Allowing space for packet headers in Wintun Tx/Rx

[Daniel Lenski]

On Wed, Apr 7, 2021 at 4:49 AM David Woodhouse <dwmw2 at infradead.org> wrote:
> If WintunSendPacket took an additional 'offset' argument to disregard a
> certain number of bytes at the beginning of the buffer, that would
> probably suffice. Or is it possible to simply add to the pointer
> returned by WintunAllocateSendPacket()?

To expand on this possibility a little bit, I had proposed kludging a
“shift” into the allocation for the outgoing packet:

    /* Always use this instead for an outgoing packet, instead of
     * malloc(sizeof(struct pkt) + payload_len */
    BYTE *tun_pkt = WintunAllocateSendPacket(
        vpninfo->wintun_session,
        paylod_len                 /* packet payload size */
        + sizeof(struct pkt)     /* OpenConnect's internal packet header size */
    );

    /* Then after we build and populate the outgoing packet, just tell
     * Wintun to send from an offset that's NOT at the beginning of the
     * buffer it allocated for us. No memcpy! */
    WintunSendPacket(vpninfo->wintun_session, tun_pkt + sizeof(struct pkt));

The concern here is that Wintun may not have been written with this
possibility in mind, and might not always like sending from an address
other than the exact start of a buffer it's allocated for us.

Thanks,
Dan