Subject: Re: [devel-ipsec] [PATCH ipsec-next v5 11/17] xfrm: iptfs: add fragmenting of larger than MTU user packets

From: Christian Hopps <[email-masked]>
Date: Wed, 17 Jul 2024 22:57:52 -0700

Simon Horman via Devel <[email-masked]> writes:

> On Sun, Jul 14, 2024 at 04:22:39PM -0400, Christian Hopps wrote:
>> From: Christian Hopps <[email-masked]>
>>
>> Add support for tunneling user (inner) packets that are larger than the
>> tunnel's path MTU (outer) using IP-TFS fragmentation.
>>
>> Signed-off-by: Christian Hopps <[email-masked]>
>> ---
>>  net/xfrm/xfrm_iptfs.c | 401 +++++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 375 insertions(+), 26 deletions(-)
>>
>> diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
>
> ...
>
>> +static int iptfs_copy_create_frags(struct sk_buff **skbp,
>> +				   struct xfrm_iptfs_data *xtfs, u32 mtu)
>> +{
>> +	struct skb_seq_state skbseq;
>> +	struct list_head sublist;
>> +	struct sk_buff *skb = *skbp;
>> +	struct sk_buff *nskb = *skbp;
>> +	u32 copy_len, offset;
>> +	u32 to_copy = skb->len - mtu;
>> +	u32 blkoff = 0;
>> +	int err = 0;
>> +
>> +	INIT_LIST_HEAD(&sublist);
>> +
>> +	BUG_ON(skb->len <= mtu);
>> +	skb_prepare_seq_read(skb, 0, skb->len, &skbseq);
>> +
>> +	/* A trimmed `skb` will be sent as the first fragment, later. */
>> +	offset = mtu;
>> +	to_copy = skb->len - offset;
>> +	while (to_copy) {
>> +		/* Send all but last fragment to allow agg. append */
>> +		list_add_tail(&nskb->list, &sublist);
>> +
>> +		/* FUTURE: if the packet has an odd/non-aligning length we could
>> +		 * send less data in the penultimate fragment so that the last
>> +		 * fragment then ends on an aligned boundary.
>> +		 */
>> +		copy_len = to_copy <= mtu ? to_copy : mtu;
>
> nit: this looks like it could be expressed using min()
>
>      Flagged by Coccinelle

Changed.

>
>> +		nskb = iptfs_copy_create_frag(&skbseq, offset, copy_len);
>> +		if (IS_ERR(nskb)) {
>> +			XFRM_INC_STATS(dev_net(skb->dev),
>> +				       LINUX_MIB_XFRMOUTERROR);
>> +			skb_abort_seq_read(&skbseq);
>> +			err = PTR_ERR(nskb);
>> +			nskb = NULL;
>> +			break;
>> +		}
>> +		iptfs_output_prepare_skb(nskb, to_copy);
>> +		offset += copy_len;
>> +		to_copy -= copy_len;
>> +		blkoff = to_copy;
>
> blkoff is set but otherwise unused in this function.
>
> Flagged by W=1 x86_64 allmodconfig builds with gcc-14 and clang 18.

This value is used in a trace point call in this function.

>
>> +	}
>> +	skb_abort_seq_read(&skbseq);
>> +
>> +	/* return last fragment that will be unsent (or NULL) */
>> +	*skbp = nskb;
>> +
>> +	/* trim the original skb to MTU */
>> +	if (!err)
>> +		err = pskb_trim(skb, mtu);
>> +
>> +	if (err) {
>> +		/* Free all frags. Don't bother sending a partial packet we will
>> +		 * never complete.
>> +		 */
>> +		kfree_skb(nskb);
>> +		list_for_each_entry_safe(skb, nskb, &sublist, list) {
>> +			skb_list_del_init(skb);
>> +			kfree_skb(skb);
>> +		}
>> +		return err;
>> +	}
>> +
>> +	/* prepare the initial fragment with an iptfs header */
>> +	iptfs_output_prepare_skb(skb, 0);
>> +
>> +	/* Send all but last fragment, if we fail to send a fragment then free
>> +	 * the rest -- no point in sending a packet that can't be reassembled.
>> +	 */
>> +	list_for_each_entry_safe(skb, nskb, &sublist, list) {
>> +		skb_list_del_init(skb);
>> +		if (!err)
>> +			err = xfrm_output(NULL, skb);
>> +		else
>> +			kfree_skb(skb);
>> +	}
>> +	if (err)
>> +		kfree_skb(*skbp);
>> +	return err;
>> +}
>> +
>> +/**
>> + * iptfs_first_should_copy() - determine if we should copy packet data.
>> + * @first_skb: the first skb in the packet
>> + * @mtu: the MTU.
>> + *
>> + * Determine if we should create subsequent skbs to hold the remaining data from
>> + * a large inner packet by copying the packet data, or cloning the original skb
>> + * and adjusting the offsets.
>> + */
>> +static bool iptfs_first_should_copy(struct sk_buff *first_skb, u32 mtu)
>> +{
>> +	u32 frag_copy_max;
>> +
>> +	/* If we have less than frag_copy_max for remaining packet we copy
>> +	 * those tail bytes as it is more efficient.
>> +	 */
>> +	frag_copy_max = mtu <= IPTFS_FRAG_COPY_MAX ? mtu : IPTFS_FRAG_COPY_MAX;
>
> Likewise, it looks like min could be used here too.

Changed.

Thanks!
Chris.
>
>> +	if ((int)first_skb->len - (int)mtu < (int)frag_copy_max)
>> +		return true;
>> +
>> +	/* If we have non-linear skb just use copy */
>> +	if (skb_is_nonlinear(first_skb))
>> +		return true;
>> +
>> +	/* So we have a simple linear skb, easy to clone and share */
>> +	return false;
>> +}
>
> ...