Re: [PATCH bpf-next v6 4/5] bpf: verifier: Support eliding map lookup nullness

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-12-19 at 21:09 -0700, Daniel Xu wrote:

lgtm, but please see a note below.

[...]

> +/* Returns constant key value if possible, else negative error */
> +static s64 get_constant_map_key(struct bpf_verifier_env *env,
> +				struct bpf_reg_state *key,
> +				u32 key_size)
> +{
> +	struct bpf_func_state *state = func(env, key);
> +	struct bpf_reg_state *reg;
> +	int slot, spi, off;
> +	int spill_size = 0;
> +	int zero_size = 0;
> +	int stack_off;
> +	int i, err;
> +	u8 *stype;
> +
> +	if (!env->bpf_capable)
> +		return -EOPNOTSUPP;
> +	if (key->type != PTR_TO_STACK)
> +		return -EOPNOTSUPP;
> +	if (!tnum_is_const(key->var_off))
> +		return -EOPNOTSUPP;
> +
> +	stack_off = key->off + key->var_off.value;
> +	slot = -stack_off - 1;
> +	spi = slot / BPF_REG_SIZE;
> +	off = slot % BPF_REG_SIZE;
> +	stype = state->stack[spi].slot_type;
> +
> +	/* First handle precisely tracked STACK_ZERO */
> +	for (i = off; i >= 0 && stype[i] == STACK_ZERO; i--)
> +		zero_size++;
> +	if (zero_size >= key_size)
> +		return 0;
> +
> +	/* Check that stack contains a scalar spill of expected size */
> +	if (!is_spilled_scalar_reg(&state->stack[spi]))
> +		return -EOPNOTSUPP;
> +	for (i = off; i >= 0 && stype[i] == STACK_SPILL; i--)
> +		spill_size++;
> +	if (spill_size != key_size)
> +		return -EOPNOTSUPP;
> +
> +	reg = &state->stack[spi].spilled_ptr;
> +	if (!tnum_is_const(reg->var_off))
> +		/* Stack value not statically known */
> +		return -EOPNOTSUPP;
> +
> +	/* We are relying on a constant value. So mark as precise
> +	 * to prevent pruning on it.
> +	 */
> +	bt_set_frame_slot(&env->bt, env->cur_state->curframe, spi);

I think env->cur_state->curframe is not always correct here.
It should be key->frameno, as key might point a few stack frames up.

> +	err = mark_chain_precision_batch(env);
> +	if (err < 0)
> +		return err;
> +
> +	return reg->var_off.value;
> +}
> +
>  static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>  			  struct bpf_call_arg_meta *meta,
>  			  const struct bpf_func_proto *fn,

[...]