[dpdk-dev] [PATCH 1/2] net/tap: add tun support

[Ferruh Yigit]

On 4/23/2018 1:58 PM, Varghese, Vipin wrote:
> Hi Ophir,
> 
> Can you help me with the investigation with the following information?
> 1) The kernel or distro in which the TAP proto flag set breaks the logic?

Hi Vipin,

I guess Ophir's point is not this is broken with some kernels but a valid field
set wrong for tap, perhaps someone can be using a custom kernel module to use
those fields, we can't know it.

Instead of duplicating [rt]x_burst() functions, I suggest creating a variable to
set if this is tun or tap and set pi.proto only for tun, this will lead less
comparison for tap and correct proto value.

> 2) Is the above still valid even after applying the patch ' https://dpdk.org/dev/patchwork/patch/37986/'?

I guess his concern is for tap, that some MAC addresses cause wrong pi.proto,
not for tun which your patch fixes.

> 
> Note: I am testing with 3.13.0, 4.4.0 and 4.13.0.
> 
> Thanks
> Vipin Varghese
> 
>> -----Original Message-----
>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Varghese, Vipin
>> Sent: Saturday, April 21, 2018 8:40 PM
>> To: Ophir Munk <ophirmu at mellanox.com>; dev at dpdk.org;
>> pascal.mazon at 6wind.com; Yigit, Ferruh <ferruh.yigit at intel.com>; Thomas
>> Monjalon <thomas at monjalon.net>; Olga Shern <olgas at mellanox.com>;
>> Shahaf Shuler <shahafs at mellanox.com>
>> Subject: Re: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
>>
>> Hi Ophir,
>>
>> <Snip>
>>
>>> Hi Vipin,
>>> I missed your point:
>>> You claim that TAP should work regardless of any pi.proto values.
>>> Can you confirm that for ALL kernels versions (past and future)?
>>
>> I have tested with 3.13.0 , 4.4.0 with patch fix.
>>
>>>
>>>> -----Original Message-----
>>>> From: Ophir Munk
>>>> Sent: Saturday, April 21, 2018 12:49 AM
>>>> To: Varghese, Vipin <vipin.varghese at intel.com>; dev at dpdk.org;
>>>> pascal.mazon at 6wind.com; Yigit, Ferruh <ferruh.yigit at intel.com>;
>>>> Thomas Monjalon <thomas at monjalon.net>; Olga Shern
>>>> <olgas at mellanox.com>; Shahaf Shuler <shahafs at mellanox.com>
>>>> Subject: RE: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
>>>>
>>>> Hi Vipin,
>>>>
>>>> Please find comments inline.
>>>>
>>>>> -----Original Message-----
>>>>> From: Varghese, Vipin [mailto:vipin.varghese at intel.com]
>>>>> Sent: Friday, April 13, 2018 6:18 AM
>>>>> To: Ophir Munk <ophirmu at mellanox.com>; dev at dpdk.org;
>>>>> pascal.mazon at 6wind.com; Yigit, Ferruh <ferruh.yigit at intel.com>;
>>>>> Thomas Monjalon <thomas at monjalon.net>; Olga Shern
>>>>> <olgas at mellanox.com>; Shahaf Shuler <shahafs at mellanox.com>
>>>>> Subject: RE: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
>>>>>
>>
>> <Snip>
>>
>>>>>> 1. Accessing the first byte here assumes it is the first IP
>>>>>> header byte (layer 3) which is correct for TUN.
>>>>>> For TAP however the first byte belongs to Ethernet destination
>>>>>> address (layer 2).
>>>>>> Please explain how this logic will work for TAP.
>>>>>
>>>>> Based on linux code base '/driver/net/tap.c' and '/driver/net/tun.c'
>>>>> from 3.13. to  4.16,
>>>>>
>>>>> Please find my observation below
>>>>> 1. File: tun.c, function: tun_get_user, check for 'tun->flags &
>>>>> TUN_TYPE_MASK' is done and if non ip is taken counter 'rx_dropped'
>>>>> is updated.
>>>>> 2. File: tap.c, there are no checks for 'tap->flags' for IFF_NO_PI
>>>>> in rx data path. Counter 'rx_dropped' is updated in 'tap_handle_frame'.
>>>>>
>>>>
>>>> I understand that in kernel implementation there is no check for
>>>> tap->flags in file tap.c, however I think there is a bug in dpdk
>>>> tap->rte_eth_tap.c
>>> file.
>>>> Please find below an example which demonstrates this claim.
>>>>
>>>>> Please find my reasoning below
>>>>> 1. First approach was to have separate function for tap and tun TX and
>> RX.
>>>>> But this will introduce code duplication, hence reworked the code
>>>>> as
>>>> above.
>>>>
>>>> I agree. Avoiding code duplication is a good approach.
>>>>
>>>>> 2. During my internal testing assigning dummy value for protocol
>>>>> field in TAP packets, did not show a difference in behaviour. May
>>>>> be there are some specific cases this failing.
>>>>>
>>>>> If there difference in behaviour, can please share the same?
>>>>>
>>>>
>>>> Please consider the following example:
>>>> I am running testpmd with a TAP device, --forward-mode=csum.
>>>> I am injecting a TCP packet, which is forwarded back (mac addresses
>>>> swapped) to the sender.
>>>> Using gdb I set a breakpoint at pmd_tx_burst() in file rte_eth_tap.c
>>>>
>>>> Looking at the following code inside pmd_tx_burst():
>>>>
>>>> 527                 char *buff_data = rte_pktmbuf_mtod(seg, void *);
>>>> 528                 j = (*buff_data & 0xf0);
>>>> 529                 pi.proto = (j == 0x40) ? 0x0008 :
>>>> 530                                 (j == 0x60) ? 0xdd86 : 0x00;
>>>>
>>>> I am printing the first 20 bytes of buff_data in line 527:
>>>>
>>>> (gdb) p/x *(unsigned char *)buff_data at 20
>>>> $3 = {0x0, 0x25, 0x88, 0x10, 0x66, 0x2, 0xf4, 0x52, 0x14, 0x7a,
>>>> 0x59, 0x81, 0x8, 0x0, 0x45, 0x0, 0x4, 0xdf, 0x0, 0x1}
>>>>
>>>> The gdb printout refers to:
>>>> 6 bytes of destination MAC address: 0x0, 0x25, 0x88, 0x10, 0x66, 0x2
>>>> 6 bytes of source MAC address: 0xf4, 0x52, 0x14, 0x7a, 0x59, 0x81
>>>> 2 bytes of Ethernet type: 0x8, 0x0 - (IPv4) IP header starting with 0x45, ...
>>>> which is the byte (0x45) that "j" should have looked at
>>>>
>>>> In the case of TAP - buff_data starts with the destination MAC
>>>> address of the sender (0x0, ...).
>>>> The code in line 528 expects that buff_data would start with an IP
>>>> header protocol (e.g. 0x45), but it is not the case for TAP.
>>>> In my case j=0x0 (line 528) which is harmless (as it ends up with
>>>> setting pi.proto=0x00, which is correct for TAP).
>>>> However, if the sender had an Intel NIC - the destination MAC
>>>> address could have started with:
>>>> $3 = {0x40, 0x25, 0xC2, ...
>>>> Or-
>>>> $3 = {0x64, 0xD4, 0xDA, ...
>>>>
>>>> as 4025C2 and 64D4DA are reserved prefixes for Intel Ethernet MAC
>>>> addresses, see: http://www.coffer.com/mac_find/?string=intel
>>>>
>>>> In this case pi.proto could end up with 0x0008 or 0xdd86 instead of
>>>> 0x0 as expected for TAP.
>>>>
>>>> I hope that this example clarifies the bug I am referring to.
>>>>
>>
>> Thanks for sharing detailed example overview. But as you mentioned this will
>> break ' 4025C2' and ' 64D4DA', This will not solve for the correction patch  '
>> https://dpdk.org/dev/patchwork/patch/37986/'.
>>
>> Only choice left is separate tx_burst for TAP and TUN PMD, as we do not
>> want to check PMD type on each call.
>>
>> Questions:
>> 1) Is this ok to split tx_burst and have redundant code?
>> 2) Does applications transparently send packets coming from Physical NIC to
>> TAP interface? Does not the application Modifies the DEST MAC addr to TAP
>> interface?
>>
>>>>>>
>>>>>> 2. If the first TUN byte contains 0x2X (which is neither IPv4
>>>>>> nor
>>>>>> IPv6) it will end up by setting ip.proto as 0xdd86.
>>>>>> Please explain how this logic will work for non-IP packets in
>>>>>> TUN
>>>>>
>>>>> I see your point. You are correct about this. Thanks for pointing
>>>>> out, may I send correction for this as
>>>>>
>>>>> """
>>>>> -		if (j & (0x40 | 0x60))
>>>>> -			pi.proto = (j == 0x40) ? 0x0008 : 0xdd86;
>>>>> +		pi.proto = (j == 0x40) ? 0x0008 :
>>>>> +					(j == 0x60) ? 0xdd86 :
>>>>> +					0x00;
>>>>> """