Subject: Re: [PATCH bpf-next 5/8] bpf: Add link-based BPF program attachment to network namespace

From: Jakub Sitnicki <[email-masked]>
Date: Thu, 28 May 2020 14:26:42 +0200

On Thu, May 28, 2020 at 07:56 AM CEST, Andrii Nakryiko wrote:
> On Wed, May 27, 2020 at 12:16 PM Jakub Sitnicki <[email-masked]> wrote:
>>
>> Add support for bpf() syscall subcommands that operate on
>> bpf_link (LINK_CREATE, LINK_UPDATE, OBJ_GET_INFO) for attach points tied to
>> network namespaces (that is flow dissector at the moment).
>>
>> Link-based and prog-based attachment can be used interchangeably, but only
>> one can be in use at a time. Attempts to attach a link when a prog is
>> already attached directly, and the other way around, will be met with
>> -EBUSY.
>>
>> Attachment of multiple links of same attach type to one netns is not
>> supported, with the intention to lift it when a use-case presents
>> itself. Because of that attempts to create a netns link, when one already
>> exists result in -E2BIG error, signifying that there is no space left for
>> another attachment.
>>
>> Link-based attachments to netns don't keep a netns alive by holding a ref
>> to it. Instead links get auto-detached from netns when the latter is being
>> destroyed by a pernet pre_exit callback.
>>
>> When auto-detached, link lives in defunct state as long there are open FDs
>> for it. -ENOLINK is returned if a user tries to update a defunct link.
>>
>> Because bpf_link to netns doesn't hold a ref to struct net, special care is
>> taken when releasing the link. The netns might be getting torn down when
>> the release function tries to access it to detach the link.
>>
>> To ensure the struct net object is alive when release function accesses it
>> we rely on the fact that cleanup_net(), struct net destructor, calls
>> synchronize_rcu() after invoking pre_exit callbacks. If auto-detach from
>> pre_exit happens first, link release will not attempt to access struct net.
>>
>> Same applies the other way around, network namespace doesn't keep an
>> attached link alive because by not holding a ref to it. Instead bpf_links
>> to netns are RCU-freed, so that pernet pre_exit callback can safely access
>> and auto-detach the link when racing with link release/free.
>>
>> Signed-off-by: Jakub Sitnicki <[email-masked]>
>> ---
>>  include/linux/bpf-netns.h      |   8 +
>>  include/net/netns/bpf.h        |   1 +
>>  include/uapi/linux/bpf.h       |   5 +
>>  kernel/bpf/net_namespace.c     | 257 ++++++++++++++++++++++++++++++++-
>>  kernel/bpf/syscall.c           |   3 +
>>  net/core/filter.c              |   1 +
>>  tools/include/uapi/linux/bpf.h |   5 +
>>  7 files changed, 278 insertions(+), 2 deletions(-)
>>
>
> [...]
>
>>  struct netns_bpf {
>>         struct bpf_prog __rcu *progs[MAX_NETNS_BPF_ATTACH_TYPE];
>> +       struct bpf_link __rcu *links[MAX_NETNS_BPF_ATTACH_TYPE];
>>  };
>>
>
> [...]
>
>>
>> -/* Protects updates to netns_bpf */
>> +struct bpf_netns_link {
>> +       struct bpf_link link;
>> +       enum bpf_attach_type type;
>> +       enum netns_bpf_attach_type netns_type;
>> +
>> +       /* struct net is not RCU-freed but we treat it as such because
>> +        * our pre_exit callback will NULL this pointer before
>> +        * cleanup_net() calls synchronize_rcu().
>> +        */
>> +       struct net __rcu *net;
>
> It feels to me (see comments below), that if you use mutex
> consistently, then this shouldn't be __rcu and you won't even need
> rcu_read_lock() when working with this pointer, because auto_detach
> and everything else won't be racing: if you got mutex in release() and
> you see non-null net pointer, auto-detach either didn't happen yet, or
> is happening at the same time, but is blocked on mutex. If you got
> mutex and see net == NULL, ok, auto-detach succeeded before release,
> so ignore net clean-up. Easy, no?

Yes, right. Grabbing the mutex in auto_detach changes everything.

I think I went about it the wrong way by trying to avoid a mutex there.
I'll go with you advice and see if I can get rid of RCU pointers in v2.

>
>> +
>> +       /* bpf_netns_link is RCU-freed for pre_exit callback invoked
>> +        * by cleanup_net() to safely access the link.
>> +        */
>> +       struct rcu_head rcu;
>> +};
>> +
>> +/* Protects updates to netns_bpf. */
>>  DEFINE_MUTEX(netns_bpf_mutex);
>>
>> +static inline struct bpf_netns_link *to_bpf_netns_link(struct bpf_link *link)
>> +{
>> +       return container_of(link, struct bpf_netns_link, link);
>> +}
>> +
>> +/* Called with RCU read lock. */
>> +static void __net_exit
>> +bpf_netns_link_auto_detach(struct net *net, enum netns_bpf_attach_type type)
>> +{
>> +       struct bpf_netns_link *net_link;
>> +       struct bpf_link *link;
>> +
>> +       link = rcu_dereference(net->bpf.links[type]);
>> +       if (!link)
>> +               return;
>> +       net_link = to_bpf_netns_link(link);
>> +       RCU_INIT_POINTER(net_link->net, NULL);
>
> Given link attach and release is done under netns_bpf_mutex, shouldn't
> this be done under the same mutex? You are modifying link concurrently
> with update, but you are not synchronizing that access.

After attaching the link to netns, there are no other updaters to
net_link->net field. Link update has to modify link->prog, and update
net->bpf.progs, but doesn't need to write to net_link->net, just read
it.

That's why I don't grab the netns_bpf_mutex on auto_detach.

>
>> +}
>> +
>> +static void bpf_netns_link_release(struct bpf_link *link)
>> +{
>> +       struct bpf_netns_link *net_link = to_bpf_netns_link(link);
>> +       enum netns_bpf_attach_type type = net_link->netns_type;
>> +       struct net *net;
>> +
>> +       /* Link auto-detached by dying netns. */
>> +       if (!rcu_access_pointer(net_link->net))
>> +               return;
>> +
>> +       mutex_lock(&netns_bpf_mutex);
>> +
>> +       /* Recheck after potential sleep. We can race with cleanup_net
>> +        * here, but if we see a non-NULL struct net pointer pre_exit
>> +        * and following synchronize_rcu() has not happened yet, and
>> +        * we have until the end of grace period to access net.
>> +        */
>> +       rcu_read_lock();
>> +       net = rcu_dereference(net_link->net);
>> +       if (net) {
>> +               RCU_INIT_POINTER(net->bpf.links[type], NULL);
>> +               RCU_INIT_POINTER(net->bpf.progs[type], NULL);
>
> bpf.progs[type] is supposed to be NULL already, why setting it again here?
>
>> +       }
>> +       rcu_read_unlock();
>> +
>> +       mutex_unlock(&netns_bpf_mutex);
>> +}
>> +
>> +static void bpf_netns_link_dealloc(struct bpf_link *link)
>> +{
>> +       struct bpf_netns_link *net_link = to_bpf_netns_link(link);
>> +
>> +       /* Delay kfree in case we're racing with cleanup_net. */
>> +       kfree_rcu(net_link, rcu);
>
> It feels to me like this RCU stuff for links is a bit overcomplicated.
> If I understand your changes correctly (and please correct me if I'm
> wrong), netns_bpf's progs are sort of like "effective progs" for
> cgroup. Regardless if attachment was bpf_link-based or straight
> bpf_prog-based, prog will always be set. link[type] would be set
> additionally only if bpf_link-based attachment was done. And that
> makes sense to make dissector hot path faster and simpler.

Correct. netns link is heavily modelled after cgroup
link. net->bpf.progs is like "effecttive progs". In fact, the next step
would be to turn it into bpf_prog_array.

> But if that's the case, link itself is always (except for auto-detach,
> which I think should be fixed) accessed under mutex and doesn't
> need/rely on rcu_read_lock() at all. So __rcu annotation for links is
> not necessary, all the rcu dereferences for links are unnecessary, and
> this kfree_rcu() is unnecessary. If that's not the case, please help
> me understand why not.

__rcu annotation for net->bpf.links is for auto_detach to access links
with grace period guarantee, in its current form. If we can get around
that, then I'm with you on that net->bpf.links doesn't need to an RCU
protected pointer.

I think you're right that grabbing netns_bpf_mutex in auto_detach will
guarantee that bpf_link stays alive while we access it from there. We
either see a non-NULL net->bpf.links[type] and know that release can't
run before we finish working on it.

Or we see a NULL net->bpf.links[type] and there is nothing to do.
Thanks makes __rcu annotation and kfree_rcu not needed any more.

Thanks, that is a great suggestion.

>
>> +}
>> +
>> +static int bpf_netns_link_update_prog(struct bpf_link *link,
>> +                                     struct bpf_prog *new_prog,
>> +                                     struct bpf_prog *old_prog)
>> +{
>> +       struct bpf_netns_link *net_link = to_bpf_netns_link(link);
>> +       struct net *net;
>> +       int ret = 0;
>> +
>> +       if (old_prog && old_prog != link->prog)
>> +               return -EPERM;
>> +       if (new_prog->type != link->prog->type)
>> +               return -EINVAL;
>> +
>> +       mutex_lock(&netns_bpf_mutex);
>> +       rcu_read_lock();
>> +
>> +       net = rcu_dereference(net_link->net);
>> +       if (!net || !check_net(net)) {
>> +               /* Link auto-detached or netns dying */
>> +               ret = -ENOLINK;
>
> This is an interesting error code. If we are going to adopt this, we
> should change it for similar cgroup link situation as well.

Credit goes to Lorenz for suggesting a different error code than EINVAL
for this situation so that user-space has a way to distinguish.

I'm happy to patch cgroup BPF link, if you support this change.

>
>> +               goto out_unlock;
>> +       }
>> +
>> +       old_prog = xchg(&link->prog, new_prog);
>> +       bpf_prog_put(old_prog);
>> +
>> +out_unlock:
>> +       rcu_read_unlock();
>> +       mutex_unlock(&netns_bpf_mutex);
>> +
>> +       return ret;
>> +}
>> +
>> +static int bpf_netns_link_fill_info(const struct bpf_link *link,
>> +                                   struct bpf_link_info *info)
>> +{
>> +       const struct bpf_netns_link *net_link;
>> +       unsigned int inum;
>> +       struct net *net;
>> +
>> +       net_link = container_of(link, struct bpf_netns_link, link);
>
> you use to_bpf_netns_link() in few places above, but straight
> container_of() here. Let's do this consistently (I'd rather stick to
> straight container_of, but that's minor).

Ah, yes. That's because bpf_link has const attribute here.  I think I
can turn to_bpf_netns_link() into a macro to get around that.

>
>> +
>> +       rcu_read_lock();
>> +       net = rcu_dereference(net_link->net);
>> +       if (net)
>> +               inum = net->ns.inum;
>> +       rcu_read_unlock();
>> +
>> +       info->netns.netns_ino = inum;
>> +       info->netns.attach_type = net_link->type;
>> +       return 0;
>> +}
>> +
>> +static void bpf_netns_link_show_fdinfo(const struct bpf_link *link,
>> +                                      struct seq_file *seq)
>> +{
>> +       struct bpf_link_info info = {};
>> +
>> +       bpf_netns_link_fill_info(link, &info);
>> +       seq_printf(seq,
>> +                  "netns_ino:\t%u\n"
>> +                  "attach_type:\t%u\n",
>> +                  info.netns.netns_ino,
>> +                  info.netns.attach_type);
>> +}
>> +
>> +static const struct bpf_link_ops bpf_netns_link_ops = {
>> +       .release = bpf_netns_link_release,
>> +       .dealloc = bpf_netns_link_dealloc,
>> +       .update_prog = bpf_netns_link_update_prog,
>> +       .fill_link_info = bpf_netns_link_fill_info,
>> +       .show_fdinfo = bpf_netns_link_show_fdinfo,
>> +};
>> +
>>  int netns_bpf_prog_query(const union bpf_attr *attr,
>>                          union bpf_attr __user *uattr)
>>  {
>> @@ -67,6 +213,13 @@ int netns_bpf_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>>
>>         net = current->nsproxy->net_ns;
>>         mutex_lock(&netns_bpf_mutex);
>> +
>> +       /* Attaching prog directly is not compatible with links */
>> +       if (rcu_access_pointer(net->bpf.links[type])) {
>> +               ret = -EBUSY;
>
> EEXIST would be returned if another prog is attached. Given in this
> case attaching prog or link has same semantics (one cannot replace
> attached program, unlike for cgroups), should we keep it consistent
> and return EEXIST here as well?

Makes sense, will switch to EEXIST.

>
>> +               goto unlock;
>> +       }
>> +
>>         switch (type) {
>>         case NETNS_BPF_FLOW_DISSECTOR:
>>                 ret = flow_dissector_bpf_prog_attach(net, prog);
>> @@ -75,6 +228,7 @@ int netns_bpf_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>>                 ret = -EINVAL;
>>                 break;
>>         }
>> +unlock:
>>         mutex_unlock(&netns_bpf_mutex);
>>
>>         return ret;
>> @@ -85,6 +239,10 @@ static int __netns_bpf_prog_detach(struct net *net,
>>  {
>>         struct bpf_prog *attached;
>>
>> +       /* Progs attached via links cannot be detached */
>> +       if (rcu_access_pointer(net->bpf.links[type]))
>> +               return -EBUSY;
>
> This is more of -EINVAL?

No strong feelings here. The intention of returning EBUSY on
attach/detach was to signal that the attach point is in use, busy, by
another incompatible attachment.

EINVAL sends a message that I did a bad job passing argument to the
syscall. But this is just my interpretation.

>
>> +
>>         /* No need for update-side lock when net is going away. */
>>         attached = rcu_dereference_protected(net->bpf.progs[type],
>>                                              !check_net(net) ||
>> @@ -112,14 +270,109 @@ int netns_bpf_prog_detach(const union bpf_attr *attr)
>>         return ret;
>>  }
>>
>> +static int __netns_bpf_link_attach(struct net *net, struct bpf_link *link,
>> +                                  enum netns_bpf_attach_type type)
>> +{
>> +       int err;
>> +
>> +       /* Allow attaching only one prog or link for now */
>> +       if (rcu_access_pointer(net->bpf.links[type]))
>> +               return -E2BIG;
>> +       /* Links are not compatible with attaching prog directly */
>> +       if (rcu_access_pointer(net->bpf.progs[type]))
>> +               return -EBUSY;
>
> Same as above. Do we need three different error codes instead of
> consistently using EEXIST?

Ok, EEXIST seems fine to me well.

>
>
>> +
>> +       switch (type) {
>> +       case NETNS_BPF_FLOW_DISSECTOR:
>> +               err = flow_dissector_bpf_prog_attach(net, link->prog);
>> +               break;
>> +       default:
>> +               err = -EINVAL;
>> +               break;
>> +       }
>> +       if (!err)
>> +               rcu_assign_pointer(net->bpf.links[type], link);
>> +       return err;
>> +}
>> +
>
> [...]
>
>> +       err = bpf_link_prime(&net_link->link, &link_primer);
>> +       if (err) {
>> +               kfree(net_link);
>> +               goto out_put_net;
>> +       }
>> +
>> +       err = netns_bpf_link_attach(net, &net_link->link, netns_type);
>> +       if (err) {
>> +               bpf_link_cleanup(&link_primer);
>> +               goto out_put_net;
>> +       }
>> +
>> +       err = bpf_link_settle(&link_primer);
>
> This looks a bit misleading. bpf_link_settle() cannot fail and returns
> FD, but here it looks like it might return error. I think it would be
> more straightforward to just:
>
>     put_net(net);
>     return bpf_link_settle(&link_primer);
>
> out_put_net:
>     put_net(net);
>     return err;

That reads better. Will change. Thanks.

Appreciate the review. Thanks for all the feedback.

-jkbs

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