udhcpc: use more correct, and more importantly, more understandable BPF code
Signed-off-by: Vladislav Grishenko <themiron@mail.ru>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 7e5ab61..d97a404 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -714,22 +714,25 @@
*
* TODO: make conditional?
*/
-#define SERVER_AND_CLIENT_PORTS ((67 << 16) + 68)
static const struct sock_filter filter_instr[] = {
- /* check for udp */
+ /* load 9th byte (protocol) */
BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 9),
- BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, IPPROTO_UDP, 2, 0), /* L5, L1, is UDP? */
- /* ugly check for arp on ethernet-like and IPv4 */
- BPF_STMT(BPF_LD|BPF_W|BPF_ABS, 2), /* L1: */
- BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, 0x08000604, 3, 4), /* L3, L4 */
- /* skip IP header */
- BPF_STMT(BPF_LDX|BPF_B|BPF_MSH, 0), /* L5: */
- /* check udp source and destination ports */
- BPF_STMT(BPF_LD|BPF_W|BPF_IND, 0),
- BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, SERVER_AND_CLIENT_PORTS, 0, 1), /* L3, L4 */
- /* returns */
- BPF_STMT(BPF_RET|BPF_K, 0x0fffffff ), /* L3: pass */
- BPF_STMT(BPF_RET|BPF_K, 0), /* L4: reject */
+ /* jump to L1 if it is IPPROTO_UDP, else to L4 */
+ BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, IPPROTO_UDP, 0, 6),
+ /* L1: load halfword from offset 6 (flags and frag offset) */
+ BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 6),
+ /* jump to L4 if any bits in frag offset field are set, else to L2 */
+ BPF_JUMP(BPF_JMP|BPF_JSET|BPF_K, 0x1fff, 4, 0),
+ /* L2: skip IP header (load index reg with header len) */
+ BPF_STMT(BPF_LDX|BPF_B|BPF_MSH, 0),
+ /* load udp destination port from halfword[header_len + 2] */
+ BPF_STMT(BPF_LD|BPF_H|BPF_IND, 2),
+ /* jump to L3 if udp dport is CLIENT_PORT, else to L4 */
+ BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, 68, 0, 1),
+ /* L3: accept packet */
+ BPF_STMT(BPF_RET|BPF_K, 0xffffffff),
+ /* L4: discard packet */
+ BPF_STMT(BPF_RET|BPF_K, 0),
};
static const struct sock_fprog filter_prog = {
.len = sizeof(filter_instr) / sizeof(filter_instr[0]),
@@ -742,18 +745,19 @@
fd = xsocket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP));
log1("Got raw socket fd %d", fd); //log2?
- if (SERVER_PORT == 67 && CLIENT_PORT == 68) {
- /* Use only if standard ports are in use */
+ sock.sll_family = AF_PACKET;
+ sock.sll_protocol = htons(ETH_P_IP);
+ sock.sll_ifindex = ifindex;
+ xbind(fd, (struct sockaddr *) &sock, sizeof(sock));
+
+ if (CLIENT_PORT == 68) {
+ /* Use only if standard port is in use */
/* Ignoring error (kernel may lack support for this) */
if (setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER, &filter_prog,
sizeof(filter_prog)) >= 0)
log1("Attached filter to raw socket fd %d", fd); // log?
}
- sock.sll_family = AF_PACKET;
- sock.sll_protocol = htons(ETH_P_IP);
- sock.sll_ifindex = ifindex;
- xbind(fd, (struct sockaddr *) &sock, sizeof(sock));
log1("Created raw socket");
return fd;