net: Generic XDP

This provides a generic SKB based non-optimized XDP path which is used
if either the driver lacks a specific XDP implementation, or the user
requests it via a new IFLA_XDP_FLAGS value named XDP_FLAGS_SKB_MODE.

It is arguable that perhaps I should have required something like
this as part of the initial XDP feature merge.

I believe this is critical for two reasons:

1) Accessibility.  More people can play with XDP with less
   dependencies.  Yes I know we have XDP support in virtio_net, but
   that just creates another depedency for learning how to use this
   facility.

   I wrote this to make life easier for the XDP newbies.

2) As a model for what the expected semantics are.  If there is a pure
   generic core implementation, it serves as a semantic example for
   driver folks adding XDP support.

One thing I have not tried to address here is the issue of
XDP_PACKET_HEADROOM, thanks to Daniel for spotting that.  It seems
incredibly expensive to do a skb_cow(skb, XDP_PACKET_HEADROOM) or
whatever even if the XDP program doesn't try to push headers at all.
I think we really need the verifier to somehow propagate whether
certain XDP helpers are used or not.

v5:
 - Handle both negative and positive offset after running prog
 - Fix mac length in XDP_TX case (Alexei)
 - Use rcu_dereference_protected() in free_netdev (kbuild test robot)

v4:
 - Fix MAC header adjustmnet before calling prog (David Ahern)
 - Disable LRO when generic XDP is installed (Michael Chan)
 - Bypass qdisc et al. on XDP_TX and record the event (Alexei)
 - Do not perform generic XDP on reinjected packets (DaveM)

v3:
 - Make sure XDP program sees packet at MAC header, push back MAC
   header if we do XDP_TX.  (Alexei)
 - Elide GRO when generic XDP is in use.  (Alexei)
 - Add XDP_FLAG_SKB_MODE flag which the user can use to request generic
   XDP even if the driver has an XDP implementation.  (Alexei)
 - Report whether SKB mode is in use in rtnl_xdp_fill() via XDP_FLAGS
   attribute.  (Daniel)

v2:
 - Add some "fall through" comments in switch statements based
   upon feedback from Andrew Lunn
 - Use RCU for generic xdp_prog, thanks to Johannes Berg.

Tested-by: Andy Gospodarek <andy@greyhouse.net>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 187 insertions, 22 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 5d5267febd56..46d220c2bf92 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1905,9 +1905,17 @@ struct net_device {
         struct lock_class_key   *qdisc_tx_busylock;
         struct lock_class_key   *qdisc_running_key;
         bool                    proto_down;
+        struct bpf_prog __rcu   *xdp_prog;
 };
 #define to_net_dev(d) container_of(d, struct net_device, dev)
 
+static inline bool netif_elide_gro(const struct net_device *dev)
+{
+        if (!(dev->features & NETIF_F_GRO) || dev->xdp_prog)
+                return true;
+        return false;
+}
+
 #define NETDEV_ALIGN            32
 
 static inline
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 8b405afb2376..633aa0276d32 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -887,7 +887,9 @@ enum {
 /* XDP section */
 
 #define XDP_FLAGS_UPDATE_IF_NOEXIST     (1U << 0)
-#define XDP_FLAGS_MASK                  (XDP_FLAGS_UPDATE_IF_NOEXIST)
+#define XDP_FLAGS_SKB_MODE              (2U << 0)
+#define XDP_FLAGS_MASK                  (XDP_FLAGS_UPDATE_IF_NOEXIST | \
+                                         XDP_FLAGS_SKB_MODE)
 
 enum {
         IFLA_XDP_UNSPEC,
diff --git a/net/core/dev.c b/net/core/dev.c
index db6e31564d06..1b3317c026c6 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -95,6 +95,7 @@
 #include <linux/notifier.h>
 #include <linux/skbuff.h>
 #include <linux/bpf.h>
+#include <linux/bpf_trace.h>
 #include <net/net_namespace.h>
 #include <net/sock.h>
 #include <net/busy_poll.h>
@@ -4251,6 +4252,125 @@ static int __netif_receive_skb(struct sk_buff *skb)
         return ret;
 }
 
+static struct static_key generic_xdp_needed __read_mostly;
+
+static int generic_xdp_install(struct net_device *dev, struct netdev_xdp *xdp)
+{
+        struct bpf_prog *new = xdp->prog;
+        int ret = 0;
+
+        switch (xdp->command) {
+        case XDP_SETUP_PROG: {
+                struct bpf_prog *old = rtnl_dereference(dev->xdp_prog);
+
+                rcu_assign_pointer(dev->xdp_prog, new);
+                if (old)
+                        bpf_prog_put(old);
+
+                if (old && !new) {
+                        static_key_slow_dec(&generic_xdp_needed);
+                } else if (new && !old) {
+                        static_key_slow_inc(&generic_xdp_needed);
+                        dev_disable_lro(dev);
+                }
+                break;
+        }
+
+        case XDP_QUERY_PROG:
+                xdp->prog_attached = !!rcu_access_pointer(dev->xdp_prog);
+                break;
+
+        default:
+                ret = -EINVAL;
+                break;
+        }
+
+        return ret;
+}
+
+static u32 netif_receive_generic_xdp(struct sk_buff *skb,
+                                     struct bpf_prog *xdp_prog)
+{
+        struct xdp_buff xdp;
+        u32 act = XDP_DROP;
+        void *orig_data;
+        int hlen, off;
+        u32 mac_len;
+
+        /* Reinjected packets coming from act_mirred or similar should
+         * not get XDP generic processing.
+         */
+        if (skb_cloned(skb))
+                return XDP_PASS;
+
+        if (skb_linearize(skb))
+                goto do_drop;
+
+        /* The XDP program wants to see the packet starting at the MAC
+         * header.
+         */
+        mac_len = skb->data - skb_mac_header(skb);
+        hlen = skb_headlen(skb) + mac_len;
+        xdp.data = skb->data - mac_len;
+        xdp.data_end = xdp.data + hlen;
+        xdp.data_hard_start = skb->data - skb_headroom(skb);
+        orig_data = xdp.data;
+
+        act = bpf_prog_run_xdp(xdp_prog, &xdp);
+
+        off = xdp.data - orig_data;
+        if (off > 0)
+                __skb_pull(skb, off);
+        else if (off < 0)
+                __skb_push(skb, -off);
+
+        switch (act) {
+        case XDP_TX:
+                __skb_push(skb, mac_len);
+                /* fall through */
+        case XDP_PASS:
+                break;
+
+        default:
+                bpf_warn_invalid_xdp_action(act);
+                /* fall through */
+        case XDP_ABORTED:
+                trace_xdp_exception(skb->dev, xdp_prog, act);
+                /* fall through */
+        case XDP_DROP:
+        do_drop:
+                kfree_skb(skb);
+                break;
+        }
+
+        return act;
+}
+
+/* When doing generic XDP we have to bypass the qdisc layer and the
+ * network taps in order to match in-driver-XDP behavior.
+ */
+static void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog)
+{
+        struct net_device *dev = skb->dev;
+        struct netdev_queue *txq;
+        bool free_skb = true;
+        int cpu, rc;
+
+        txq = netdev_pick_tx(dev, skb, NULL);
+        cpu = smp_processor_id();
+        HARD_TX_LOCK(dev, txq, cpu);
+        if (!netif_xmit_stopped(txq)) {
+                rc = netdev_start_xmit(skb, dev, txq, 0);
+                if (dev_xmit_complete(rc))
+                        free_skb = false;
+        }
+        HARD_TX_UNLOCK(dev, txq);
+        if (free_skb) {
+                trace_xdp_exception(dev, xdp_prog, XDP_TX);
+                kfree_skb(skb);
+        }
+}
+
 static int netif_receive_skb_internal(struct sk_buff *skb)
 {
         int ret;
@@ -4262,6 +4382,21 @@ static int netif_receive_skb_internal(struct sk_buff *skb)
 
         rcu_read_lock();
 
+        if (static_key_false(&generic_xdp_needed)) {
+                struct bpf_prog *xdp_prog = rcu_dereference(skb->dev->xdp_prog);
+
+                if (xdp_prog) {
+                        u32 act = netif_receive_generic_xdp(skb, xdp_prog);
+
+                        if (act != XDP_PASS) {
+                                rcu_read_unlock();
+                                if (act == XDP_TX)
+                                        generic_xdp_tx(skb, xdp_prog);
+                                return NET_RX_DROP;
+                        }
+                }
+        }
+
 #ifdef CONFIG_RPS
         if (static_key_false(&rps_needed)) {
                 struct rps_dev_flow voidflow, *rflow = &voidflow;
@@ -4494,7 +4629,7 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
         enum gro_result ret;
         int grow;
 
-        if (!(skb->dev->features & NETIF_F_GRO))
+        if (netif_elide_gro(skb->dev))
                 goto normal;
 
         if (skb->csum_bad)
@@ -6723,6 +6858,7 @@ EXPORT_SYMBOL(dev_change_proto_down);
  */
 int dev_change_xdp_fd(struct net_device *dev, int fd, u32 flags)
 {
+        int (*xdp_op)(struct net_device *dev, struct netdev_xdp *xdp);
         const struct net_device_ops *ops = dev->netdev_ops;
         struct bpf_prog *prog = NULL;
         struct netdev_xdp xdp;
@@ -6730,14 +6866,16 @@ int dev_change_xdp_fd(struct net_device *dev, int fd, u32 flags)
 
         ASSERT_RTNL();
 
-        if (!ops->ndo_xdp)
-                return -EOPNOTSUPP;
+        xdp_op = ops->ndo_xdp;
+        if (!xdp_op || (flags & XDP_FLAGS_SKB_MODE))
+                xdp_op = generic_xdp_install;
+
         if (fd >= 0) {
                 if (flags & XDP_FLAGS_UPDATE_IF_NOEXIST) {
                         memset(&xdp, 0, sizeof(xdp));
                         xdp.command = XDP_QUERY_PROG;
 
-                        err = ops->ndo_xdp(dev, &xdp);
+                        err = xdp_op(dev, &xdp);
                         if (err < 0)
                                 return err;
                         if (xdp.prog_attached)
@@ -6753,7 +6891,7 @@ int dev_change_xdp_fd(struct net_device *dev, int fd, u32 flags)
         xdp.command = XDP_SETUP_PROG;
         xdp.prog = prog;
 
-        err = ops->ndo_xdp(dev, &xdp);
+        err = xdp_op(dev, &xdp);
         if (err < 0 && prog)
                 bpf_prog_put(prog);
 
@@ -7793,6 +7931,7 @@ EXPORT_SYMBOL(alloc_netdev_mqs);
 void free_netdev(struct net_device *dev)
 {
         struct napi_struct *p, *n;
+        struct bpf_prog *prog;
 
         might_sleep();
         netif_free_tx_queues(dev);
@@ -7811,6 +7950,12 @@ void free_netdev(struct net_device *dev)
         free_percpu(dev->pcpu_refcnt);
         dev->pcpu_refcnt = NULL;
 
+        prog = rcu_dereference_protected(dev->xdp_prog, 1);
+        if (prog) {
+                bpf_prog_put(prog);
+                static_key_slow_dec(&generic_xdp_needed);
+        }
+
         /*  Compatibility with error handling in drivers */
         if (dev->reg_state == NETREG_UNINITIALIZED) {
                 netdev_freemem(dev);
diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
index c98bbfbd26b8..814e58a3ce8b 100644
--- a/net/core/gro_cells.c
+++ b/net/core/gro_cells.c
@@ -13,7 +13,7 @@ int gro_cells_receive(struct gro_cells *gcells, struct sk_buff *skb)
         struct net_device *dev = skb->dev;
         struct gro_cell *cell;
 
-        if (!gcells->cells || skb_cloned(skb) || !(dev->features & NETIF_F_GRO))
+        if (!gcells->cells || skb_cloned(skb) || netif_elide_gro(dev))
                 return netif_rx(skb);
 
         cell = this_cpu_ptr(gcells->cells);
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 088f9c8b4196..9031a6c8bfa7 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -896,15 +896,13 @@ static size_t rtnl_port_size(const struct net_device *dev,
                 return port_self_size;
 }
 
-static size_t rtnl_xdp_size(const struct net_device *dev)
+static size_t rtnl_xdp_size(void)
 {
         size_t xdp_size = nla_total_size(0) +   /* nest IFLA_XDP */
-                          nla_total_size(1);    /* XDP_ATTACHED */
+                          nla_total_size(1) +   /* XDP_ATTACHED */
+                          nla_total_size(4);    /* XDP_FLAGS */
 
-        if (!dev->netdev_ops->ndo_xdp)
-                return 0;
-        else
-                return xdp_size;
+        return xdp_size;
 }
 
 static noinline size_t if_nlmsg_size(const struct net_device *dev,
@@ -943,7 +941,7 @@ static noinline size_t if_nlmsg_size(const struct net_device *dev,
                + nla_total_size(MAX_PHYS_ITEM_ID_LEN) /* IFLA_PHYS_PORT_ID */
                + nla_total_size(MAX_PHYS_ITEM_ID_LEN) /* IFLA_PHYS_SWITCH_ID */
                + nla_total_size(IFNAMSIZ) /* IFLA_PHYS_PORT_NAME */
-               + rtnl_xdp_size(dev) /* IFLA_XDP */
+               + rtnl_xdp_size() /* IFLA_XDP */
                + nla_total_size(1); /* IFLA_PROTO_DOWN */
 
 }
@@ -1251,23 +1249,35 @@ static int rtnl_fill_link_ifmap(struct sk_buff *skb, struct net_device *dev)
 
 static int rtnl_xdp_fill(struct sk_buff *skb, struct net_device *dev)
 {
-        struct netdev_xdp xdp_op = {};
         struct nlattr *xdp;
+        u32 xdp_flags = 0;
+        u8 val = 0;
         int err;
 
-        if (!dev->netdev_ops->ndo_xdp)
-                return 0;
         xdp = nla_nest_start(skb, IFLA_XDP);
         if (!xdp)
                 return -EMSGSIZE;
-        xdp_op.command = XDP_QUERY_PROG;
-        err = dev->netdev_ops->ndo_xdp(dev, &xdp_op);
-        if (err)
-                goto err_cancel;
-        err = nla_put_u8(skb, IFLA_XDP_ATTACHED, xdp_op.prog_attached);
+        if (rcu_access_pointer(dev->xdp_prog)) {
+                xdp_flags = XDP_FLAGS_SKB_MODE;
+                val = 1;
+        } else if (dev->netdev_ops->ndo_xdp) {
+                struct netdev_xdp xdp_op = {};
+
+                xdp_op.command = XDP_QUERY_PROG;
+                err = dev->netdev_ops->ndo_xdp(dev, &xdp_op);
+                if (err)
+                        goto err_cancel;
+                val = xdp_op.prog_attached;
+        }
+        err = nla_put_u8(skb, IFLA_XDP_ATTACHED, val);
         if (err)
                 goto err_cancel;
 
+        if (xdp_flags) {
+                err = nla_put_u32(skb, IFLA_XDP_FLAGS, xdp_flags);
+                if (err)
+                        goto err_cancel;
+        }
         nla_nest_end(skb, xdp);
         return 0;