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gnrc_ipv6: remove obsolete and harmful reception code

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When reworking the reception of IPv6 packets I reset a previously set
`ipv6` snip as follows  when the IPv6 extension handler returns a
packet (see first hunk of this commit):

```C
ipv6 = pkt->next->next
```

With `gnrc_ipv6_ext` this makes *somewhat* sense, `pkt->next` was
previously equal to `ipv6` and after the function call `pkt->next`
is the marked extension header, while `pkt->next->next` is the IPv6
header. However, since `ipv6` is already write-protected i.e.
`ipv6->users == 1` (see ll. 665-675), any additional call of
`gnrc_pktbuf_start_write()` [won't][start-write-doc] duplicate the
packet. In fact, the only `gnrc_pktbuf_start_write()` in
`gnrc_ipv6_ext` is used to send the *result* to the subscribers of that
extension header type, leaving the original packet unchanged for the
caller. As such `ipv6` remains the pointer to the IPv6 header whether
we set it in the line above or not. So we actually don't need that
line.

However, the extension header handling also returns a packet when
`gnrc_ipv6_ext` is not compiled in. In that case it is just a dummy
define that returns the packet you give provide it which means that
this still holds true: `pkt->next == ipv6`.
So setting `ipv6` in this case is actually harmful, as `ipv6` now
points to the NETIF header [following the IPv6 header][pkt-structure]
in the packet and this causes the `user` counter of that NETIF header
`hdr` to be decremented if `hdr->users > 1` in the write-protection I
removed in hunk 2 of this commit:

```C
/* pkt might not be writable yet, if header was given above */
ipv6 = gnrc_pktbuf_start_write(ipv6);
if (ipv6 == NULL) {
    DEBUG("ipv6: unable to get write access to packet: dropping it\n");
    gnrc_pktbuf_release(pkt);
    return;
}
```

But as we already established, `ipv6->users` is already 1, so we don't
actually need the write protection here either.

Since the packet stays unchanged after the `ipv6` snip, we also don't
need to re-search for `netif_hdr` after the other two lines are
removed.

[start-write-doc]: https://doc.riot-os.org/group__net__gnrc__pktbuf.html#ga640418467294ae3d408c109ab27bd617
[pkt-structure]: https://doc.riot-os.org/group__net__gnrc__pkt.html#ga278e783e56a5ee6f1bd7b81077ed82a7
This commit is contained in:
Martine S. Lenders 2019-06-22 01:26:37 +02:00
parent b8269316e6
commit ea449f3f9e
1 changed files with 1 additions and 13 deletions
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14
sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c
View File

@ -724,10 +724,7 @@ static void _receive(gnrc_pktsnip_t *pkt)
ipv6_addr_to_str(addr_str, &(hdr->dst), sizeof(addr_str)),
first_nh, byteorder_ntohs(hdr->len));
if ((pkt = gnrc_ipv6_ext_process_hopopt(pkt, &first_nh)) != NULL) {
ipv6 = pkt->next->next;
}
else {
if ((pkt = gnrc_ipv6_ext_process_hopopt(pkt, &first_nh)) == NULL) {
DEBUG("ipv6: packet's extension header was errorneous or packet was "
"consumed due to it\n");
return;
@ -762,16 +759,7 @@ static void _receive(gnrc_pktsnip_t *pkt)
else if (--(hdr->hl) > 0) { /* drop packets that *reach* Hop Limit 0 */
DEBUG("ipv6: forward packet to next hop\n");
/* pkt might not be writable yet, if header was given above */
ipv6 = gnrc_pktbuf_start_write(ipv6);
if (ipv6 == NULL) {
DEBUG("ipv6: unable to get write access to packet: dropping it\n");
gnrc_pktbuf_release(pkt);
return;
}
/* remove L2 headers around IPV6 */
netif_hdr = gnrc_pktsnip_search_type(pkt, GNRC_NETTYPE_NETIF);
if (netif_hdr != NULL) {
gnrc_pktbuf_remove_snip(pkt, netif_hdr);
}