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Date: Fri, 28 Jun 2019 19:33:44 +0200
From: Björn Töpel <bjorn.topel@...il.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Alexei Starovoitov <ast@...nel.org>,
Netdev <netdev@...r.kernel.org>,
Björn Töpel <bjorn.topel@...el.com>,
"Karlsson, Magnus" <magnus.karlsson@...el.com>,
Bruce Richardson <bruce.richardson@...el.com>,
Song Liu <songliubraving@...com>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 1/2] xsk: remove AF_XDP socket from map when
the socket is released
On Fri, 28 Jun 2019 at 02:33, Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 06/20/2019 12:06 PM, Björn Töpel wrote:
> > From: Björn Töpel <bjorn.topel@...el.com>
> >
> > When an AF_XDP socket is released/closed the XSKMAP still holds a
> > reference to the socket in a "released" state. The socket will still
> > use the netdev queue resource, and block newly created sockets from
> > attaching to that queue, but no user application can access the
> > fill/complete/rx/tx queues. This results in that all applications need
> > to explicitly clear the map entry from the old "zombie state"
> > socket. This should be done automatically.
> >
> > After this patch, when a socket is released, it will remove itself
> > from all the XSKMAPs it resides in, allowing the socket application to
> > remove the code that cleans the XSKMAP entry.
> >
> > This behavior is also closer to that of SOCKMAP, making the two socket
> > maps more consistent.
> >
> > Suggested-by: Bruce Richardson <bruce.richardson@...el.com>
> > Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
>
> Sorry for the bit of delay in reviewing, few comments inline:
>
No worries!
> > ---
> > include/net/xdp_sock.h | 3 ++
> > kernel/bpf/xskmap.c | 101 +++++++++++++++++++++++++++++++++++------
> > net/xdp/xsk.c | 25 ++++++++++
> > 3 files changed, 116 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > index ae0f368a62bb..011a1b08d7c9 100644
> > --- a/include/net/xdp_sock.h
> > +++ b/include/net/xdp_sock.h
> > @@ -68,6 +68,8 @@ struct xdp_sock {
> > */
> > spinlock_t tx_completion_lock;
> > u64 rx_dropped;
> > + struct list_head map_list;
> > + spinlock_t map_list_lock;
> > };
> >
> > struct xdp_buff;
> > @@ -87,6 +89,7 @@ struct xdp_umem_fq_reuse *xsk_reuseq_swap(struct xdp_umem *umem,
> > struct xdp_umem_fq_reuse *newq);
> > void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq);
> > struct xdp_umem *xdp_get_umem_from_qid(struct net_device *dev, u16 queue_id);
> > +void xsk_map_delete_from_node(struct xdp_sock *xs, struct list_head *node);
> >
> > static inline char *xdp_umem_get_data(struct xdp_umem *umem, u64 addr)
> > {
> > diff --git a/kernel/bpf/xskmap.c b/kernel/bpf/xskmap.c
> > index ef7338cebd18..af802c89ebab 100644
> > --- a/kernel/bpf/xskmap.c
> > +++ b/kernel/bpf/xskmap.c
> > @@ -13,8 +13,58 @@ struct xsk_map {
> > struct bpf_map map;
> > struct xdp_sock **xsk_map;
> > struct list_head __percpu *flush_list;
> > + spinlock_t lock;
> > };
> >
> > +/* Nodes are linked in the struct xdp_sock map_list field, and used to
> > + * track which maps a certain socket reside in.
> > + */
> > +struct xsk_map_node {
> > + struct list_head node;
> > + struct xsk_map *map;
> > + struct xdp_sock **map_entry;
> > +};
> > +
> > +static struct xsk_map_node *xsk_map_node_alloc(void)
> > +{
> > + return kzalloc(sizeof(struct xsk_map_node), GFP_ATOMIC | __GFP_NOWARN);
> > +}
> > +
> > +static void xsk_map_node_free(struct xsk_map_node *node)
> > +{
> > + kfree(node);
> > +}
> > +
> > +static void xsk_map_node_init(struct xsk_map_node *node,
> > + struct xsk_map *map,
> > + struct xdp_sock **map_entry)
> > +{
> > + node->map = map;
> > + node->map_entry = map_entry;
> > +}
> > +
> > +static void xsk_map_add_node(struct xdp_sock *xs, struct xsk_map_node *node)
> > +{
> > + spin_lock_bh(&xs->map_list_lock);
> > + list_add_tail(&node->node, &xs->map_list);
> > + spin_unlock_bh(&xs->map_list_lock);
> > +}
> > +
> > +static void xsk_map_del_node(struct xdp_sock *xs, struct xdp_sock **map_entry)
> > +{
> > + struct xsk_map_node *n, *tmp;
> > +
> > + spin_lock_bh(&xs->map_list_lock);
> > + list_for_each_entry_safe(n, tmp, &xs->map_list, node) {
> > + if (map_entry == n->map_entry) {
> > + list_del(&n->node);
> > + xsk_map_node_free(n);
> > + }
> > + }
> > + spin_unlock_bh(&xs->map_list_lock);
> > +
> > +}
> > +
> > static struct bpf_map *xsk_map_alloc(union bpf_attr *attr)
> > {
> > struct xsk_map *m;
> > @@ -34,6 +84,7 @@ static struct bpf_map *xsk_map_alloc(union bpf_attr *attr)
> > return ERR_PTR(-ENOMEM);
> >
> > bpf_map_init_from_attr(&m->map, attr);
> > + spin_lock_init(&m->lock);
> >
> > cost = (u64)m->map.max_entries * sizeof(struct xdp_sock *);
> > cost += sizeof(struct list_head) * num_possible_cpus();
> > @@ -76,15 +127,16 @@ static void xsk_map_free(struct bpf_map *map)
> > bpf_clear_redirect_map(map);
> > synchronize_net();
> >
> > + spin_lock_bh(&m->lock);
> > for (i = 0; i < map->max_entries; i++) {
> > - struct xdp_sock *xs;
> > -
> > - xs = m->xsk_map[i];
> > - if (!xs)
> > - continue;
> > + struct xdp_sock **map_entry = &m->xsk_map[i];
> > + struct xdp_sock *old_xs;
> >
> > - sock_put((struct sock *)xs);
> > + old_xs = xchg(map_entry, NULL);
> > + if (old_xs)
> > + xsk_map_del_node(old_xs, map_entry);
> > }
> > + spin_unlock_bh(&m->lock);
> >
> > free_percpu(m->flush_list);
> > bpf_map_area_free(m->xsk_map);
> > @@ -166,7 +218,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
> > {
> > struct xsk_map *m = container_of(map, struct xsk_map, map);
> > u32 i = *(u32 *)key, fd = *(u32 *)value;
> > - struct xdp_sock *xs, *old_xs;
> > + struct xdp_sock *xs, *old_xs, **entry;
> > + struct xsk_map_node *node;
> > struct socket *sock;
> > int err;
> >
> > @@ -193,11 +246,20 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
> > return -EOPNOTSUPP;
> > }
> >
> > - sock_hold(sock->sk);
> > + node = xsk_map_node_alloc();
> > + if (!node) {
> > + sockfd_put(sock);
> > + return -ENOMEM;
> > + }
> >
> > - old_xs = xchg(&m->xsk_map[i], xs);
> > + spin_lock_bh(&m->lock);
> > + entry = &m->xsk_map[i];
> > + xsk_map_node_init(node, m, entry);
> > + xsk_map_add_node(xs, node);
> > + old_xs = xchg(entry, xs);
> > if (old_xs)
> > - sock_put((struct sock *)old_xs);
> > + xsk_map_del_node(old_xs, entry);
> > + spin_unlock_bh(&m->lock);
> >
> > sockfd_put(sock);
> > return 0;
> > @@ -206,19 +268,32 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
> > static int xsk_map_delete_elem(struct bpf_map *map, void *key)
> > {
> > struct xsk_map *m = container_of(map, struct xsk_map, map);
> > - struct xdp_sock *old_xs;
> > + struct xdp_sock *old_xs, **map_entry;
> > int k = *(u32 *)key;
> >
> > if (k >= map->max_entries)
> > return -EINVAL;
> >
> > - old_xs = xchg(&m->xsk_map[k], NULL);
> > + spin_lock_bh(&m->lock);
> > + map_entry = &m->xsk_map[k];
> > + old_xs = xchg(map_entry, NULL);
> > if (old_xs)
> > - sock_put((struct sock *)old_xs);
> > + xsk_map_del_node(old_xs, map_entry);
> > + spin_unlock_bh(&m->lock);
> >
> > return 0;
> > }
> >
> > +void xsk_map_delete_from_node(struct xdp_sock *xs, struct list_head *node)
> > +{
> > + struct xsk_map_node *n = list_entry(node, struct xsk_map_node, node);
> > +
> > + spin_lock_bh(&n->map->lock);
> > + *n->map_entry = NULL;
> > + spin_unlock_bh(&n->map->lock);
> > + xsk_map_node_free(n);
> > +}
> > +
> > const struct bpf_map_ops xsk_map_ops = {
> > .map_alloc = xsk_map_alloc,
> > .map_free = xsk_map_free,
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index a14e8864e4fa..1931d98a7754 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -335,6 +335,27 @@ static int xsk_init_queue(u32 entries, struct xsk_queue **queue,
> > return 0;
> > }
> >
> > +static struct list_head *xsk_map_list_pop(struct xdp_sock *xs)
> > +{
> > + struct list_head *node = NULL;
> > +
> > + spin_lock_bh(&xs->map_list_lock);
> > + if (!list_empty(&xs->map_list)) {
> > + node = xs->map_list.next;
> > + list_del(node);
> > + }
> > + spin_unlock_bh(&xs->map_list_lock);
> > + return node;
> > +}
> > +
> > +static void xsk_delete_from_maps(struct xdp_sock *xs)
> > +{
> > + struct list_head *node;
> > +
> > + while ((node = xsk_map_list_pop(xs)))
> > + xsk_map_delete_from_node(xs, node);
> > +}
> > +
>
> I stared at this set for a while and I think there are still two
> issues in the design unless I'm missing something obvious.
>
> 1) xs teardown and parallel map update:
>
> - CPU0 is in xsk_release(), calls xsk_delete_from_maps().
> - CPU1 is in xsk_map_update_elem(), both access the same map slot.
> - CPU0 does the xsk_map_list_pop() for that given slot, gets
> interrupted before calling into xsk_map_delete_from_node().
> - CPU1 takes m->lock in updates, *entry = xs to the new sock,
> does xsk_map_del_node() to check on the xs (which CPU0 tears
> down). Given this was popped off the list, it doesn't do
> anything here, all good. It unlocks m->lock and succeeds.
> - CPU0 now continues in xsk_map_delete_from_node(), takes
> m->lock, zeroes *n->map_entry, releases m->lock, and frees
> n. However, at this point *n->map_entry contains the xs that
> we've just updated on CPU1. So zero'ing it will 1) remove
> the wrong entry, and ii) leak it since it goes out of reach.
>
Argh. No, you're (again) not missing anything. Thanks for catching this.
> 2) Inconsistent use of xchg() and friends:
>
> - AF_XDP fast-path is doing READ_ONCE(m->xsk_map[key]) without
> taking m->lock. This is also why you have xchg() for example
> inside m->lock region since both protect different things (should
> probably be commented). However, this is not consistently used.
> E.g. xsk_map_delete_from_node() or xsk_map_update_elem() have
> plain assignment, so compiler could in theory happily perform
> store tearing and the READ_ONCE() would see garbage. This needs
> to be consistently paired.
>
Indeed. Back to the drawing board. Thanks for your time, Daniel.
Björn
> > static int xsk_release(struct socket *sock)
> > {
> > struct sock *sk = sock->sk;
> > @@ -354,6 +375,7 @@ static int xsk_release(struct socket *sock)
> > sock_prot_inuse_add(net, sk->sk_prot, -1);
> > local_bh_enable();
> >
> > + xsk_delete_from_maps(xs);
> > if (xs->dev) {
> > struct net_device *dev = xs->dev;
> >
> > @@ -767,6 +789,9 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
> > mutex_init(&xs->mutex);
> > spin_lock_init(&xs->tx_completion_lock);
> >
> > + INIT_LIST_HEAD(&xs->map_list);
> > + spin_lock_init(&xs->map_list_lock);
> > +
> > mutex_lock(&net->xdp.lock);
> > sk_add_node_rcu(sk, &net->xdp.list);
> > mutex_unlock(&net->xdp.lock);
> >
>
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