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packet: fix reserve calculation

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Commit b84bbaf7a6 ("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: b84bbaf7a6 ("packet: in packet_snd start writing at link layer allocation")
Reported-by: Tariq Toukan <[email protected]>
Signed-off-by: Willem de Bruijn <[email protected]>
Acked-by: Soheil Hassas Yeganeh <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
This commit is contained in:
Willem de Bruijn 2018-05-24 18:10:30 -04:00 • committed by David S. Miller
commit 9aad13b087
1 changed files with 1 additions and 1 deletions
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2
net/packet/af_packet.c
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@ -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 */