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Date:   Wed, 16 Dec 2020 09:28:31 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Dmytro Shytyi <dmytro@...tyi.net>
Cc:     "David Miller" <davem@...emloft.net>,
        "yoshfuji" <yoshfuji@...ux-ipv6.org>,
        "kuznet" <kuznet@....inr.ac.ru>,
        "liuhangbin" <liuhangbin@...il.com>,
        "netdev" <netdev@...r.kernel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next V8] net: Variable SLAAC: SLAAC with prefixes of
 arbitrary length in PIO

On Wed, 16 Dec 2020 15:01:33 +0100 Dmytro Shytyi wrote:
> Hello David,
> 
> Thank you for your comment. 
> Asnwers in-line.
> 
> Take care,
>                               
> Dmytro SHYTYI
> 
> 
> ---- On Wed, 16 Dec 2020 01:00:49 +0100 David Miller <davem@...emloft.net> wrote ----
> 
>  > From: Dmytro Shytyi <dmytro@...tyi.net> 
>  > Date: Wed, 09 Dec 2020 04:27:54 +0100 
>  >    
>  > > Variable SLAAC [Can be activated via sysctl]: 
>  > > SLAAC with prefixes of arbitrary length in PIO (randomly 
>  > > generated hostID or stable privacy + privacy extensions). 
>  > > The main problem is that SLAAC RA or PD allocates a /64 by the Wireless 
>  > > carrier 4G, 5G to a mobile hotspot, however segmentation of the /64 via 
>  > > SLAAC is required so that downstream interfaces can be further subnetted. 
>  > > Example: uCPE device (4G + WI-FI enabled) receives /64 via Wireless, and 
>  > > assigns /72 to VNF-Firewall, /72 to WIFI, /72 to VNF-Router, /72 to 
>  > > Load-Balancer and /72 to wired connected devices. 
>  > > IETF document that defines problem statement: 
>  > > draft-mishra-v6ops-variable-slaac-problem-stmt 
>  > > IETF document that specifies variable slaac: 
>  > > draft-mishra-6man-variable-slaac 
>  > > 
>  > > Signed-off-by: Dmytro Shytyi <dmytro@...tyi.net> 
>  > > --- 
>  > > diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h 
>  > > index dda61d150a13..67ca3925463c 100644 
>  > > --- a/include/linux/ipv6.h 
>  > > +++ b/include/linux/ipv6.h 
>  > > @@ -75,6 +75,7 @@ struct ipv6_devconf { 
>  > >      __s32        disable_policy; 
>  > >      __s32           ndisc_tclass; 
>  > >      __s32        rpl_seg_enabled; 
>  > > +    __s32        variable_slaac; 
>  > > 
>  > >      struct ctl_table_header *sysctl_header; 
>  > >  }; 
>  > > diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h 
>  > > index 13e8751bf24a..f2af4f9fba2d 100644 
>  > > --- a/include/uapi/linux/ipv6.h 
>  > > +++ b/include/uapi/linux/ipv6.h 
>  > > @@ -189,7 +189,8 @@ enum { 
>  > >      DEVCONF_ACCEPT_RA_RT_INFO_MIN_PLEN, 
>  > >      DEVCONF_NDISC_TCLASS, 
>  > >      DEVCONF_RPL_SEG_ENABLED, 
>  > > -    DEVCONF_MAX 
>  > > +    DEVCONF_MAX, 
>  > > +    DEVCONF_VARIABLE_SLAAC 
>  > >  }; 
>  > > 
>  > > 
>  > > diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c 
>  > > index eff2cacd5209..07afe4ce984e 100644 
>  > > --- a/net/ipv6/addrconf.c 
>  > > +++ b/net/ipv6/addrconf.c 
>  > > @@ -236,6 +236,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = { 
>  > >      .addr_gen_mode        = IN6_ADDR_GEN_MODE_EUI64, 
>  > >      .disable_policy        = 0, 
>  > >      .rpl_seg_enabled    = 0, 
>  > > +    .variable_slaac        = 0, 
>  > >  }; 
>  > > 
>  > >  static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = { 
>  > > @@ -291,6 +292,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = { 
>  > >      .addr_gen_mode        = IN6_ADDR_GEN_MODE_EUI64, 
>  > >      .disable_policy        = 0, 
>  > >      .rpl_seg_enabled    = 0, 
>  > > +    .variable_slaac        = 0, 
>  > >  }; 
>  > > 
>  > >  /* Check if link is ready: is it up and is a valid qdisc available */ 
>  > > @@ -1340,9 +1342,15 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block) 
>  > >          goto out; 
>  > >      } 
>  > >      in6_ifa_hold(ifp); 
>  > > -    memcpy(addr.s6_addr, ifp->addr.s6_addr, 8); 
>  > > -    ipv6_gen_rnd_iid(&addr); 
>  > > 
>  > > +    if (ifp->prefix_len == 64) { 
>  > > +        memcpy(addr.s6_addr, ifp->addr.s6_addr, 8); 
>  > > +        ipv6_gen_rnd_iid(&addr); 
>  > > +    } else if (ifp->prefix_len > 0 && ifp->prefix_len <= 128 && 
>  > > +           idev->cnf.variable_slaac) { 
>  > > +        get_random_bytes(addr.s6_addr, 16); 
>  > > +        ipv6_addr_prefix_copy(&addr, &ifp->addr, ifp->prefix_len); 
>  > > +    } 
>  > >      age = (now - ifp->tstamp) / HZ; 
>  > > 
>  > >      regen_advance = idev->cnf.regen_max_retry * 
>  > > @@ -2569,6 +2577,37 @@ static bool is_addr_mode_generate_stable(struct inet6_dev *idev) 
>  > >             idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_RANDOM; 
>  > >  } 
>  > > 
>  > > +static struct inet6_ifaddr *ipv6_cmp_rcvd_prsnt_prfxs(struct inet6_ifaddr *ifp, 
>  > > +                              struct inet6_dev *in6_dev, 
>  > > +                              struct net *net, 
>  > > +                              const struct prefix_info *pinfo) 
>  > > +{ 
>  > > +    struct inet6_ifaddr *result_base = NULL; 
>  > > +    struct inet6_ifaddr *result = NULL; 
>  > > +    bool prfxs_equal; 
>  > > + 
>  > > +    result_base = result;   
>  >  
>  > This is NULL, are you sure you didn't mewan to init this to 'ifp' 
>  >  or similar instead?   
> 
> [Dmytro] I put the entire function to comment below the instructions.
> [Dmytro]:
> +static struct inet6_ifaddr *ipv6_cmp_rcvd_prsnt_prfxs(struct inet6_ifaddr *ifp,
> +                         struct inet6_dev *in6_dev,
> +                         struct net *net,
> +                         const struct prefix_info *pinfo)
> +{
> +    struct inet6_ifaddr *result_base = NULL;
> +    struct inet6_ifaddr *result = NULL;
> +    bool prfxs_equal;
> +
> +    result_base = result;
> +    rcu_read_lock();
> +    list_for_each_entry_rcu(ifp, &in6_dev->addr_list, if_list) {
> +        if (!net_eq(dev_net(ifp->idev->dev), net))
> +            continue;
> +        prfxs_equal =
> +            ipv6_prefix_equal(&pinfo->prefix, &ifp->addr, pinfo->prefix_len);
> +        if (prfxs_equal && pinfo->prefix_len == ifp->prefix_len) {
> +            result = ifp;
> +            in6_ifa_hold(ifp);
> +            break;
> +        }
> +    }
> +    rcu_read_unlock();
> +    if (result_base != result)
> +        ifp = result;
> +    else
> +        ifp = NULL;
> +
> +    return ifp;
> +}
> +
> 
> [Dmytro]:
> 1st initial stage is :
> +    result_base = result;
> 
> 2nd stage is (as you mention, 'result' will be assigned to 'ifp', in the process):
> +            result = ifp;
> 
> 3rd stage is to compare if  "result_base" and "result" are not equal (and take required action).
>  if (result_base != result)
> +        ifp = result;
> +    else
> +        ifp = NULL;
> 
> Looks more/less ok for me.

I think I see what you're trying to do here. Use result_base as a
"marker" or the base value?

But I'd say it makes the code harder to follow. It looks like this:

	result_base = NULL;
	result = NULL;

	result_base = result
	lock()
	for ...
		/* search logic */
	unlock()
	
	if (result == result_base)
		ifp = result
	else
		ifp = NULL
	return NULL

This would be a lot simpler, and functionally equivalent:

	result = NULL

	lock()
	for ...
		/* search logic */
	unlock()

	return result

Right?

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