dhcp: shorten BPF code a bit

yuwata · yuwata · commit ea51deb188e5 · 2021-10-21T22:39:53.000+09:00
- replace `A = k` and `X = A` with `X = k`,
- replace `A ^= X` and `A == 0` with `A == X`.
- use UINT32_MAX when accept a packet
diff --git a/src/libsystemd-network/dhcp-network.c b/src/libsystemd-network/dhcp-network.c
@@ -58,24 +58,20 @@ static int _bind_raw_socket(int ifindex, union sockaddr_union *link,
 
                 /* We only support MAC address length to be either 0 or 6 (ETH_ALEN). Optionally
                  * compare chaddr for ETH_ALEN bytes. */
-                BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETH_ALEN, 0, 12),                                  /* A (the MAC address length) == ETH_ALEN ? */
-                BPF_STMT(BPF_LD + BPF_IMM, unaligned_read_be32(&eth_mac->ether_addr_octet[0])),        /* A <- 4 bytes of client's MAC */
-                BPF_STMT(BPF_MISC + BPF_TAX, 0),                                                       /* X <- A */
+                BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETH_ALEN, 0, 8),                                   /* A (the MAC address length) == ETH_ALEN ? */
+                BPF_STMT(BPF_LDX + BPF_IMM, unaligned_read_be32(&eth_mac->ether_addr_octet[0])),       /* X <- 4 bytes of client's MAC */
                 BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(DHCPPacket, dhcp.chaddr)),                 /* A <- 4 bytes of MAC from dhcp.chaddr */
-                BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0),                                                /* A xor X */
-                BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                                          /* A == 0 ? */
+                BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_X, 0, 1, 0),                                          /* A == X ? */
                 BPF_STMT(BPF_RET + BPF_K, 0),                                                          /* ignore */
-                BPF_STMT(BPF_LD + BPF_IMM, unaligned_read_be16(&eth_mac->ether_addr_octet[4])),        /* A <- remainder of client's MAC */
-                BPF_STMT(BPF_MISC + BPF_TAX, 0),                                                       /* X <- A */
+                BPF_STMT(BPF_LDX + BPF_IMM, unaligned_read_be16(&eth_mac->ether_addr_octet[4])),       /* X <- remainder of client's MAC */
                 BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, dhcp.chaddr) + 4),             /* A <- remainder of MAC from dhcp.chaddr */
-                BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0),                                                /* A xor X */
-                BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                                          /* A == 0 ? */
+                BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_X, 0, 1, 0),                                          /* A == X ? */
                 BPF_STMT(BPF_RET + BPF_K, 0),                                                          /* ignore */
 
                 BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(DHCPPacket, dhcp.magic)),  /* A <- DHCP magic cookie */
                 BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, DHCP_MAGIC_COOKIE, 1, 0),          /* cookie == DHCP magic cookie ? */
                 BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
-                BPF_STMT(BPF_RET + BPF_K, 65535),                                      /* return all */
+                BPF_STMT(BPF_RET + BPF_K, UINT32_MAX),                                 /* accept */
         };
         struct sock_fprog fprog = {
                 .len = ELEMENTSOF(filter),
