Re: [PATCH 1/3] libdtrace: resolve BTF type tags when importing BTF into CTF

[Kris Van Hees <kris.van.hees@oracle.com>]

On Mon, Jul 20, 2026 at 03:30:58PM +0200, Jacob Carlborg wrote:
> Without this fix, running any DTrace commands results in:
> 
> ```
> dtrace: invalid probe specifier ::::
> "/usr/lib64/dtrace/6.12.0/procfs.d", line 131: operator -> cannot be
> applied to pointer to type "void"; must be applied to a struct or
> union pointer
> ```
> 
> The issue occurs on kernels which have BTF with type tags. The fix is
> to treat `BTF_KIND_TYPE_TAG` as a transparent modifier, resolving
> straight to `type->type`.

Thanks for catching this.  There is another place that needs fixing as far
as I can see...  In dt_btf_real_type_by_id() we also need to handle
BTF_KIND_TYPE_TAG as a transparent modifier, I believe.

> 
> Signed-off-by: Jacob Carlborg <doob@me.com>
> ---
>  libdtrace/dt_btf.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/libdtrace/dt_btf.c b/libdtrace/dt_btf.c
> index 1f793397..1414dd1e 100644
> --- a/libdtrace/dt_btf.c
> +++ b/libdtrace/dt_btf.c
> @@ -672,10 +672,28 @@ dt_btf_add_to_ctf(dtrace_hdl_t *dtp, dt_btf_t *btf, ctf_dict_t *ctf,
> 
>                 return ctfid == CTF_ERR ? dt_ctf_error(dtp, ctf) : ctfid;
>         }
> +       case BTF_KIND_TYPE_TAG: {
> +               /*
> +                * A type tag (e.g. __rcu, __user, __percpu) is a transparent
> +                * annotation on the type it wraps.  CTF has no equivalent, so
> +                * we resolve straight through to the underlying type, just as
> +                * we do for const/volatile/restrict.  Treating it as ignored
> +                * (mapping it to void) would leave every tagged member -- such
> +                * as task_struct.real_parent (__rcu) -- resolving to "void",
> +                * which breaks translator compilation on kernels whose BTF
> +                * carries type tags.
> +                */
> +               ctfid = dt_btf_add_to_ctf(dtp, btf, ctf, type->type);
> +               if (ctfid == CTF_ERR)
> +                       return CTF_ERR;         /* errno already set */
> +
> +               btf->ctfids[type_id] = ctfid;
> +
> +               return ctfid;
> +       }
>         case BTF_KIND_VAR:
>         case BTF_KIND_DATASEC:
>         case BTF_KIND_DECL_TAG:
> -       case BTF_KIND_TYPE_TAG:
>         case BTF_KIND_ENUM64:
>         case BTF_KIND_FUNC:
>                 return btf->ctfids[0];          /* Ignored for CTF */
> --
> 2.50.1 (Apple Git-155)
> 
> 
>