Subject: Re: [PATCH 3/3] Interface to query tun/tap features.

From: Max Krasnyansky <[email-masked]>
Date: Thu, 07 Feb 2008 21:07:02 -0800

Hi Rusty,

Sorry for delay in reply. I totally missed this one. Need to fix my mail filters. 
See below.

Rusty Russell wrote:
> (No real change, just updated with new bits)
> 
> 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 <[email-masked]>
> ---
>  drivers/net/tun.c      |    9 +++++++++
>  include/linux/if_tun.h |    3 +++
>  2 files changed, 12 insertions(+)
> 
> diff -r c0e7a8b99325 drivers/net/tun.c
> --- a/drivers/net/tun.c	Wed Jan 23 20:12:51 2008 +1100
> +++ b/drivers/net/tun.c	Wed Jan 23 20:17:28 2008 +1100
> @@ -790,6 +790,15 @@ 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.  This was introduced with IFF_GSO_HDR, so if a
> +		 * kernel doesn't have this ioctl, it doesn't have GSO header
> +		 * support. */
> +		return put_user(IFF_ALL_FLAGS, (unsigned int __user*)argp);
> +	}
> +
>  	if (!tun)
>  		return -EBADFD;
>  
> diff -r c0e7a8b99325 include/linux/if_tun.h
> --- a/include/linux/if_tun.h	Wed Jan 23 20:12:51 2008 +1100
> +++ b/include/linux/if_tun.h	Wed Jan 23 20:17:28 2008 +1100
> @@ -82,6 +82,7 @@ struct tun_struct {
>  #define TUNSETOWNER   _IOW('T', 204, int)
>  #define TUNSETLINK    _IOW('T', 205, int)
>  #define TUNSETGROUP   _IOW('T', 206, int)
> +#define TUNGETFEATURES _IOR('T', 207, unsigned int)
>  
>  /* TUNSETIFF ifr flags */
>  #define IFF_TUN		0x0001
> @@ -91,6 +92,8 @@ struct tun_struct {
>  #define IFF_VIRTIO_HDR	0x4000
>  #define IFF_RECV_CSUM	0x8000
>  #define IFF_RECV_GSO	0x0800
> +#define IFF_ALL_FLAGS (IFF_TUN | IFF_TAP | IFF_NO_PI | IFF_ONE_QUEUE | \
> +		       IFF_VIRTIO_HDR | IFF_RECV_CSUM | IFF_RECV_GSO)
>  
>  struct tun_pi {
>  	unsigned short flags;

Definitely Ack this one. Query interface makes perfect sense.
I'll reply to the GSO itself shortly.

I was going to create git tree for tun changes for awhile now. Looks like the time has come.
I'll do that asap.

Thanx
Max