Re: [PATCH net-next 1/3] net: dsa: don't pass cloned skb's to drivers xmit function

[Christian Eggers <ceggers@arri.de>]

Hi Vladimir,

On Saturday, 17 October 2020, 02:48:16 CEST, Vladimir Oltean wrote:
> On Fri, Oct 16, 2020 at 10:02:24PM +0200, Christian Eggers wrote:
> > Ensure that the skb is not cloned and has enough tail room for the tail
> > tag. This code will be removed from the drivers in the next commits.
> > 
> > Signed-off-by: Christian Eggers <ceggers@arri.de>
> > ---
> 
> Does 1588 work for you using this change, or you haven't finished
> implementing it yet? If you haven't, I would suggest finishing that
> part first.
Yes it does. Just after finishing this topic, I would to sent the patches for
PTP. Maybe I'll do it in parallel, anything but the combination of L2/E2E/SLOB
seems to work.

> The post-reallocation skb looks nothing like the one before.
> 
> Before:
> skb len=68 headroom=2 headlen=68 tailroom=186
> mac=(2,14) net=(16,-1) trans=-1
> shinfo(txflags=1 nr_frags=0 gso(size=0 type=0 segs=0))
> csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
> hash(0x9d6927ec sw=1 l4=0) proto=0x88f7 pkttype=0 iif=0
> dev name=swp2 feat=0x0x0002000000005020
> sk family=17 type=3 proto=0
> 
> After:
> skb len=68 headroom=2 headlen=68 tailroom=186
> mac=(2,16) net=(18,-17) trans=1
> shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
> csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
> hash(0x0 sw=0 l4=0) proto=0x0000 pkttype=0 iif=0
> 
> Notice how you've changed shinfo(txflags), among other things.

I get a similar output when placing the two skb_dump() calls in the current
ksz_common_xmit() code:

[ 5052.662168] old:skb len=58 headroom=2 headlen=58 tailroom=68
[ 5052.662168] mac=(2,14) net=(16,-1) trans=-1
[ 5052.662168] shinfo(txflags=1 nr_frags=0 gso(size=0 type=0 segs=0))
[ 5052.662168] csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
[ 5052.662168] hash(0x0 sw=0 l4=0) proto=0x88f7 pkttype=0 iif=0
[ 5052.676360] old:dev name=lan0 feat=0x0x0002000000005220
[ 5052.679001] old:sk family=17 type=3 proto=0
[ 5052.681140] old:skb linear:   00000000: 01 1b 19 00 00 00 52 d9 a9 5d a1 40 88 f7 01 02
[ 5052.685236] old:skb linear:   00000010: 00 2c 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 5052.689342] old:skb linear:   00000020: 00 00 52 d9 a9 ff fe 5d a1 40 00 01 00 00 01 7f
[ 5052.693418] old:skb linear:   00000030: 00 00 00 00 00 00 00 00 00 00
[ 5052.696843] new:skb len=65 headroom=2 headlen=65 tailroom=61
[ 5052.696843] mac=(2,16) net=(18,-17) trans=1
[ 5052.696843] shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
[ 5052.696843] csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
[ 5052.696843] hash(0x0 sw=0 l4=0) proto=0x0000 pkttype=0 iif=0
[ 5052.711215] new:skb linear:   00000000: 01 1b 19 00 00 00 52 d9 a9 5d a1 40 88 f7 01 02
[ 5052.715305] new:skb linear:   00000010: 00 2c 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 5052.719407] new:skb linear:   00000020: 00 00 52 d9 a9 ff fe 5d a1 40 00 01 00 00 01 7f
[ 5052.723484] new:skb linear:   00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 5052.727587] new:skb linear:   00000040: 00


Note that whilst some skb members differ, the two hexdumps look correct.

> Which proves that you can't just copy&paste whatever you found in
> tag_trailer.c.
I did. tag_trailer and tag_ksz are quite similar here, so I took a combination of them.

> I am not yet sure whether there is any helper that can be used instead
> of this crazy open-coding. Right now, not having tested anything yet, my
> candidates of choice would be pskb_expand_head or __pskb_pull_tail. You
> should probably also try to cater here for the potential reallocation
> done in the skb_cow_head() of non-tail taggers. Which would lean the
> balance towards pskb_expand_head(), I believe.
The "open coding" is from the existing code (which doesn't say that it is
correct). I will investigate why the copied skb is different and whether 
psk_expand_head can do better.

I don't like to touch the non-tail taggers, this is too much out of the scope
of my current work.

> Also, if the result is going to be longer than ~20 lines of code, I
> strongly suggest moving the reallocation to a separate function so you
> don't clutter dsa_slave_xmit.
As Florian requested I'll likely put the code into a separate function in
slave.c and call it from the individual tail-taggers in order not to put 
extra conditionals in dsa_slave_xmit.

regards
Christian