netdev - [PATCH net] packet: fix reserve calculation

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Message-Id: <20180524221030.158150-1-willemdebruijn.kernel@gmail.com>
Date:   Thu, 24 May 2018 18:10:30 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net] packet: fix reserve calculation

From: Willem de Bruijn <willemb@...gle.com>

Commit b84bbaf7a6c8 ("packet: in packet_snd start writing at link
layer allocation") ensures that packet_snd always starts writing
the link layer header in reserved headroom allocated for this
purpose.

This is needed because packets may be shorter than hard_header_len,
in which case the space up to hard_header_len may be zeroed. But
that necessary padding is not accounted for in skb->len.

The fix, however, is buggy. It calls skb_push, which grows skb->len
when moving skb->data back. But in this case packet length should not
change.

Instead, call skb_reserve, which moves both skb->data and skb->tail
back, without changing length.

Fixes: b84bbaf7a6c8 ("packet: in packet_snd start writing at link layer allocation")
Reported-by: Tariq Toukan <tariqt@...lanox.com>
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
---
 net/packet/af_packet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index e9422fe45179..acb7b86574cd 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2911,7 +2911,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 		if (unlikely(offset < 0))
 			goto out_free;
 	} else if (reserve) {
-		skb_push(skb, reserve);
+		skb_reserve(skb, -reserve);
 	}

 	/* Returns -EFAULT on error */
-- 
2.17.0.921.gf22659ad46-goog