libctf: lookup_by_name: do not return success for nonexistent pointer types

The recent work allowing lookups of pointers in child dicts when the
pointed-to type is in the parent dict broke the case where a pointer
type that does not exist at all is looked up: we mistakenly return the
pointed-to type, which is likely not a pointer at all.  This causes
considerable confusion.

Fixed, with a new testcase.

libctf/ChangeLog
2021-01-19  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-lookup.c (ctf_lookup_by_name_internal): Do not return the
	base type if looking up a nonexistent pointer type.
	* testsuite/libctf-regression/pptrtab*: Test it.

libctf/ChangeLog

@@ -1,3 +1,9 @@
+2021-01-19  Nick Alcock  <nick.alcock@oracle.com>
+
+	* ctf-lookup.c (ctf_lookup_by_name_internal): Do not return the
+	base type if looking up a nonexistent pointer type.
+	* testsuite/libctf-regression/pptrtab*: Test it.
+
 2021-01-13  Alan Modra  <amodra@gmail.com>
 
 	* Makefile.in: Regenerate.

libctf/ctf-lookup.c

@@ -184,24 +184,36 @@ ctf_lookup_by_name_internal (ctf_dict_t *fp, ctf_dict_t *child,
 	     from resolving the type down to its base type and use that instead.
 	     This helps with cases where the CTF data includes "struct foo *"
 	     but not "foo_t *" and the user tries to access "foo_t *" in the
-	     debugger.  */
+	     debugger.
+
+	     There is extra complexity here because uninitialized elements in
+	     the pptrtab and ptrtab are set to zero, but zero (as the type ID
+	     meaning the unimplemented type) is a valid return type from
+	     ctf_lookup_by_name.  (Pointers to types are never of type 0, so
+	     this is unambiguous, just fiddly to deal with.)  */
 
 	  uint32_t idx = LCTF_TYPE_TO_INDEX (fp, type);
 	  int in_child = 0;
 
-	  ntype = type;
+	  ntype = CTF_ERR;
 	  if (child && idx <= child->ctf_pptrtab_len)
 	    {
 	      ntype = child->ctf_pptrtab[idx];
 	      if (ntype)
 		in_child = 1;
+	      else
+		ntype = CTF_ERR;
 	    }
 
-	  if (ntype == 0)
+	  if (ntype == CTF_ERR)
-	    ntype = fp->ctf_ptrtab[idx];
+	    {
+	      ntype = fp->ctf_ptrtab[idx];
+	      if (ntype == 0)
+		ntype = CTF_ERR;
+	    }
 
 	  /* Try resolving to its base type and check again.  */
-	  if (ntype == 0)
+	  if (ntype == CTF_ERR)
 	    {
 	      if (child)
 		ntype = ctf_type_resolve_unsliced (child, type);
@@ -213,16 +225,22 @@ ctf_lookup_by_name_internal (ctf_dict_t *fp, ctf_dict_t *child,
 
 	      idx = LCTF_TYPE_TO_INDEX (fp, ntype);
 
-	      ntype = 0;
+	      ntype = CTF_ERR;
 	      if (child && idx <= child->ctf_pptrtab_len)
 		{
 		  ntype = child->ctf_pptrtab[idx];
 		  if (ntype)
 		    in_child = 1;
+		  else
+		    ntype = CTF_ERR;
 		}
 
-	      if (ntype == 0)
+	      if (ntype == CTF_ERR)
-		ntype = fp->ctf_ptrtab[idx];
+		{
+		  ntype = fp->ctf_ptrtab[idx];
+		  if (ntype == 0)
+		    ntype = CTF_ERR;
+		}
 	      if (ntype == CTF_ERR)
 		goto notype;
 	    }

libctf/testsuite/libctf-regression/pptrtab-a.c

@@ -1,3 +1,5 @@
 typedef long a_t;
+typedef long b_t;
 
 a_t *a;
+b_t ignore2;

libctf/testsuite/libctf-regression/pptrtab-b.c

@@ -1,4 +1,5 @@
 typedef long a_t;
+typedef long b_t;
 
 a_t b;
-
+b_t ignore1;

libctf/testsuite/libctf-regression/pptrtab.c

@@ -23,13 +23,18 @@ main (int argc, char *argv[])
     goto open_err;
 
   /* Make sure we can look up a_t * by name in all non-parent dicts, even though
-     the a_t * and the type it points to are in distinct dicts.  */
+     the a_t * and the type it points to are in distinct dicts; make sure we
+     cannot look up b_t *.  */
 
   while ((fp = ctf_archive_next (ctf, &i, &arcname, 1, &err)) != NULL)
     {
       if ((type = ctf_lookup_by_name (fp, "a_t *")) == CTF_ERR)
 	goto err;
 
+      if ((ctf_lookup_by_name (fp, "b_t *")) != CTF_ERR ||
+          ctf_errno (fp) != ECTF_NOTYPE)
+	goto noerr;
+
       if (ctf_type_reference (fp, type) == CTF_ERR)
 	goto err;
 
@@ -51,4 +56,7 @@ main (int argc, char *argv[])
  err:
   fprintf (stderr, "Lookup failed in %s: %s\n", arcname, ctf_errmsg (ctf_errno (fp)));
   return 1;
+ noerr:
+  fprintf (stderr, "Lookup unexpectedly succeeded in %s\n", arcname);
+  return 1;
 }