Re: [PATCH bpf-next 2/2] selftests/bpf: add exception handling selftests for tp_bpf program

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Nov 4, 2021 at 3:56 PM Alan Maguire <alan.maguire@oracle.com> wrote:
>
>
>
> On Wed, 3 Nov 2021, Andrii Nakryiko wrote:
>
> > On Wed, Nov 3, 2021 at 2:50 AM Alan Maguire <alan.maguire@oracle.com> wrote:
> > >
> > > Exception handling is triggered in BPF tracing programs when
> > > a NULL pointer is dereferenced; the exception handler zeroes the
> > > target register and execution of the BPF program progresses.
> > >
> > > To test exception handling then, we need to trigger a NULL pointer
> > > dereference for a field which should never be zero; if it is, the
> > > only explanation is the exception handler ran.  The skb->sk is
> > > the NULL pointer chosen (for a ping received for 127.0.0.1 there
> > > is no associated socket), and the sk_sndbuf size is chosen as the
> > > "should never be 0" field.  Test verifies sk is NULL and sk_sndbuf
> > > is zero.
> > >
> > > Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> > > ---
> > >  tools/testing/selftests/bpf/prog_tests/exhandler.c | 45 ++++++++++++++++++++++
> > >  tools/testing/selftests/bpf/progs/exhandler_kern.c | 35 +++++++++++++++++
> > >  2 files changed, 80 insertions(+)
> > >  create mode 100644 tools/testing/selftests/bpf/prog_tests/exhandler.c
> > >  create mode 100644 tools/testing/selftests/bpf/progs/exhandler_kern.c
> > >
> > > diff --git a/tools/testing/selftests/bpf/prog_tests/exhandler.c b/tools/testing/selftests/bpf/prog_tests/exhandler.c
> > > new file mode 100644
> > > index 0000000..5999498
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/prog_tests/exhandler.c
> <snip>
> > > +
> > > +       bss = skel->bss;
> >
> > nit: you don't need to have a separate variable for that,
> > skel->bss->exception_triggered in below check would be just as
> > readable
> >
>
> sure, will do.
>
> > > +
> > > +       err = exhandler_kern__attach(skel);
> > > +       if (CHECK(err, "attach", "attach failed: %d\n", err))
> > > +               goto cleanup;
> > > +
> > > +       if (CHECK(SYSTEM("ping -c 1 127.0.0.1"),
> >
> > Is there some other tracepoint or kernel function that could be used
> > for testing and triggered without shelling out to ping binary? This
> > hurts test isolation and will make it or some other ping-using
> > selftests spuriously fail when running in parallel test mode (i.e.,
> > sudo ./test_progs -j).
>
> I've got a new version of this working which uses a fork() in
> combination with tp_btf/task_newtask ; the new task will have
> a NULL task->task_works pointer, but if it wasn't NULL it
> would have to point at a struct callback_head containing a
> non-NULL callback function. So we can verify that
> task->task_works and task->task_works->func are NULL to ensure
> exception triggered instead.  That should interfere
> less with other parallel tests hopefully?

Yeah, tracing a fork would be better, thanks!. Make sure you are
filtering by pid, to avoid accidentally tripping on some unrelated
fork.

>
> >
> > > +                 "ping localhost",
> > > +                 "ping localhost failed\n"))
> > > +               goto cleanup;
> > > +
> > > +       if (CHECK(bss->exception_triggered == 0,
> >
> > please use ASSERT_EQ() instead, CHECK()s are kind of deprecated for new tests
> >
>
>
> sure, will do.
>
> > > diff --git a/tools/testing/selftests/bpf/progs/exhandler_kern.c b/tools/testing/selftests/bpf/progs/exhandler_kern.c
> > > new file mode 100644
> > > index 0000000..4049450
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/progs/exhandler_kern.c
> > > @@ -0,0 +1,35 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/* Copyright (c) 2021, Oracle and/or its affiliates. */
> > > +
> > > +#include "vmlinux.h"
> > > +
> > > +#include <bpf/bpf_helpers.h>
> > > +#include <bpf/bpf_tracing.h>
> > > +#include <bpf/bpf_core_read.h>
> > > +
> > > +char _license[] SEC("license") = "GPL";
> > > +
> > > +unsigned int exception_triggered;
> > > +
> > > +/* TRACE_EVENT(netif_rx,
> > > + *         TP_PROTO(struct sk_buff *skb),
> > > + */
> > > +SEC("tp_btf/netif_rx")
> > > +int BPF_PROG(trace_netif_rx, struct sk_buff *skb)
> > > +{
> > > +       struct sock *sk;
> > > +       int sndbuf;
> > > +
> > > +       /* To verify we hit an exception we dereference skb->sk->sk_sndbuf;
> > > +        * sndbuf size should never be zero, so if it is we know the exception
> > > +        * handler triggered and zeroed the destination register.
> > > +        */
> > > +       __builtin_preserve_access_index(({
> > > +               sk = skb->sk;
> > > +               sndbuf = sk->sk_sndbuf;
> > > +       }));
> >
> > you don't need __builtin_preserve_access_index(({ }) region, because
> > vmlinux.h already annotates all the types with preserve_access_index
> > attribute
> >
>
> ah, great, I missed that somehow. Thanks!
>
> Alan