Re: [PATCH] net: Reset skb to network header in neigh_hh_output

On Wed, 2016-10-26 at 18:53 +0200, Abdelrhman Ahmed wrote:

I think it's at the right place as the current one is a little different from the
commit e1f165032c8bade3a6bdf546f8faf61fda4dd01c.

In the next lines, skb_push is called after copying the hardware header and there
is no change to the data pointer inside the retry loop. We only need to reset
before this loop.

__skb_pull(skb, skb_network_offset(skb));

do {
	seq = read_seqbegin(&hh->hh_lock);
	hh_len = hh->hh_len;
	if (likely(hh_len <= HH_DATA_MOD)) {
		/* this is inlined by gcc */
		memcpy(skb->data - HH_DATA_MOD, hh->hh_data, HH_DATA_MOD);
	} else {
		int hh_alen = HH_DATA_ALIGN(hh_len);

		memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
	}
} while (read_seqretry(&hh->hh_lock, seq));

skb_push(skb, hh_len);

In the commit e1f165032c8bade3a6bdf546f8faf61fda4dd01c, dev_hard_header which
calls create method for adding hardware header (uses skb_push) so it was
required to reset to network header in the beginning of the retry loop.

Right you are, thanks for the clarification !

Back to the cause of the bug then.

If netfilter is the only case this might be needed, can't this be fixed
in netfilter ?

neigh_hh_output() is in fast path, it is quite annoying adding this
operation.