Re: [PATCH 4/9] bpf: Allow nested BTF object to be refferenced by BTF object + offset

Andrii Nakryiko
2020-05-14T22:32:42Z

On Wed, May 6, 2020 at 6:31 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding btf_struct_address function that takes 2 BTF objects
> and offset as arguments and checks wether object A is nested
> in object B on given offset.
>
> This function is be used when checking the helper function
> PTR_TO_BTF_ID arguments. If the argument has an offset value,
> the btf_struct_address will check if the final address is
> the expected BTF ID.
>
> This way we can access nested BTF objects under PTR_TO_BTF_ID
> pointer type and pass them to helpers, while they still point
> to valid kernel BTF objects.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> include/linux/bpf.h | 3 ++
> kernel/bpf/btf.c | 69 +++++++++++++++++++++++++++++++++++++++++++
> kernel/bpf/verifier.c | 32 +++++++++++++-------
> 3 files changed, 94 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 1262ec460ab3..bc589cdd8c34 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1213,6 +1213,9 @@ int btf_struct_access(struct bpf_verifier_log *log,
> const struct btf_type *t, int off, int size,
> enum bpf_access_type atype,
> u32 *next_btf_id);
> +int btf_struct_address(struct bpf_verifier_log *log,
> + const struct btf_type *t,
> + u32 off, u32 exp_id);
> int btf_resolve_helper_id(struct bpf_verifier_log *log,
> const struct bpf_func_proto *fn, int);
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index a2cfba89a8e1..07f22469acab 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -4004,6 +4004,75 @@ int btf_struct_access(struct bpf_verifier_log *log,
> return -EINVAL;
> }
>
> +int btf_struct_address(struct bpf_verifier_log *log,
> + const struct btf_type *t,
> + u32 off, u32 exp_id)

The logic in this function is quite tricky and overlaps heavily with
btf_struct_access. You are already missing flexible array support that
Yonghong added recently. Let's see if it's possible to extract all
this "find field in a struct by offset" logic into reusable function?

> +{
> + u32 i, moff, mtrue_end, msize = 0;
> + const struct btf_member *member;
> + const struct btf_type *mtype;
> + const char *tname, *mname;
> +
> +again:
> + tname = __btf_name_by_offset(btf_vmlinux, t->name_off);
> + if (!btf_type_is_struct(t)) {
> + bpf_log(log, "Type '%s' is not a struct\n", tname);
> + return -EINVAL;
> + }
> +
> + if (off > t->size) {

>=, actually?

> + bpf_log(log, "address beyond struct %s at off %u size %u\n",
> + tname, off, t->size);
> + return -EACCES;
> + }
> +

[...]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 70ad009577f8..b988df5ada20 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3665,6 +3665,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> {
> struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
> enum bpf_reg_type expected_type, type = reg->type;
> + const struct btf_type *btf_type;
> int err = 0;
>
> if (arg_type == ARG_DONTCARE)
> @@ -3743,17 +3744,28 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> expected_type = PTR_TO_BTF_ID;
> if (type != expected_type)
> goto err_type;
> - if (reg->btf_id != meta->btf_id) {
> - verbose(env, "Helper has type %s got %s in R%d\n",
> - kernel_type_name(meta->btf_id),
> - kernel_type_name(reg->btf_id), regno);
> + if (reg->off) {

well, off==0 can still point to (arbitrarily) nested structs that are
first member of outer struct... So I guess it would be better to
search for inner struct if btf_id is unexpected regardless of off
value?

> + btf_type = btf_type_by_id(btf_vmlinux, reg->btf_id);
> + if (btf_struct_address(&env->log, btf_type, reg->off, meta->btf_id)) {
> + verbose(env, "Helper has type %s got %s in R%d, off %d\n",
> + kernel_type_name(meta->btf_id),
> + kernel_type_name(reg->btf_id), regno, reg->off);
>
> - return -EACCES;
> - }
> - if (!tnum_is_const(reg->var_off) || reg->var_off.value || reg->off) {
> - verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
> - regno);
> - return -EACCES;
> + return -EACCES;
> + }
> + } else {
> + if (reg->btf_id != meta->btf_id) {
> + verbose(env, "Helper has type %s got %s in R%d\n",
> + kernel_type_name(meta->btf_id),
> + kernel_type_name(reg->btf_id), regno);
> +
> + return -EACCES;
> + }
> + if (!tnum_is_const(reg->var_off) || reg->var_off.value) {
> + verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
> + regno);
> + return -EACCES;
> + }
> }
> } else if (arg_type == ARG_PTR_TO_SPIN_LOCK) {
> if (meta->func_id == BPF_FUNC_spin_lock) {
> --
> 2.25.4
>