In the course of tracking down a defect in some existing software, I've found the failure demonstrated by the short program below. Essentially, a cBPF program that just rejects every frame (ie always returns zero) and is attached to a socket using setsockopt(SOL_SOCKET, SO_ATTACH_FILTER, ...) still occasionally lets frames through to userspace.
The code is based on the first example in Documentation/networking/filter.txt, except that I've changed the content of the filter program and added a timeout on the socket. To reproduce the problem: # gcc test.c -o test # sudo ./test ... and in another console start a large network operation. In my case, I copied a ~300MB core file I had lying around to another host on the LAN. The test code should print the string "Failed to read from socket" 100 times. In practice, it produces about 10% "Received packet with ethertype..." messages. I've observed the same result on Ubuntu amd64 glibc system running a 5.9.0 kernel and also on Alpine arm64v8 muslc system running a 4.9.1 kernel. I've written test code in both C and Python. I'm fairly sure this is not something I'm doing wrong - but very keen to have things thrown at me if it is. Regards, Tom Cook #include <stdio.h> #include <sys/socket.h> #include <sys/types.h> #include <arpa/inet.h> #include <linux/if_ether.h> #include <linux/filter.h> #include <stdint.h> #include <unistd.h> struct sock_filter code[] = { { 0x06, 0, 0, 0x00 } /* BPF_RET | BPF_K 0 0 0 */ }; struct sock_fprog bpf = { .len = 1, .filter = code, }; void test() { uint8_t buf[2048]; int sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL)); if (sock < 0) { printf("Failed to open socket\n"); return; } int ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf)); if (ret < 0) { printf("Failed to set socket filter\n"); return; } struct timeval tv = { .tv_sec = 1 }; ret = setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv)); if (ret < 0) { printf("Failed to set socket timeout\n"); return; } ssize_t count = recv(sock, buf, 2048, 0); if (count <= 0) { printf("Failed to read from socket\n"); return; } close(sock); uint16_t *ethertype = (short*)(buf + 12); uint8_t *proto = (unsigned char *)(buf + 23); uint16_t *dport = (uint16_t *)(buf + 14 + 20); printf("Received packet with ethertype 0x%04hu, protocol 0x%02hhu and dport 0x%04hu\n", *ethertype, *proto, *dport); } int main() { for (size_t ii = 0; ii < 100; ++ii) { test(); } }