bpf: xdp: allow offloads to store into rx_queue_index

It's fairly easy for offloaded XDP programs to select the RX queue
packets go to.  We need a way of expressing this in the software.
Allow write to the rx_queue_index field of struct xdp_md for
device-bound programs.

Skip convert_ctx_access callback entirely for offloads.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NQuentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

include/linux/bpf.h

@@ -627,7 +627,7 @@ bool bpf_offload_dev_match(struct bpf_prog *prog, struct bpf_map *map);
 #if defined(CONFIG_NET) && defined(CONFIG_BPF_SYSCALL)
 int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr);
 
-static inline bool bpf_prog_is_dev_bound(struct bpf_prog_aux *aux)
+static inline bool bpf_prog_is_dev_bound(const struct bpf_prog_aux *aux)
 {
 	return aux->offload_requested;
 }

kernel/bpf/verifier.c

@@ -5215,7 +5215,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		}
 	}
 
-	if (!ops->convert_ctx_access)
+	if (!ops->convert_ctx_access || bpf_prog_is_dev_bound(env->prog->aux))
 		return 0;
 
 	insn = env->prog->insnsi + delta;

net/core/filter.c

@@ -4645,8 +4645,15 @@ static bool xdp_is_valid_access(int off, int size,
 				const struct bpf_prog *prog,
 				struct bpf_insn_access_aux *info)
 {
-	if (type == BPF_WRITE)
+	if (type == BPF_WRITE) {
+		if (bpf_prog_is_dev_bound(prog->aux)) {
+			switch (off) {
+			case offsetof(struct xdp_md, rx_queue_index):
+				return __is_valid_xdp_access(off, size);
+			}
+		}
 		return false;
+	}
 
 	switch (off) {
 	case offsetof(struct xdp_md, data):