dhcp-network: make clear that we are ANDing Fragment offset field with mask

Reading BPF assembly written as C macros is inherently difficult. Don't
make it harder than necessary and provide clearer explanation in the
comment.

diff --git a/src/libsystemd-network/dhcp-network.c b/src/libsystemd-network/dhcp-network.c
index 17020b0a00cb846a8560ee0fe34fad070dee5fc1..16411cc41a384aa082d29047d7a9f517cb1bf42b 100644 (file)
--- a/src/libsystemd-network/dhcp-network.c
+++ b/src/libsystemd-network/dhcp-network.c
@@ -46,7 +46,7 @@ int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link,
             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                          /* A == 0 ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, ip.frag_off)), /* A <- Flags + Fragment offset */
             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                          /* A == 0 ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, ip.frag_off)), /* A <- Flags + Fragment offset */

-            BPF_STMT(BPF_ALU + BPF_AND + BPF_K, 0x1fff),                           /* A <- A & 0x1fff */
+            BPF_STMT(BPF_ALU + BPF_AND + BPF_K, 0x1fff),                           /* A <- A & 0x1fff (Fragment offset) */

             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                          /* A == 0 ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, udp.dest)),    /* A <- UDP destination port */
             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                          /* A == 0 ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, udp.dest)),    /* A <- UDP destination port */