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

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

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?

> +#undef SK_WRITABLE_FIELD
> --
> 2.36.0.464.gb9c8b46e94-goog
>