[PATCH v2 1/3] libdtrace: resolve BTF type tags transparently

[Jacob Carlborg <doob@me.com>]

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`.

Signed-off-by: Jacob Carlborg <doob@me.com>
---
v2: also handle BTF_KIND_TYPE_TAG as a transparent modifier in
    dt_btf_real_type_by_id() (per review feedback).

 libdtrace/dt_btf.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/libdtrace/dt_btf.c b/libdtrace/dt_btf.c
index 1f793397..3ec60dcb 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 */
@@ -860,6 +878,7 @@ dt_btf_real_type_by_id(dtrace_hdl_t *dtp, const dt_btf_t *btf, int32_t id)
 		case BTF_KIND_RESTRICT:
 		case BTF_KIND_TYPEDEF:
 		case BTF_KIND_VOLATILE:
+		case BTF_KIND_TYPE_TAG:
 			type = dt_btf_type_by_id(dtp, btf, type->type);
 		default:
 			return type;
-- 
2.50.1 (Apple Git-155)