Re: [PATCH bpf v2] xdp: Fix handling of devmap in generic XDP

On 2018/06/14 17:49, Jesper Dangaard Brouer wrote:

On Thu, 14 Jun 2018 11:07:42 +0900
Toshiaki Makita [off-list ref] wrote:

quoted

Commit 67f29e07e131 ("bpf: devmap introduce dev_map_enqueue") changed
the return value type of __devmap_lookup_elem() from struct net_device *
to struct bpf_dtab_netdev * but forgot to modify generic XDP code
accordingly.
Thus generic XDP incorrectly used struct bpf_dtab_netdev where struct
net_device is expected, then skb->dev was set to invalid value.

v2:
- Fix compiler warning without CONFIG_BPF_SYSCALL.

Fixes: 67f29e07e131 ("bpf: devmap introduce dev_map_enqueue")
Signed-off-by: Toshiaki Makita <redacted>

Thanks for catching this!

Acked-by: Jesper Dangaard Brouer <redacted>

Notice, that the current code works (and does not crash), but it is
pure luck.  Because struct bpf_dtab_netdev happen to have the
net_device as the first member.

struct bpf_dtab_netdev {
	struct net_device *dev; /* must be first member, due to tracepoint */
	struct bpf_dtab *dtab;
	unsigned int bit;
	struct xdp_bulk_queue __percpu *bulkq;
	struct rcu_head rcu;
};

Actually no, the current code does not work and can crash, because we
need to dereference the pointer, i.e. need fwd->dev (IOW *fwd) not fwd.

-- 
Toshiaki Makita