netdev - Re: [PATCH net v2] net: handle ARPHRD_PPP in dev_is_mac_header

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Date: Thu, 3 Aug 2023 11:37:00 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Guillaume Nault <gnault@...hat.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Eric Dumazet <edumazet@...gle.com>, Daniel Borkmann <daniel@...earbox.net>,
 Alexei Starovoitov <ast@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
 bpf@...r.kernel.org, stable@...r.kernel.org,
 Siwar Zitouni <siwar.zitouni@...nd.com>
Subject: Re: [PATCH net v2] net: handle ARPHRD_PPP in dev_is_mac_header_xmit()

Le 03/08/2023 à 10:46, Guillaume Nault a écrit :
> On Wed, Aug 02, 2023 at 02:21:06PM +0200, Nicolas Dichtel wrote:
>> This kind of interface doesn't have a mac header.
> 
> Well, PPP does have a link layer header.
It has a link layer, but not an ethernet header.

> Do you instead mean that PPP automatically adds it?
> 
>> This patch fixes bpf_redirect() to a ppp interface.
> 
> Can you give more details? Which kind of packets are you trying to
> redirect to PPP interfaces?
My ebpf program redirect an IP packet (eth / ip) from a physical ethernet device
at ingress to a ppp device at egress. In this case, the bpf_redirect() function
should remove the ethernet header from the packet before calling the xmit ppp
function. Before my patch, the ppp xmit function adds a ppp header (protocol IP
/ 0x0021) before the ethernet header. It results to a corrupted packet. After
the patch, the ppp xmit function encapsulates the IP packet, as expected.

> 
> To me this looks like a hack to work around the fact that
> ppp_start_xmit() automatically adds a PPP header. Maybe that's the
It's not an hack, it works like for other kind of devices managed by the
function bpf_redirect() / dev_is_mac_header_xmit().

Hope it's more clear.


Regards,
Nicolas

> best we can do given the current state of ppp_generic.c, but the
> commit message should be clear about what the real problem is and
> why the patch takes this approach to fix or work around it.
> 
>> CC: stable@...r.kernel.org
>> Fixes: 27b29f63058d ("bpf: add bpf_redirect() helper")
>> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
>> Tested-by: Siwar Zitouni <siwar.zitouni@...nd.com>
>> ---
>>
>> v1 -> v2:
>>  - I forgot the 'Tested-by' tag in the v1 :/
>>
>>  include/linux/if_arp.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/include/linux/if_arp.h b/include/linux/if_arp.h
>> index 1ed52441972f..8efbe29a6f0c 100644
>> --- a/include/linux/if_arp.h
>> +++ b/include/linux/if_arp.h
>> @@ -53,6 +53,7 @@ static inline bool dev_is_mac_header_xmit(const struct net_device *dev)
>>  	case ARPHRD_NONE:
>>  	case ARPHRD_RAWIP:
>>  	case ARPHRD_PIMREG:
>> +	case ARPHRD_PPP:
>>  		return false;
>>  	default:
>>  		return true;
>> -- 
>> 2.39.2
>>
>>
>