Re: [PATCH v8 bpf-next 07/13] bpf: Add btf_struct_ids_match function

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

On Wed, Jul 22, 2020 at 2:13 PM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding btf_struct_ids_match function to check if given address provided
> by BTF object + offset is also address of another nested BTF object.
>
> This allows to pass an argument to helper, which is defined via parent
> BTF object + offset, like for bpf_d_path (added in following changes):
>
>   SEC("fentry/filp_close")
>   int BPF_PROG(prog_close, struct file *file, void *id)
>   {
>     ...
>     ret = bpf_d_path(&file->f_path, ...
>
> The first bpf_d_path argument is hold by verifier as BTF file object
> plus offset of f_path member.
>
> The btf_struct_ids_match function will walk the struct file object and
> check if there's nested struct path object on the given offset.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/linux/bpf.h   |  2 ++
>  kernel/bpf/btf.c      | 29 +++++++++++++++++++++++++++++
>  kernel/bpf/verifier.c | 18 ++++++++++++------
>  3 files changed, 43 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index bae557ff2da8..c981e258fed3 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1306,6 +1306,8 @@ 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);
> +bool btf_struct_ids_match(struct bpf_verifier_log *log,
> +                         int off, u32 id, u32 mid);
>  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 1ab5fd5bf992..562d4453fad3 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -4140,6 +4140,35 @@ int btf_struct_access(struct bpf_verifier_log *log,
>         return -EINVAL;
>  }
>
> +bool btf_struct_ids_match(struct bpf_verifier_log *log,
> +                         int off, u32 id, u32 mid)
> +{
> +       const struct btf_type *type;
> +       u32 nid;
> +       int err;
> +

mid and nid are terrible names, especially as an input argument name.
mid == need_type_id? nid == cur_type_id or something along those
lines?

> +       do {
> +               type = btf_type_by_id(btf_vmlinux, id);
> +               if (!type)
> +                       return false;
> +               err = btf_struct_walk(log, type, off, 1, &nid);
> +               if (err < 0)
> +                       return false;
> +
> +               /* We found nested struct object. If it matches
> +                * the requested ID, we're done. Otherwise let's
> +                * continue the search with offset 0 in the new
> +                * type.
> +                */
> +               if (err == walk_struct && mid == nid)
> +                       return true;
> +               off = 0;
> +               id = nid;
> +       } while (err == walk_struct);

This seems like a slightly more obvious control flow:

again:

   ...

   if (err != walk_struct)
      return false;

   if (mid != nid) {
      off = 0;
      id = nid;
      goto again;
   }

   return true;

> +
> +       return false;
> +}
> +
>  int btf_resolve_helper_id(struct bpf_verifier_log *log,
>                           const struct bpf_func_proto *fn, int arg)
>  {

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