[PATCH net-next 04/11] tuntap: simplify error handling in tun_build_skb()

[Jason Wang <jasowang@xxxxxxxxxx>]

There's no need to duplicate page get logic in each action. So this
patch tries to get page and calculate the offset before processing XDP
actions, and undo them when meet errors (we don't care the performance
on errors). This will be used for factoring out XDP logic.

Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
 drivers/net/tun.c | 37 ++++++++++++++++---------------------
 1 file changed, 16 insertions(+), 21 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 372caf7d67d9..f8cdcfa392c3 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1642,7 +1642,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 				     int len, int *skb_xdp)
 {
 	struct page_frag *alloc_frag = &current->task_frag;
-	struct sk_buff *skb;
+	struct sk_buff *skb = NULL;
 	struct bpf_prog *xdp_prog;
 	int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 	unsigned int delta = 0;
@@ -1668,6 +1668,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 	if (copied != len)
 		return ERR_PTR(-EFAULT);
 
+	get_page(alloc_frag->page);
+	alloc_frag->offset += buflen;
+
 	/* There's a small window that XDP may be set after the check
 	 * of xdp_prog above, this should be rare and for simplicity
 	 * we do XDP on skb in case the headroom is not enough.
@@ -1695,23 +1698,15 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 
 		switch (act) {
 		case XDP_REDIRECT:
-			get_page(alloc_frag->page);
-			alloc_frag->offset += buflen;
 			err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
 			xdp_do_flush_map();
 			if (err)
-				goto err_redirect;
-			rcu_read_unlock();
-			local_bh_enable();
-			return NULL;
+				goto err_xdp;
+			goto out;
 		case XDP_TX:
-			get_page(alloc_frag->page);
-			alloc_frag->offset += buflen;
 			if (tun_xdp_tx(tun->dev, &xdp) < 0)
-				goto err_redirect;
-			rcu_read_unlock();
-			local_bh_enable();
-			return NULL;
+				goto err_xdp;
+			goto out;
 		case XDP_PASS:
 			delta = orig_data - xdp.data;
 			len = xdp.data_end - xdp.data;
@@ -1730,23 +1725,23 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 	local_bh_enable();
 
 	skb = build_skb(buf, buflen);
-	if (!skb)
-		return ERR_PTR(-ENOMEM);
+	if (!skb) {
+		skb = ERR_PTR(-ENOMEM);
+		goto out;
+	}
 
 	skb_reserve(skb, pad - delta);
 	skb_put(skb, len);
-	get_page(alloc_frag->page);
-	alloc_frag->offset += buflen;
 
 	return skb;
 
-err_redirect:
-	put_page(alloc_frag->page);
 err_xdp:
+	alloc_frag->offset -= buflen;
+	put_page(alloc_frag->page);
+out:
 	rcu_read_unlock();
 	local_bh_enable();
-	this_cpu_inc(tun->pcpu_stats->rx_dropped);
-	return NULL;
+	return skb;
 }
 
 /* Get packet from user space buffer */
-- 
2.17.1