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Message-ID: <e29f8d2a-f9b3-36d3-13aa-d5bec16cf61b@blackwall.org>
Date:   Fri, 13 May 2022 19:19:24 +0300
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Subject: Re: [PATCH net-next v2] bond: add mac filter option for balance-xor
Content-Language: en-US
To:     Jonathan Toppins <jtoppins@redhat.com>, netdev@vger.kernel.org
Cc:     Long Xin <lxin@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Jay Vosburgh <j.vosburgh@gmail.com>,
        Veaceslav Falico <vfalico@gmail.com>,
        Andy Gospodarek <andy@greyhouse.net>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>
References: <6227427ef3b57d7de6d4d95e9dd7c9b222a37bf6.1651689665.git.jtoppins@redhat.com>
 <f85a0a66-d3b8-9d20-9abb-fc9fa5e84eab@blackwall.org>
 <d2696dab-2490-feb5-ccb2-96906fc652f0@redhat.com>
From:   Nikolay Aleksandrov <razor@blackwall.org>
In-Reply-To: <d2696dab-2490-feb5-ccb2-96906fc652f0@redhat.com>
Content-Type: text/plain; charset=UTF-8
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On 13/05/2022 18:42, Jonathan Toppins wrote:
> Hi Nik, thanks for the review. Responses below.
> 
> On 5/5/22 08:14, Nikolay Aleksandrov wrote:
>> On 04/05/2022 21:47, Jonathan Toppins wrote:
>>> Implement a MAC filter that prevents duplicate frame delivery when
>>> handling BUM traffic. This attempts to partially replicate OvS SLB
>>> Bonding[1] like functionality without requiring significant change
>>> in the Linux bridging code.
>>>
>>> A typical network setup for this feature would be:
>>>
>>>              .--------------------------------------------.
>>>              |         .--------------------.             |
>>>              |         |                    |             |
>>>         .-------------------.               |             |
>>>         |    | Bond 0  |    |               |             |
>>>         | .--'---. .---'--. |               |             |
>>>    .----|-| eth0 |-| eth1 |-|----.    .-----+----.   .----+------.
>>>    |    | '------' '------' |    |    | Switch 1 |   | Switch 2  |
>>>    |    '---,---------------'    |    |          +---+           |
>>>    |       /                     |    '----+-----'   '----+------'
>>>    |  .---'---.    .------.      |         |              |
>>>    |  |  br0  |----| VM 1 |      |      ~~~~~~~~~~~~~~~~~~~~~
>>>    |  '-------'    '------'      |     (                     )
>>>    |      |        .------.      |     ( Rest of Network     )
>>>    |      '--------| VM # |      |     (_____________________)
>>>    |               '------'      |
>>>    |  Host 1                     |
>>>    '-----------------------------'
>>>
>>> Where 'VM1' and 'VM#' are hosts connected to a Linux bridge, br0, with
>>> bond0 and its associated links, eth0 & eth1, provide ingress/egress. One
>>> can assume bond0, br1, and hosts VM1 to VM# are all contained in a
>>> single box, as depicted. Interfaces eth0 and eth1 provide redundant
>>> connections to the data center with the requirement to use all bandwidth
>>> when the system is functioning normally. Switch 1 and Switch 2 are
>>> physical switches that do not implement any advanced L2 management
>>> features such as MLAG, Cisco's VPC, or LACP.
>>>
>>> Combining this feature with vlan+srcmac hash policy allows a user to
>>> create an access network without the need to use expensive switches that
>>> support features like Cisco's VCP.
>>>
>>> [1] https://docs.openvswitch.org/en/latest/topics/bonding/#slb-bonding
>>>
>>> Co-developed-by: Long Xin <lxin@redhat.com>
>>> Signed-off-by: Long Xin <lxin@redhat.com>
>>> Signed-off-by: Jonathan Toppins <jtoppins@redhat.com>
>>> ---
>>>
>>> Notes:
>>>      v2:
>>>       * dropped needless abstraction functions and put code in module init
>>>       * renamed variable "rc" to "ret" to stay consistent with most of the
>>>         code
>>>       * fixed parameter setting management, when arp-monitor is turned on
>>>         this feature will be turned off similar to how miimon and arp-monitor
>>>         interact
>>>       * renamed bond_xor_recv to bond_mac_filter_recv for a little more
>>>         clarity
>>>       * it appears the implied default return code for any bonding recv probe
>>>         must be `RX_HANDLER_ANOTHER`. Changed the default return code of
>>>         bond_mac_filter_recv to use this return value to not break skb
>>>         processing when the skb dev is switched to the bond dev:
>>>           `skb->dev = bond->dev`
>>>
>>>   Documentation/networking/bonding.rst  |  19 +++
>>>   drivers/net/bonding/Makefile          |   2 +-
>>>   drivers/net/bonding/bond_mac_filter.c | 201 ++++++++++++++++++++++++++
>>>   drivers/net/bonding/bond_mac_filter.h |  39 +++++
>>>   drivers/net/bonding/bond_main.c       |  27 ++++
>>>   drivers/net/bonding/bond_netlink.c    |  13 ++
>>>   drivers/net/bonding/bond_options.c    |  86 ++++++++++-
>>>   drivers/net/bonding/bonding_priv.h    |   1 +
>>>   include/net/bond_options.h            |   1 +
>>>   include/net/bonding.h                 |   3 +
>>>   include/uapi/linux/if_link.h          |   1 +
>>>   11 files changed, 390 insertions(+), 3 deletions(-)
>>>   create mode 100644 drivers/net/bonding/bond_mac_filter.c
>>>   create mode 100644 drivers/net/bonding/bond_mac_filter.h
>>>
>>
>> Hi Jonathan,
>> I must mention that this is easily solvable with two very simple ebpf programs, one on egress
>> to track source macs and one on ingress to filter them, it can also easily be solved by a
>> user-space agent that adds macs for filtering in many different ways, after all these VMs
>> run on the host and you don't need bond-specific knowledge to do this. Also you have no visibility
>> into what is currently being filtered, so it will be difficult to debug. With the above solutions
>> you already have that. I don't think the bond should be doing any learning or filtering, this is
>> deviating a lot from its purpose and adds unnecessary complexity.
>> That being said, if you decide to continue with the set, comments are below...
> 
> This is an excellent observation, it does appear this could likely be done with eBPF. However, the delivery of such a solution to a user would be the difficult part. There appears to be no standard way for attaching a program to an interface, it still seems customary to write your own custom loader. Where would the user run this loader? In Debian likely in a post up hook with ifupdown, in Fedora one would have to write a locally custom dispatcher script (assuming Network Manager) that only ran the loader for a given interface. In short I do not see a reasonably appropriate way to deploy an eBPF program to users with the current infrastructure. Also, I am not aware of the bpf syscall supporting signed program loading. Signing kernel modules seems popular with some distros to identify limits of support and authentication of an unmodified system. I suspect similar bpf support might be needed to identify support and authentication for deployed programs.
> 

A great deal of the distributions (almost all major ones) out there already use eBPF for various tasks, so I can't see
how any of these arguments apply nowadays. There are standard ways to load eBPF programs that have been around
for quite some time and most of the different software needed to achieve that is already packaged
for all major distributions (and has been for a long time). Anyway getting into the details of "how" the user would load the program
is not really pertinent to the discussion, that doesn't warrant adding so much new complexity in the bonding driver
which will have to be maintained forever. Honestly, I don't like the idea of adding learning to the bonding at all,
I think it's the wrong place for it, especially when the solution can easily be achieved with already available means.
It might not even be eBPF, you can do it with a user-space agent that uses nftables or some other filtering mechanism,
I'm sure you can think of many other ways to solve it which don't require this new infrastructure. All of these ways
to solve it have many advantages over this (e.g. visibility into the current entries being filtered, control over them and so on).

That's my opinion of course, it'd be nice to get feedback from others as well.

Cheers,
 Nik

> [...]
> 
>>> diff --git a/drivers/net/bonding/bond_mac_filter.c b/drivers/net/bonding/bond_mac_filter.c
>>> new file mode 100644
>>> index 000000000000..e86b2b475df3
>>> --- /dev/null
>>> +++ b/drivers/net/bonding/bond_mac_filter.c
>>> @@ -0,0 +1,201 @@
>>> +// SPDX-License-Identifier: GPL-2.0-only
>>> +/*
>>> + * Filter received frames based on MAC addresses "behind" the bond.
>>> + */
>>> +
>>> +#include "bonding_priv.h"
>>> +
>>> +static const struct rhashtable_params bond_rht_params = {
>>> +    .head_offset         = offsetof(struct bond_mac_cache_entry, rhnode),
>>> +    .key_offset          = offsetof(struct bond_mac_cache_entry, key),
>>> +    .key_len             = sizeof(struct mac_addr),
>>> +    .automatic_shrinking = true,
>>> +};
>>> +
>>> +static inline unsigned long hold_time(const struct bonding *bond)
>>
>> no inlines in .c files, let the compiler do its job
>>
>>> +{
>>> +    return msecs_to_jiffies(5000);
>>> +}
>>> +
>>> +static bool has_expired(const struct bonding *bond,
>>> +            struct bond_mac_cache_entry *mac)
>>> +{
>>> +    return time_before_eq(mac->used + hold_time(bond), jiffies);
>>> +}
>>> +
>>> +static void mac_delete_rcu(struct callback_head *head)
>>> +{
>>> +    kmem_cache_free(bond_mac_cache,
>>> +            container_of(head, struct bond_mac_cache_entry, rcu));
>>> +}
>>> +
>>> +static int mac_delete(struct bonding *bond,
>>> +              struct bond_mac_cache_entry *entry)
>>> +{
>>> +    int ret;
>>> +
>>> +    ret = rhashtable_remove_fast(bond->mac_filter_tbl,
>>> +                     &entry->rhnode,
>>> +                     bond->mac_filter_tbl->p);
>>> +    set_bit(BOND_MAC_DEAD, &entry->flags);
>>
>> you don't need the atomic bitops, these flags are all modified and checked
>> under the entry lock
> 
> I need to keep the atomic set_bit if I remove the [use-after-free] idiomatic issue later in the file.
> 
>>
>>> +    call_rcu(&entry->rcu, mac_delete_rcu);
>>
>> all of these entries are queued to be freed, what happens if we unload the bonding
>> driver before that?
> 
> [...]
> 
>>
>>> +
>>> +    rhashtable_walk_enter(bond->mac_filter_tbl, &iter);
>>> +    rhashtable_walk_start(&iter);
>>> +    while ((entry = rhashtable_walk_next(&iter)) != NULL) {
>>> +        if (IS_ERR(entry))
>>> +            continue;
>>> +
>>> +        spin_lock_irqsave(&entry->lock, flags);
>>> +        if (has_expired(bond, entry))
>>> +            mac_delete(bond, entry);
>>> +        spin_unlock_irqrestore(&entry->lock, flags);
>>
>> deleting entries while holding their own lock is not very idiomatic
> 
> [use-after-free] To fix this I made has_expired take the lock, making has_expired atomic. Now there is no need to have the critical section above and mac_delete can be outside the critical section. This also removed the use-after-free bug that would appear if the code were not using RCU and cache malloc.
> 
>>
>>> +    bond->mac_filter_tbl = kzalloc(sizeof(*bond->mac_filter_tbl),
>>> +                       GFP_KERNEL);
>>> +    if (!bond->mac_filter_tbl)
>>> +        return -ENOMEM;
>>> +
>>> +    ret = rhashtable_init(bond->mac_filter_tbl, &bond_rht_params);
>>> +    if (ret)
>>> +        kfree(bond->mac_filter_tbl);
>>
>> on error this is freed, but the pointer is stale and on bond destruction
>> will be accessed and potentially freed again
> 
> set to NULL.
> 
> [...]
> 
>>> +static int mac_create(struct bonding *bond, const u8 *addr)
>>> +{
>>> +    struct bond_mac_cache_entry *entry;
>>> +    int ret;
>>> +
>>> +    entry = kmem_cache_alloc(bond_mac_cache, GFP_ATOMIC);
>>> +    if (!entry)
>>> +        return -ENOMEM;
>>> +    spin_lock_init(&entry->lock);
>>> +    memcpy(&entry->key, addr, sizeof(entry->key));
>>> +    entry->used = jiffies;
>>
>> you must zero the old fields, otherwise you can find stale values from old
>> structs which were freed
> 
> good point, have done.
> 
> [...]
> 
>>> diff --git a/drivers/net/bonding/bond_mac_filter.h b/drivers/net/bonding/bond_mac_filter.h
>>> new file mode 100644
>>> index 000000000000..7c968d41b456
>>> --- /dev/null
>>> +++ b/drivers/net/bonding/bond_mac_filter.h
>>> @@ -0,0 +1,39 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-only
>>> + *
>>> + * Filter received frames based on MAC addresses "behind" the bond.
>>> + */
>>> +
>>> +#ifndef _BOND_MAC_FILTER_H
>>> +#define _BOND_MAC_FILTER_H
>>> +#include <net/bonding.h>
>>> +#include <linux/spinlock.h>
>>> +#include <linux/rhashtable.h>
>>> +
>>> +enum {
>>> +    BOND_MAC_DEAD,
>>> +    BOND_MAC_LOCKED,
>>> +    BOND_MAC_STATIC,
>>
>> What are BOND_MAC_LOCKED or STATIC ? I didn't see them used anywhere.
> 
> Stale, was going to use them to allow the user to manually add entries but never got around to it. Removed.
> 
> [...]
> 
>>> diff --git a/drivers/net/bonding/bond_options.c b/drivers/net/bonding/bond_options.c
>>> index 64f7db2627ce..d295903a525b 100644
>>> --- a/drivers/net/bonding/bond_options.c
>>> +++ b/drivers/net/bonding/bond_options.c
> 
> [...]
> 
>>> @@ -1035,6 +1075,44 @@ static int bond_option_use_carrier_set(struct bonding *bond,
>>>       return 0;
>>>   }
>>>   +static int bond_option_mac_filter_set(struct bonding *bond,
>>> +                      const struct bond_opt_value *newval)
>>> +{
>>> +    int rc = 0;
>>> +    u8 prev = bond->params.mac_filter;
>>
>> reverse xmas tree
>>
>>> +
>>> +    if (newval->value && bond->params.arp_interval) {
>>
>> what happens if we set arp_interval after enabling this, the table will be
>> freed while the bond is up and is using it, also the queued work is still enabled
> 
> This is a good observation. To simplify the option setting I moved the init/destroy of the hash table to bond_open/close respectively. This allowed me to simply set the value of mac_filter. The only catch is in bond_option_arp_interval_set() if mac_filter is set and the interface is up, the user will receive an -EBUSY. This was the minimal amount of configuration behavioral change I could think of.
> 
> Thanks,
> -Jon
>