On 16/02/2026 17:13, Sabrina Dubroca wrote:
2026-02-05, 16:10:36 +0100, Ralf Lici wrote:
When processing TCP stream data in ovpn_tcp_recv, we receive large
cloned skbs from __strp_rcv that may contain multiple coalesced packets.
The current implementation has two bugs:
1. Header offset overflow: Using pskb_pull with large offsets on
coalesced skbs causes skb->data - skb->head to exceed the u16 storage
of skb->network_header. This causes skb_reset_network_header to fail
on the inner decapsulated packet, resulting in packet drops.
2. Unaligned protocol headers: Extracting packets from arbitrary
positions within the coalesced TCP stream provides no alignment
guarantees for the packet data causing performance penalties on
architectures without efficient unaligned access. Additionally,
openvpn's 2-byte length prefix on TCP packets causes the subsequent
4-byte opcode and packet ID fields to be inherently misaligned.
Fix both issues by allocating a new skb for each openvpn packet and
using skb_copy_bits to extract only the packet content into the new
buffer, skipping the 2-byte length prefix. Also, check the length before
invoking the function that performs the allocation to avoid creating an
invalid skb.
If the packet has to be forwarded to userspace the 2-byte prefix can be
pushed to the head safely, without misalignment.
As a side effect, this approach also avoids the expensive linearization
that pskb_pull triggers on cloned skbs with page fragments. In testing,
this resulted in TCP throughput improvements of up to 74%.
Fixes: 11851cbd60ea ("ovpn: implement TCP transport")
Signed-off-by: Ralf Lici <[email protected]>
---
Changes since v1:
- updated the __skb_push usage for consistency with similar operations,
such as in ovpn_tcp_send_skb
Reviewed-by: Sabrina Dubroca <[email protected]>
Thanks a lot, Sabrina!
I was particularly excited for this patch due to the speed boost given
to TCP.
Sorry for the delay.
No worries - it seems Jakub et al are still busy with the merge window -
the other patches are still waiting in the line too.
Will submit this as well ASAP!
Regards,
--
Antonio Quartulli
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