netdev - [PATCH RFC v4 4/9] tap: Pad virtio header with zero

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Message-Id: <20240924-rss-v4-4-84e932ec0e6c@daynix.com>
Date: Tue, 24 Sep 2024 11:01:09 +0200
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, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: [PATCH RFC v4 4/9] tap: Pad virtio header with zero

tap used to simply advance iov_iter when it needs to pad virtio header,
which leaves the garbage in the buffer as is. This is especially
problematic when tap starts to allow enabling the hash reporting
feature; even if the feature is enabled, the packet may lack a hash
value and may contain a hole in the virtio header because the packet
arrived before the feature gets enabled or does not contain the
header fields to be hashed. If the hole is not filled with zero, it is
impossible to tell if the packet lacks a hash value.

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