Subject: Re: [PATCH v3 1/9] ptp: Add generic ptp v2 header parsing function

From: Russell King - ARM Linux admin <[email-masked]>
Date: Wed, 5 Aug 2020 00:14:29 +0100

On Wed, Aug 05, 2020 at 01:04:31AM +0300, Grygorii Strashko wrote:
> On 05/08/2020 00:44, Russell King - ARM Linux admin wrote:
> > On Wed, Aug 05, 2020 at 12:34:47AM +0300, Grygorii Strashko wrote:
> > > On 05/08/2020 00:07, Russell King - ARM Linux admin wrote:
> > > > On Tue, Aug 04, 2020 at 11:56:12PM +0300, Grygorii Strashko wrote:
> > > > > 
> > > > > 
> > > > > On 31/07/2020 13:06, Kurt Kanzenbach wrote:
> > > > > > On Thu Jul 30 2020, Petr Machata wrote:
> > > > > > > Kurt Kanzenbach <[email-masked]> writes:
> > > > > > > 
> > > > > > > > @@ -107,6 +107,37 @@ unsigned int ptp_classify_raw(const struct sk_buff *skb)
> > > > > > > >     }
> > > > > > > >     EXPORT_SYMBOL_GPL(ptp_classify_raw);
> > > > > > > > +struct ptp_header *ptp_parse_header(struct sk_buff *skb, unsigned int type)
> > > > > > > > +{
> > > > > > > > +	u8 *data = skb_mac_header(skb);
> > > > > > > > +	u8 *ptr = data;
> > > > > > > 
> > > > > > > One of the "data" and "ptr" variables is superfluous.
> > > > > > 
> > > > > > Yeah. Can be shortened to u8 *ptr = skb_mac_header(skb);
> > > > > 
> > > > > Actually usage of skb_mac_header(skb) breaks CPTS RX time-stamping on
> > > > > am571x platform PATCH 6.
> > > > > 
> > > > > The CPSW RX timestamp requested after full packet put in SKB, but
> > > > > before calling eth_type_trans().
> > > > > 
> > > > > So, skb->data pints on Eth header, but skb_mac_header() return garbage.
> > > > > 
> > > > > Below diff fixes it for me.
> > > > 
> > > > However, that's likely to break everyone else.
> > > > 
> > > > For example, anyone calling this from the mii_timestamper rxtstamp()
> > > > method, the skb will have been classified with the MAC header pushed
> > > > and restored, so skb->data points at the network header.
> > > > 
> > > > Your change means that ptp_parse_header() expects the MAC header to
> > > > also be pushed.
> > > > 
> > > > Is it possible to adjust CPTS?
> > > > 
> > > > Looking at:
> > > > drivers/net/ethernet/ti/cpsw.c... yes.
> > > > drivers/net/ethernet/ti/cpsw_new.c... yes.
> > > > drivers/net/ethernet/ti/netcp_core.c... unclear.
> > > > 
> > > > If not, maybe cpts should remain unconverted - I don't see any reason
> > > > to provide a generic function for one user.
> > > > 
> > > 
> > > Could it be an option to pass "u8 *ptr" instead of "const struct sk_buff *skb" as
> > > input parameter to ptp_parse_header()?
> > 
> > It needs to read from the buffer, and in order to do that, it needs to
> > validate that the buffer contains sufficient data.  So, at minimum it
> > needs to be a pointer and size of valid data.
> > 
> > I was thinking about suggesting that as a core function, with a wrapper
> > for the existing interface.
> > 
> 
> Then length can be added.

Actually, it needs more than that, because skb->data..skb->len already
may contain the eth header or may not.

> Otherwise not only CPTS can't benefit from this new API, but also
> drivers like oki-semi/pch_gbe/pch_gbe_main.c -> pch_ptp_match()

Again, this looks like it can be solved easily by swapping the position
of these two calls:

                        pch_rx_timestamp(adapter, skb);

                        skb->protocol = eth_type_trans(skb, netdev);

> or have to two have two APIs (name?).
> 
> ptp_parse_header1(struct sk_buff *skb, unsigned int type)
> {
> 	u8 *data = skb_mac_header(skb);
> 
> ptp_parse_header2(struct sk_buff *skb, unsigned int type)
> {
> 	u8 *data = skb->data;
> 
> everything else is the same.

Actually, I really don't think we want 99% of users doing:

	hdr = ptp_parse_header(skb_mac_header(skb), skb->data, skb->len, type)

or

	hdr = ptp_parse_header(skb_mac_header(skb), skb->data + skb->len, type);

because that is what it will take, and this is starting to look
really very horrid.

So, I repeat my question again: can netcp_core.c be adjusted to
ensure that the skb mac header field is correctly set by calling
eth_type_trans() prior to calling the rx hooks?  The other two
cpts cases look easy to change, and the oki-semi also looks the
same.

Otherwise, the easiest thing may be to just revert the change to
cpts.

-- 
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