netdev - [PATCH RFC v3 4/9] tap: Pad virtio header with zero

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Message-Id: <20240915-rss-v3-4-c630015db082@daynix.com>
Date: Sun, 15 Sep 2024 10:17:43 +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 4/9] tap: Pad virtio header with zero

tap 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 tap 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 tap.

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

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index 77574f7a3bd4..ba044302ccc6 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -813,7 +813,7 @@ static ssize_t tap_put_user(struct tap_queue *q,
 		    sizeof(vnet_hdr))
 			return -EFAULT;

-		iov_iter_advance(iter, vnet_hdr_len - sizeof(vnet_hdr));
+		iov_iter_zero(vnet_hdr_len - sizeof(vnet_hdr), iter);
 	}
 	total = vnet_hdr_len;
 	total += skb->len;

-- 
2.46.0