netdev - Re: [PATCH net-next v3 2/3] xdp: Always use a devmap for XDP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <877ede2s1t.fsf@toke.dk>
Date:   Mon, 04 Mar 2019 23:28:14 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH net-next v3 2/3] xdp: Always use a devmap for XDP_REDIRECT to a device

Jakub Kicinski <jakub.kicinski@...ronome.com> writes:

> On Mon, 04 Mar 2019 20:05:30 +0100, Toke Høiland-Jørgensen wrote:
>> > Hm.  I think you'll still need a lock (mutex?) on the alloc path, but
>> > the free path should be fine as long as you load the map pointer before
>> > looking at the refcnt (atomic op ensuring the barrier there).  
>> 
>> Yeah, for the per-namespace refcnt it's pretty straight forward, the
>> trouble is the global count that needs to iterate over all namespaces;
>> probably need to put that all behind a (non-spin)lock, right?
>
> Because net iteration is under RCU? You can switch to taking net_rwsem
> for that one, no? I'm probably confused again ;)

Because there's a single refcount that needs to trigger
creation/deletion of *all* the default maps. I.e.

if (atomic_dec_return(&global_refcnt))
  for_each_namespace(net)
    destroy_default_map(net);

which needs to not step on the toes of a subsequent

if (atomic_inc_return(&global_refcnt) == 1)
  for_each_namespace(net)
    create_default_map(net);

(or vice versa, of course).

Not sure there's a way to do that without wrapping both of those
constructs (including the refcnt inc/dec) in a mutex?

-Toke