xsk: remove @first_frag from xsk_build_skb()

Instead of using auxiliary boolean that tracks if we are at first frag
when gathering all elements of skb, same functionality can be achieved
with checking if skb_shared_info::nr_frags is 0.

Remove @first_frag but be careful around xsk_build_skb_zerocopy() and
NULL the skb pointer when it failed so that common error path does not
incorrectly interpret it during decision whether to call kfree_skb().

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925160009.2474816-3-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 01f258894faebb968bb2ae8509173674d37941bf..f7e0d254a7237c175d2d3df4a5c597e46498ea72 100644 (file)
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -730,13 +730,13 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
        struct xsk_tx_metadata *meta = NULL;
        struct net_device *dev = xs->dev;
        struct sk_buff *skb = xs->skb;
-       bool first_frag = false;
        int err;
 
        if (dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
                skb = xsk_build_skb_zerocopy(xs, desc);
                if (IS_ERR(skb)) {
                        err = PTR_ERR(skb);
+                       skb = NULL;
                        goto free_err;
                }
        } else {
@@ -747,8 +747,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
                len = desc->len;
 
                if (!skb) {
-                       first_frag = true;
-
                        hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(dev->needed_headroom));
                        tr = dev->needed_tailroom;
                        skb = sock_alloc_send_skb(&xs->sk, hr + len + tr, 1, &err);
@@ -798,7 +796,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
                        list_add_tail(&xsk_addr->addr_node, &XSKCB(skb)->addrs_list);
                }
 
-               if (first_frag && desc->options & XDP_TX_METADATA) {
+               if (!skb_shinfo(skb)->nr_frags && desc->options & XDP_TX_METADATA) {
                        if (unlikely(xs->pool->tx_metadata_len == 0)) {
                                err = -EINVAL;
                                goto free_err;
@@ -840,7 +838,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
        return skb;
 
 free_err:
-       if (first_frag && skb)
+       if (skb && !skb_shinfo(skb)->nr_frags)
                kfree_skb(skb);
 
        if (err == -EOVERFLOW) {