netdev - Re: cBPF socket filters failing

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Date:   Fri, 15 Jan 2021 15:16:37 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Tom Cook <tom.k.cook@...il.com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: cBPF socket filters failing - inexplicably?

On Fri, Jan 15, 2021 at 7:52 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> Adding appropriate mailing list to cc...
>
> My wild guess is that as soon as socket got created:
> socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
> the packets were already queued to it.
> So later setsockopt() is too late to filter.
>
> Eric, thoughts?

Exactly, this is what happens.

I do not know how tcpdump and other programs deal with this.

Maybe by setting a small buffer size, or draining the queue.

>
> On Wed, Jan 6, 2021 at 6:55 AM Tom Cook <tom.k.cook@...il.com> wrote:
> >
> > 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@...il.com> 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@...il.com> 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();
> > > >     }
> > > > }