Re: [PATCH bpf-next v6 10/10] selftests/bpf: verify lsm_cgroup struct sock access

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

On Mon, May 9, 2022 at 4:38 PM Stanislav Fomichev <sdf@google.com> wrote:
>
> On Mon, May 9, 2022 at 2:54 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Fri, Apr 29, 2022 at 2:16 PM Stanislav Fomichev <sdf@google.com> wrote:
> > >
> > > sk_priority & sk_mark are writable, the rest is readonly.
> > >
> > > Add new ldx_offset fixups to lookup the offset of struct field.
> > > Allow using test.kfunc regardless of prog_type.
> > >
> > > One interesting thing here is that the verifier doesn't
> > > really force me to add NULL checks anywhere :-/
> > >
> > > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > > ---
> > >  tools/testing/selftests/bpf/test_verifier.c   | 54 ++++++++++++++++++-
> > >  .../selftests/bpf/verifier/lsm_cgroup.c       | 34 ++++++++++++
> > >  2 files changed, 87 insertions(+), 1 deletion(-)
> > >  create mode 100644 tools/testing/selftests/bpf/verifier/lsm_cgroup.c
> > >
> >
> > [...]
> >
> > > diff --git a/tools/testing/selftests/bpf/verifier/lsm_cgroup.c b/tools/testing/selftests/bpf/verifier/lsm_cgroup.c
> > > new file mode 100644
> > > index 000000000000..af0efe783511
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/verifier/lsm_cgroup.c
> > > @@ -0,0 +1,34 @@
> > > +#define SK_WRITABLE_FIELD(tp, field, size, res) \
> > > +{ \
> > > +       .descr = field, \
> > > +       .insns = { \
> > > +               /* r1 = *(u64 *)(r1 + 0) */ \
> > > +               BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, 0), \
> > > +               /* r1 = *(u64 *)(r1 + offsetof(struct socket, sk)) */ \
> > > +               BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, 0), \
> > > +               /* r2 = *(u64 *)(r1 + offsetof(struct sock, <field>)) */ \
> > > +               BPF_LDX_MEM(size, BPF_REG_2, BPF_REG_1, 0), \
> > > +               /* *(u64 *)(r1 + offsetof(struct sock, <field>)) = r2 */ \
> > > +               BPF_STX_MEM(size, BPF_REG_1, BPF_REG_2, 0), \
> > > +               BPF_MOV64_IMM(BPF_REG_0, 1), \
> > > +               BPF_EXIT_INSN(), \
> > > +       }, \
> > > +       .result = res, \
> > > +       .errstr = res ? "no write support to 'struct sock' at off" : "", \
> > > +       .prog_type = BPF_PROG_TYPE_LSM, \
> > > +       .expected_attach_type = BPF_LSM_CGROUP, \
> > > +       .kfunc = "socket_post_create", \
> > > +       .fixup_ldx = { \
> > > +               { "socket", "sk", 1 }, \
> > > +               { tp, field, 2 }, \
> > > +               { tp, field, 3 }, \
> > > +       }, \
> > > +}
> > > +
> > > +SK_WRITABLE_FIELD("sock_common", "skc_family", BPF_H, REJECT),
> > > +SK_WRITABLE_FIELD("sock", "sk_sndtimeo", BPF_DW, REJECT),
> > > +SK_WRITABLE_FIELD("sock", "sk_priority", BPF_W, ACCEPT),
> > > +SK_WRITABLE_FIELD("sock", "sk_mark", BPF_W, ACCEPT),
> > > +SK_WRITABLE_FIELD("sock", "sk_pacing_rate", BPF_DW, REJECT),
> > > +
> >
> > have you tried writing it as C program and adding the test to
> > test_progs? Does something not work there?
>
> Seems like it should work, I don't see any issues with writing 5
> programs to test each field.
> But test_verified still feels like a better fit? Any reason in
> particular you'd prefer test_progs over test_verifier?

Adding that fixup_ldx->strct special handling didn't feel like the
best fit, tbh. test_progs is generally much nicer to deal with in
terms of CI and in terms of comprehending what's going on and
supporting the code longer term.