Message-ID: <32FB9CF5-E5BB-4912-B76D-53971C6B6F98@nutanix.com>
Date: Thu, 1 May 2025 01:00:50 +0000
From: Jon Kohler <jon@...anix.com>
To: Stanislav Fomichev <stfomichev@...il.com>
CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jason Wang
	<jasowang@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
        "David S. Miller"
	<davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov
	<ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard
 Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Simon
 Horman <horms@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next v2] xdp: Add helpers for head length, headroom,
 and metadata length



> On Apr 30, 2025, at 5:09 PM, Stanislav Fomichev <stfomichev@...il.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> On 04/30, Jon Kohler wrote:
>> Introduce new XDP helpers:
>> - xdp_headlen: Similar to skb_headlen
>> - xdp_headroom: Similar to skb_headroom
>> - xdp_metadata_len: Similar to skb_metadata_len
>> 
>> Integrate these helpers into tap, tun, and XDP implementation to start.
>> 
>> No functional changes introduced.
>> 
>> Signed-off-by: Jon Kohler <jon@...anix.com>
>> ---
>> v1->v2: Integrate feedback from Willem
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__patchwork.kernel.org_project_netdevbpf_patch_20250430182921.1704021-2D1-2Djon-40nutanix.com_&d=DwIBaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=9pdxzQszX_M0K3gEPeYOyMZZYSkRR8IMvxslS8320Eoctk58y-ELCdZ5iaryF2GH&s=J-ILB7E9VQ_plo0hyjEtzGzjy6G0_o4MMMmmE_z8vvc&e= 
>> 
>> drivers/net/tap.c |  6 +++---
>> drivers/net/tun.c | 12 +++++------
>> include/net/xdp.h | 54 +++++++++++++++++++++++++++++++++++++++++++----
>> net/core/xdp.c    | 12 +++++------
>> 4 files changed, 65 insertions(+), 19 deletions(-)
>> 
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index d4ece538f1b2..a62fbca4b08f 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -1048,7 +1048,7 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
>> struct sk_buff *skb;
>> int err, depth;
>> 
>> - if (unlikely(xdp->data_end - xdp->data < ETH_HLEN)) {
>> + if (unlikely(xdp_headlen(xdp) < ETH_HLEN)) {
>> err = -EINVAL;
>> goto err;
>> }
>> @@ -1062,8 +1062,8 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
>> goto err;
>> }
>> 
>> - skb_reserve(skb, xdp->data - xdp->data_hard_start);
>> - skb_put(skb, xdp->data_end - xdp->data);
>> + skb_reserve(skb, xdp_headroom(xdp));
>> + skb_put(skb, xdp_headlen(xdp));
>> 
>> skb_set_network_header(skb, ETH_HLEN);
>> skb_reset_mac_header(skb);
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 7babd1e9a378..4c47eed71986 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1567,7 +1567,7 @@ static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
>> dev_core_stats_rx_dropped_inc(tun->dev);
>> return err;
>> }
>> - dev_sw_netstats_rx_add(tun->dev, xdp->data_end - xdp->data);
>> + dev_sw_netstats_rx_add(tun->dev, xdp_headlen(xdp));
>> break;
>> case XDP_TX:
>> err = tun_xdp_tx(tun->dev, xdp);
>> @@ -1575,7 +1575,7 @@ static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
>> dev_core_stats_rx_dropped_inc(tun->dev);
>> return err;
>> }
>> - dev_sw_netstats_rx_add(tun->dev, xdp->data_end - xdp->data);
>> + dev_sw_netstats_rx_add(tun->dev, xdp_headlen(xdp));
>> break;
>> case XDP_PASS:
>> break;
>> @@ -2355,7 +2355,7 @@ static int tun_xdp_one(struct tun_struct *tun,
>>       struct xdp_buff *xdp, int *flush,
>>       struct tun_page *tpage)
>> {
>> - unsigned int datasize = xdp->data_end - xdp->data;
>> + unsigned int datasize = xdp_headlen(xdp);
>> struct tun_xdp_hdr *hdr = xdp->data_hard_start;
>> struct virtio_net_hdr *gso = &hdr->gso;
>> struct bpf_prog *xdp_prog;
>> @@ -2415,14 +2415,14 @@ static int tun_xdp_one(struct tun_struct *tun,
>> goto out;
>> }
>> 
>> - skb_reserve(skb, xdp->data - xdp->data_hard_start);
>> - skb_put(skb, xdp->data_end - xdp->data);
>> + skb_reserve(skb, xdp_headroom(xdp));
>> + skb_put(skb, xdp_headlen(xdp));
>> 
>> /* The externally provided xdp_buff may have no metadata support, which
>> * is marked by xdp->data_meta being xdp->data + 1. This will lead to a
>> * metasize of -1 and is the reason why the condition checks for > 0.
>> */
>> - metasize = xdp->data - xdp->data_meta;
>> + metasize = xdp_metadata_len(xdp);
>> if (metasize > 0)
>> skb_metadata_set(skb, metasize);
>> 
>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>> index 48efacbaa35d..044345b18305 100644
>> --- a/include/net/xdp.h
>> +++ b/include/net/xdp.h
>> @@ -151,10 +151,56 @@ xdp_get_shared_info_from_buff(const struct xdp_buff *xdp)
>> return (struct skb_shared_info *)xdp_data_hard_end(xdp);
>> }
>> 
>> +/**
>> + * xdp_headlen - Calculate the length of the data in an XDP buffer
>> + * @xdp: Pointer to the XDP buffer structure
>> + *
>> + * Compute the length of the data contained in the XDP buffer. Does not
>> + * include frags, use xdp_get_buff_len() for that instead.
>> + *
>> + * Analogous to skb_headlen().
>> + *
>> + * Return: The length of the data in the XDP buffer in bytes.
>> + */
>> +static inline unsigned int xdp_headlen(const struct xdp_buff *xdp)
>> +{
>> + return xdp->data_end - xdp->data;
>> +}
>> +
>> +/**
>> + * xdp_headroom - Calculate the headroom available in an XDP buffer
>> + * @xdp: Pointer to the XDP buffer structure
>> + *
>> + * Compute the headroom in an XDP buffer.
>> + *
>> + * Analogous to the skb_headroom().
>> + *
>> + * Return: The size of the headroom in bytes.
>> + */
>> +static inline unsigned int xdp_headroom(const struct xdp_buff *xdp)
>> +{
>> + return xdp->data - xdp->data_hard_start;
>> +}
>> +
>> +/**
>> + * xdp_metadata_len - Calculate the length of metadata in an XDP buffer
>> + * @xdp: Pointer to the XDP buffer structure
>> + *
>> + * Compute the length of the metadata region in an XDP buffer.
>> + *
>> + * Analogous to skb_metadata_len().
>> + *
>> + * Return: The length of the metadata in bytes.
>> + */
>> +static inline unsigned int xdp_metadata_len(const struct xdp_buff *xdp)
> 
> I believe this has to return int, not unsigned int. There are places
> where we do data_meta = data + 1, and the callers check whether
> the result is signed or sunsigned.

The existing SKB APIs are the exact same return type (I copied them 1:1),
but I have a feeling that we’re never intending these values to either A) be
negative and/or B) wrap in strange ways?

> 
>> +{
>> + return xdp->data - xdp->data_meta;
>> +}
>> +
>> static __always_inline unsigned int
>> xdp_get_buff_len(const struct xdp_buff *xdp)
>> {
>> - unsigned int len = xdp->data_end - xdp->data;
>> + unsigned int len = xdp_headlen(xdp);
>> const struct skb_shared_info *sinfo;
>> 
>> if (likely(!xdp_buff_has_frags(xdp)))
>> @@ -364,8 +410,8 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
>> int metasize, headroom;

Said another way, perhaps this should be unsigned?

>> 
>> /* Assure headroom is available for storing info */
>> - headroom = xdp->data - xdp->data_hard_start;
>> - metasize = xdp->data - xdp->data_meta;
>> + headroom = xdp_headroom(xdp);
>> + metasize = xdp_metadata_len(xdp);
>> metasize = metasize > 0 ? metasize : 0;
> 
> ^^ like here

Look across the tree, seems like more are unsigned than signed

These ones use unsigned:
xdp_convert_zc_to_xdp_frame
veth_xdp_rcv_skb
xsk_construct_skb
bnxt_copy_xdp
i40e_build_skb
i40e_construct_skb_zc
ice_build_skb (this is u8)
ice_construct_skb_zc
igb_build_skb
igb_construct_skb_zc
igc_build_skb
igc_construct_skb
igc_construct_skb_zc
ixgbe_build_skb
ixgbe_construct_skb_zc
ixgbevf_build_skb
mvneta_swbm_build_skb
mlx5e_xsk_construct_skb
mana_build_skb
stmmac_construct_skb_zc
bpf_prog_run_generic_xdp
xdp_get_metalen

These ones are regular int:
xdp_build_skb_from_buff
xdp_build_skb_from_zc
xdp_update_frame_from_buff
tun_xdp_one
build_skb_from_xdp_buff

Perhaps a separate patch to convert the regulars to unsigned,
thoughts?

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