On 9/11/23 7:41 PM, Stanislav Fomichev wrote:
On 09/11, Daniel Borkmann wrote:
On 9/11/23 7:11 PM, Stanislav Fomichev wrote:
On 09/09, Martin KaFai Lau wrote:
On 9/8/23 2:00 PM, Stanislav Fomichev wrote:
Commit 151e887d8ff9 ("veth: Fixing transmit return status for dropped
packets") exposed the fact that bpf_clone_redirect is capable of
returning raw NET_XMIT_XXX return codes.
This is in the conflict with its UAPI doc which says the following:
"0 on success, or a negative error in case of failure."
Let's wrap dev_queue_xmit's return value (in __bpf_tx_skb) into
net_xmit_errno to make sure we correctly propagate NET_XMIT_DROP
as -ENOBUFS instead of 1.
Note, this is technically breaking existing UAPI where we used to
return 1 and now will do -ENOBUFS. The alternative is to
document that bpf_clone_redirect can return 1 for DROP and 2 for CN.
Reported-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
---
net/core/filter.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/net/core/filter.c b/net/core/filter.c
index a094694899c9..9e297931b02f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2129,6 +2129,9 @@ static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
ret = dev_queue_xmit(skb);
dev_xmit_recursion_dec();
+ if (ret > 0)
+ ret = net_xmit_errno(ret);
I think it is better to have bpf_clone_redirect returning -ENOBUFS instead
of leaking NET_XMIT_XXX to the uapi. The bpf_clone_redirect in the
uapi/bpf.h also mentions
* Return
* 0 on success, or a negative error in case of failure.
If -ENOBUFS is returned in __bpf_tx_skb, should the same be done for
__bpf_rx_skb? and should net_xmit_errno() only be done for
bpf_clone_redirect()? __bpf_{tx,rx}_skb is also used by skb_do_redirect()
which also calls __bpf_redirect_neigh() that returns NET_XMIT_xxx but no
caller seems to care the NET_XMIT_xxx value now.
__bpf_rx_skb seems to only add to backlog and doesn't seem to return any
of the NET_XMIT_xxx. But I might be wrong and haven't looked too deep
into that.
Daniel should know more here. I would wait for Daniel to comment.
Ack, sure!
I think my preference would be to just document it in the helper UAPI, what
Stan was suggesting below:
| Note, this is technically breaking existing UAPI where we used to
| return 1 and now will do -ENOBUFS. The alternative is to
| document that bpf_clone_redirect can return 1 for DROP and 2 for CN.
And then only adjusting the test case.
In this case, would we also need something similar to our
TCP_BPF_<state> changes? Like BUILD_BUG_ON(BPF_NET_XMIT_XXX !=
NET_XMIT_XXX)? Otherwise, we risk more leakage into the UAPI.
Merely documenting doesn't seem enough?
We could probably just mention that a positive, non-zero code indicates
that the skb clone got forwarded to the target netdevice but got dropped
from driver side. This is somewhat also driver dependent e.g. if you look
at dummy which does drop-all, it returns NETDEV_TX_OK. Anything more
specific in the helper doc such as defining BPF_NET_XMIT_* would be more
confusing.
Programs checking for ret < 0 will continue to behave as before. Technically
the bpf_clone_redirect() did its job just that on the veth side things were
dropped. Other drivers such as tun, vrf, ipvlan, bond could already have
returned NET_XMIT_DROP, so technically it's not a new situation where it is
possible. And having a ret > 0 could then also be clearly used to differentiate
that something came from driver side rather than helper side.
For the selftest, may be another option is to use a 28 bytes data_in for the
lwt program redirecting to veth? 14 bytes used by bpf_prog_test_run_skb and
leave 14 bytes for veth_xmit. It seems the original intention of the "veth
ETH_HLEN+1 packet ingress" test is expecting it to succeed also.
IIUC, you're suggesting to pass full ipv4 or ipv6 packet for veth tests
to make them actually succeed with the forwarding, right?
Sure, I can do that. But let's keep this entry with the -NOBUFS as well?
Just for the sake of ensuring that we don't export NET_XMIT_xxx from
uapi.
