Re: [PATCH RFC v4 bpf-next 09/11] tun: Support xdp in the Tx path for xdp_frames

[David Ahern <dahern@digitalocean.com>]

On 3/3/20 3:40 AM, Jesper Dangaard Brouer wrote:
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index dcae6521a39d..d3fc7e921c85 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1359,10 +1359,50 @@ static void __tun_xdp_flush_tfile(struct tun_file *tfile)
>>  	tfile->socket.sk->sk_data_ready(tfile->socket.sk);
>>  }
>>  
>> +static u32 tun_do_xdp_tx(struct tun_struct *tun, struct tun_file *tfile,
>> +			 struct xdp_frame *frame, struct xdp_txq_info *txq)
>> +{
>> +	struct bpf_prog *xdp_prog;
>> +	u32 act = XDP_PASS;
>> +
>> +	xdp_prog = rcu_dereference(tun->xdp_egress_prog);
>> +	if (xdp_prog) {
>> +		struct xdp_buff xdp;
>> +
>> +		xdp.data_hard_start = frame->data - frame->headroom;
> 
> This is correct, only because frame->headroom have been reduced with
> sizeof(*xdp_frame), as we want to avoid that the BPF-prog have access
> to xdp_frame memory.  Remember that memory storing xdp_frame in located
> in the top of the payload/page.
> 
> 
>> +		xdp.data = frame->data;
>> +		xdp.data_end = xdp.data + frame->len;
>> +		xdp_set_data_meta_invalid(&xdp);
>> +		xdp.txq = txq;
>> +
>> +		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> 
> The BPF-prog can change/adjust headroom and tailroom (tail only shrink,
> but I'm working on extending this).  Thus, you need to adjust the
> xdp_frame accordingly afterwards.

Why do I need to make any adjustments beyond what is done by
bpf_xdp_adjust_head and bpf_xdp_adjust_tail?

The frame is on its way out, so the stack will not see the frame after
any head or tail changes. (REDIRECT is not supported, only PASS or DROP)