netdev - [PATCH net v3 7/9] tap: Avoid double-tracking iov

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Message-Id: <20250116-tun-v3-7-c6b2871e97f7@daynix.com>
Date: Thu, 16 Jan 2025 17:08:10 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Jonathan Corbet <corbet@....net>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>, 
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org, kvm@...r.kernel.org, 
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
 Yuri Benditovich <yuri.benditovich@...nix.com>, 
 Andrew Melnychenko <andrew@...nix.com>, 
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com, 
 devel@...nix.com, Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: [PATCH net v3 7/9] tap: Avoid double-tracking iov_iter length
 changes

tap_get_user() used to track the length of iov_iter with another
variable. We can use iov_iter_count() to determine the current length
to avoid such chores.

Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
---
 drivers/net/tap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index 5aa41d5f7765..061c2f27dfc8 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -641,7 +641,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
 	struct sk_buff *skb;
 	struct tap_dev *tap;
 	unsigned long total_len = iov_iter_count(from);
-	unsigned long len = total_len;
+	unsigned long len;
 	int err;
 	struct virtio_net_hdr vnet_hdr = { 0 };
 	int vnet_hdr_len = 0;
@@ -655,9 +655,8 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
 		vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);

 		err = -EINVAL;
-		if (len < vnet_hdr_len)
+		if (iov_iter_count(from) < vnet_hdr_len)
 			goto err;
-		len -= vnet_hdr_len;

 		err = -EFAULT;
 		if (!copy_from_iter_full(&vnet_hdr, sizeof(vnet_hdr), from))
@@ -671,10 +670,12 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
 				 tap16_to_cpu(q, vnet_hdr.csum_start) +
 				 tap16_to_cpu(q, vnet_hdr.csum_offset) + 2);
 		err = -EINVAL;
-		if (tap16_to_cpu(q, vnet_hdr.hdr_len) > len)
+		if (tap16_to_cpu(q, vnet_hdr.hdr_len) > iov_iter_count(from))
 			goto err;
 	}

+	len = iov_iter_count(from);
+
 	err = -EINVAL;
 	if (unlikely(len < ETH_HLEN))
 		goto err;

-- 
2.47.1