netdev - [PATCH RFC v3 5/9] tun: Pad virtio header with zero

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Message-Id: <20240915-rss-v3-5-c630015db082@daynix.com>
Date: Sun, 15 Sep 2024 10:17:44 +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>, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: [PATCH RFC v3 5/9] tun: Pad virtio header with zero

tun used to simply advance iov_iter when it needs to pad virtio header.
This leaves the garbage in the buffer as is and prevents telling if the
header is padded or contains some real data.

In theory, a user of tun can fill the buffer with zero before calling
read() to avoid such a problem, but leaving the garbage in the buffer is
awkward anyway so fill the buffer in tun.

Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
---
 drivers/net/tun.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 1d06c560c5e6..9d93ab9ee58f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2073,7 +2073,7 @@ static ssize_t tun_put_user_xdp(struct tun_struct *tun,
 		if (unlikely(copy_to_iter(&gso, sizeof(gso), iter) !=
 			     sizeof(gso)))
 			return -EFAULT;
-		iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
+		iov_iter_zero(vnet_hdr_sz - sizeof(gso), iter);
 	}

 	ret = copy_to_iter(xdp_frame->data, size, iter) + vnet_hdr_sz;
@@ -2146,7 +2146,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 		if (copy_to_iter(&gso, sizeof(gso), iter) != sizeof(gso))
 			return -EFAULT;

-		iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
+		iov_iter_zero(vnet_hdr_sz - sizeof(gso), iter);
 	}

 	if (vlan_hlen) {

-- 
2.46.0