[PATCH nftables] Fix register allocation for EXPR_SET_ELEM

["Anders K. Pedersen | Cohaesio" <akp@cohaesio.com>]

From: Anders K. Pedersen <akp@cohaesio.com>

I noticed that while

 # nft add rule ip6 filter postrouting \
	flow table acct_out \{ meta iif . ip6 saddr timeout 600s counter \}

works, the opposite order for the concatenated expressions fails:

 # nft add rule ip6 filter postrouting \
	flow table acct_out \{ ip6 saddr . meta iif timeout 600s counter \}
 nft: netlink_linearize.c:634: netlink_gen_expr: Assertion `dreg < ctx->reg_low' failed.

I traced this down to get_register() and release_register(), where the
EXPR_CONCAT handling isn't hit, when it's embedded in EXPR_SET_ELEM, and
fixed it similarly to how EXPR_SET_ELEM is handled in netlink_gen_expr().

Signed-off-by: Anders K. Pedersen <akp@cohaesio.com>
---
 src/netlink_linearize.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/netlink_linearize.c b/src/netlink_linearize.c
--- a/src/netlink_linearize.c
+++ b/src/netlink_linearize.c
@@ -73,6 +73,9 @@ static void __release_register(struct netlink_linearize_ctx *ctx,
 static enum nft_registers get_register(struct netlink_linearize_ctx *ctx,
 				       const struct expr *expr)
 {
+	if (expr && expr->ops->type == EXPR_SET_ELEM)
+		return get_register(ctx, expr->key);
+
 	if (expr && expr->ops->type == EXPR_CONCAT)
 		return __get_register(ctx, expr->len);
 	else
@@ -82,6 +85,9 @@ static enum nft_registers get_register(struct netlink_linearize_ctx *ctx,
 static void release_register(struct netlink_linearize_ctx *ctx,
 			     const struct expr *expr)
 {
+	if (expr && expr->ops->type == EXPR_SET_ELEM)
+		return release_register(ctx, expr->key);
+
 	if (expr && expr->ops->type == EXPR_CONCAT)
 		__release_register(ctx, expr->len);
 	else