being root or by setting it for a single executable (similar to set-uid) using setcap.
http://linux.die.net/man/8/setcap