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

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Message-ID: <m2r0brchfr.fsf@chopps.org>
Date: Wed, 17 Jul 2024 23:56:58 -0700
From: Christian Hopps <chopps@...pps.org>
To: Simon Horman <horms@...nel.org>
Cc: Christian Hopps <chopps@...pps.org>, Steffen Klassert
 <steffen.klassert@...unet.com>, netdev@...r.kernel.org, Christian Hopps
 <chopps@...n.net>, devel@...ux-ipsec.org
Subject: Re: [devel-ipsec] [PATCH ipsec-next v5 11/17] xfrm: iptfs: add
 fragmenting of larger than MTU user packets


Christian Hopps <chopps@...pps.org> writes:

> [[PGP Signed Part:Good signature from 2E1D830ED7B83025 Christian Hopps <chopps@...il.com> (trust ultimate) created at 2024-07-17T23:02:33-0700 using RSA]]
>
> Simon Horman via Devel <devel@...ux-ipsec.org> writes:
>
>> On Sun, Jul 14, 2024 at 04:22:39PM -0400, Christian Hopps wrote:
>>> From: Christian Hopps <chopps@...n.net>
>>>
>>> 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 <chopps@...n.net>
>>> ---
>>>  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.

Moved to the later tracepoint layered commit.

Thanks,
Chris.


>
>>
>>> +	}
>>> +	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;
>>> +}
>>
>> ...
>
> [[End of PGP Signed Part]]

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