[PATCH bpf-next 4/9] xdp: Implement chain call logic to support multiple programs on one interface

["Toke Høiland-Jørgensen" <toke@redhat.com>]

From: Toke Høiland-Jørgensen <toke@redhat.com>

This adds for executing multiple XDP programs on a single interface using
the chain call map type introduced in the previous commits. The logic is
added as an extension of bpf_prog_run_xdp() which will loop through the
call sequence specified by the chain call map installed on the current
interface. The overhead when no chain map is installed is only a single
pointer dereference.

The total call sequence length is limited to 32 programs, and the call
sequence will be aborted and XDP_ABORTED returned if it is exceeded.
Likewise, if a program in the sequence returns XDP_ABORTED, the whole
sequence will be aborted immediately, on the assumption that this is a
fault somewhere in the system.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 include/linux/filter.h |   26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 2ce57645f3cd..8a79ddd4f7b5 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -693,6 +693,7 @@ static inline u32 bpf_prog_run_clear_cb(const struct bpf_prog *prog,
 	return res;
 }
 
+#define BPF_XDP_MAX_CHAIN_CALLS 32
 static __always_inline u32 bpf_prog_run_xdp(const struct bpf_prog *prog,
 					    struct xdp_buff *xdp)
 {
@@ -702,7 +703,30 @@ static __always_inline u32 bpf_prog_run_xdp(const struct bpf_prog *prog,
 	 * already takes rcu_read_lock() when fetching the program, so
 	 * it's not necessary here anymore.
 	 */
-	return BPF_PROG_RUN(prog, xdp);
+
+	int i = BPF_XDP_MAX_CHAIN_CALLS;
+	struct bpf_map *chain_map;
+	u32 ret;
+
+	chain_map = rcu_dereference(xdp->rxq->dev->xdp_chain_map);
+	if (!chain_map)
+		return BPF_PROG_RUN(prog, xdp);
+
+	do {
+		if (!--i) {
+			ret = XDP_ABORTED;
+			goto out;
+		}
+
+		ret = BPF_PROG_RUN(prog, xdp);
+		if (ret == XDP_ABORTED)
+			goto out;
+
+		prog = bpf_xdp_chain_map_get_prog(chain_map, prog->aux->id, ret);
+	} while(prog);
+
+out:
+	return ret;
 }
 
 static inline u32 bpf_prog_insn_size(const struct bpf_prog *prog)