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Date:   Thu, 12 May 2022 10:11:27 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH bpf-next v6 10/10] selftests/bpf: verify lsm_cgroup struct
 sock access

On Wed, May 11, 2022 at 8:38 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, May 10, 2022 at 10:31 AM Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> > On Mon, May 9, 2022 at 4:44 PM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Mon, May 9, 2022 at 4:38 PM Stanislav Fomichev <sdf@...gle.com> wrote:
> > > >
> > > > On Mon, May 9, 2022 at 2:54 PM Andrii Nakryiko
> > > > <andrii.nakryiko@...il.com> wrote:
> > > > >
> > > > > On Fri, Apr 29, 2022 at 2:16 PM Stanislav Fomichev <sdf@...gle.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@...gle.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.
> >
> > This is not new, right? We already have a bunch of fixup_xxx things.
>
> I'm not saying it's wrong, but we don't have to keep adding extra
> custom fixup_xxx things and having hand crafted assembly test cases if
> we can do C tests, right? BPF assembly tests are sometimes necessary
> if we need to craft some special conditions which are hard to
> guarantee from Clang side during C to BPF assembly translation. But
> this one doesn't seem to be the case.
>
> > I can try to move this into test_progs in largely the same manner if
> > you prefer, having a C file per field seems like an overkill.
>
> You don't need a separate C file for each case. See what Joanne does
> with SEC("?...") for dynptr tests, or what Kumar did for his kptr
> tests. You can put multiple negative tests as separate BPF programs in
> one file with auto-load disabled through SEC("?...") and then
> open/load skeleton each time for each program, one at a time.

I'm gonna start with keeping the assembly, but moving it into
test_progs. I think it looks a bit nicer than the fixup stuff I'm
currently doing and I like how everything is in the same place. So
please yell at me if you still don't like it next time I send it out
and I'll try to explore SEC("?...").

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