Re: cBPF socket filters failing - inexplicably?

[Tom Cook]

Another factoid to add to this:  I captured all traffic on an
interface while the test program was running using

tcpdump -i wlo1 -w capture.pcap

observing that multiple packets got through the filter.  I then built
the bpf_dbg program from the kernel source tree and ran the same
filter and capture file through it:

$ tools/bpf_dbg
> load bpf 1,6 0 0 0
> load pcap capture.pcap
> run
bpf passes:0 fails:269288

So bpf_dbg thinks the filter is correct; it's only when the filter is
attached to an actual socket that it fails occasionally.

Regards,
Tom

On Wed, Jan 6, 2021 at 10:07 AM Tom Cook <tom.k.cook@xxxxxxxxx> wrote:
>
> Just to note I have also reproduced this on a 5.10.0 kernel.
>
> On Tue, Jan 5, 2021 at 1:42 PM Tom Cook <tom.k.cook@xxxxxxxxx> wrote:
> >
> > 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();
> >     }
> > }