Interface to query tun/tap features.

The problem with introducing IFF_RECV_CSUM and IFF_RECV_GSO is that
they need to set dev->features to enable GSO and/or checksumming,
which is supposed to be done before register_netdevice(), ie. as part
of TUNSETIFF.

Unfortunately, TUNSETIFF has always just ignored flags it doesn't understand,
so there's no good way of detecting whether the kernel supports IFF_GSO_HDR.

This patch implements a TUNGETFEATURES ioctl which returns all the valid IFF
flags.  It could be extended later to include other features.

Here's an example program which uses it:

#include <linux/if_tun.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>

static struct {
	unsigned int flag;
	const char *name;
} known_flags[] = {
	{ IFF_TUN, "TUN" },
	{ IFF_TAP, "TAP" },
	{ IFF_NO_PI, "NO_PI" },
	{ IFF_ONE_QUEUE, "ONE_QUEUE" },
	{ IFF_VIRTIO_HDR, "VIRTIO_HDR" },
	{ IFF_RECV_CSUM, "RECV_CSUM" },
	{ IFF_RECV_GSO, "RECV_GSO" },
};

int main()
{
	unsigned int features, i;

	int netfd = open("/dev/net/tun", O_RDWR);
	if (netfd < 0)
		err(1, "Opening /dev/net/tun");

	if (ioctl(netfd, TUNGETFEATURES, &features) != 0) {
		printf("Kernel does not support TUNGETFEATURES, guessing\n");
		features = (IFF_TUN|IFF_TAP|IFF_NO_PI|IFF_ONE_QUEUE);
	}
	printf("Available features are: ");
	for (i = 0; i < sizeof(known_flags)/sizeof(known_flags[0]); i++) {
		if (features & known_flags[i].flag) {
			features &= ~known_flags[i].flag;
			printf("%s ", known_flags[i].name);
		}
	}
	if (features)
		printf("(UNKNOWN %#x)", features);
	printf("\n");
	return 0;
}

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/net/tun.c      |    8 ++++++++
 include/linux/if_tun.h |    1 +
 2 files changed, 9 insertions(+)

diff -r cbef1a615b90 drivers/net/tun.c
--- a/drivers/net/tun.c	Sat Apr 05 23:24:46 2008 +1100
+++ b/drivers/net/tun.c	Sat Apr 05 23:25:36 2008 +1100
@@ -952,6 +952,14 @@ static int tun_chr_ioctl(struct inode *i
 		return 0;
 	}
 
+	if (cmd == TUNGETFEATURES) {
+		/* Currently this just means: "what IFF flags are valid?".
+		 * This is needed because we never checked for invalid flags on
+		 * TUNSETIFF. */
+		return put_user(IFF_TUN | IFF_TAP | IFF_NO_PI | IFF_ONE_QUEUE,
+				(unsigned int __user*)argp);
+	}
+
 	if (!tun)
 		return -EBADFD;
 
diff -r cbef1a615b90 include/linux/if_tun.h
--- a/include/linux/if_tun.h	Sat Apr 05 23:24:46 2008 +1100
+++ b/include/linux/if_tun.h	Sat Apr 05 23:25:36 2008 +1100
@@ -44,6 +44,7 @@
 #define TUNSETGROUP   _IOW('T', 206, int)
 #define TUNSETRECVVRING _IOW('T', 207, int)
 #define TUNSETXMITVRING _IOW('T', 208, int)
+#define TUNGETFEATURES _IOR('T', 209, unsigned int)
 
 /* TUNSETIFF ifr flags */
 #define IFF_TUN		0x0001