src: add dynamic register allocation infrastructure

Starting Linux kernel 5.18-rc, operations on registers that already
contain the expected data are turned into noop.

Track operation on registers to use the same register through
nftnl_reg_get(). This patch introduces an LRU eviction strategy when all
the registers are in used.

nftnl_reg_get_scratch() is used to allocate a register as scratchpad
area: no tracking is performed in this case, although register eviction
might occur.

Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/include/regs.h b/include/regs.h
new file mode 100644
index 0000000..5312f60
--- /dev/null
+++ b/include/regs.h
@@ -0,0 +1,32 @@
+#ifndef _LIBNFTNL_REGS_INTERNAL_H_
+#define _LIBNFTNL_REGS_INTERNAL_H_
+
+enum nftnl_expr_type {
+	NFT_EXPR_UNSPEC	= 0,
+	NFT_EXPR_PAYLOAD,
+	NFT_EXPR_META,
+};
+
+struct nftnl_reg {
+	enum nftnl_expr_type				type;
+	uint32_t					len;
+	uint64_t					genid;
+	uint8_t						word;
+	union {
+		struct {
+			enum nft_meta_keys		key;
+		} meta;
+		struct {
+			enum nft_payload_bases		base;
+			uint32_t			offset;
+		} payload;
+	};
+};
+
+struct nftnl_regs {
+	uint32_t		num_regs;
+	struct nftnl_reg	*reg;
+	uint64_t		genid;
+};
+
+#endif