btf: fix resolving BTF_KIND_VAR after ARRAY, STRUCT, UNION, PTR

btf_datasec_resolve contains a bug that causes the following BTF
to fail loading:

    [1] DATASEC a size=2 vlen=2
        type_id=4 offset=0 size=1
        type_id=7 offset=1 size=1
    [2] INT (anon) size=1 bits_offset=0 nr_bits=8 encoding=(none)
    [3] PTR (anon) type_id=2
    [4] VAR a type_id=3 linkage=0
    [5] INT (anon) size=1 bits_offset=0 nr_bits=8 encoding=(none)
    [6] TYPEDEF td type_id=5
    [7] VAR b type_id=6 linkage=0

This error message is printed during btf_check_all_types:

    [1] DATASEC a size=2 vlen=2
        type_id=7 offset=1 size=1 Invalid type

By tracing btf_*_resolve we can pinpoint the problem:

    btf_datasec_resolve(depth: 1, type_id: 1, mode: RESOLVE_TBD) = 0
        btf_var_resolve(depth: 2, type_id: 4, mode: RESOLVE_TBD) = 0
            btf_ptr_resolve(depth: 3, type_id: 3, mode: RESOLVE_PTR) = 0
        btf_var_resolve(depth: 2, type_id: 4, mode: RESOLVE_PTR) = 0
    btf_datasec_resolve(depth: 1, type_id: 1, mode: RESOLVE_PTR) = -22

The last invocation of btf_datasec_resolve should invoke btf_var_resolve
by means of env_stack_push, instead it returns EINVAL. The reason is that
env_stack_push is never executed for the second VAR.

    if (!env_type_is_resolve_sink(env, var_type) &&
        !env_type_is_resolved(env, var_type_id)) {
        env_stack_set_next_member(env, i + 1);
        return env_stack_push(env, var_type, var_type_id);
    }

env_type_is_resolve_sink() changes its behaviour based on resolve_mode.
For RESOLVE_PTR, we can simplify the if condition to the following:

    (btf_type_is_modifier() || btf_type_is_ptr) && !env_type_is_resolved()

Since we're dealing with a VAR the clause evaluates to false. This is
not sufficient to trigger the bug however. The log output and EINVAL
are only generated if btf_type_id_size() fails.

    if (!btf_type_id_size(btf, &type_id, &type_size)) {
        btf_verifier_log_vsi(env, v->t, vsi, "Invalid type");
        return -EINVAL;
    }

Most types are sized, so for example a VAR referring to an INT is not a
problem. The bug is only triggered if a VAR points at a modifier. Since
we skipped btf_var_resolve that modifier was also never resolved, which
means that btf_resolved_type_id returns 0 aka VOID for the modifier.
This in turn causes btf_type_id_size to return NULL, triggering EINVAL.

To summarise, the following conditions are necessary:

- VAR pointing at PTR, STRUCT, UNION or ARRAY
- Followed by a VAR pointing at TYPEDEF, VOLATILE, CONST, RESTRICT or
  TYPE_TAG

The fix is to reset resolve_mode to RESOLVE_TBD before attempting to
resolve a VAR from a DATASEC.

Fixes: 1dc92851 ("bpf: kernel side support for BTF Var and DataSec")
Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/r/20230306112138.155352-2-lmb@isovalent.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/btf.c

@@ -4569,6 +4569,7 @@ static int btf_datasec_resolve(struct btf_verifier_env *env,
 	struct btf *btf = env->btf;
 	u16 i;
 
+	env->resolve_mode = RESOLVE_TBD;
 	for_each_vsi_from(i, v->next_member, v->t, vsi) {
 		u32 var_type_id = vsi->type, type_id, type_size = 0;
 		const struct btf_type *var_type = btf_type_by_id(env->btf,