libctf, ld: prohibit getting the size or alignment of forwards

C allows you to do only a very few things with entities of incomplete
type (as opposed to pointers to them): make pointers to them and give
them cv-quals, roughly. In particular you can't sizeof them and you
can't get their alignment.

We cannot impose all the requirements the standard imposes on CTF users,
because the deduplicator can transform any structure type into a forward
for the purposes of breaking cycles: so CTF type graphs can easily
contain things like arrays of forward type (if you want to figure out
their size or alignment, you need to chase down the types this forward
might be a forward to in child TU dicts: we will soon add API functions
to make doing this much easier).

Nonetheless, it is still meaningless to ask for the size or alignment of
forwards: but libctf didn't prohibit this and returned nonsense from
internal implementation details when you asked (it returned the kind of
the pointed-to type as both the size and alignment, because forwards
reuse ctt_type as a type kind, and ctt_type and ctt_size overlap).  So
introduce a new error, ECTF_INCOMPLETE, which is returned when you try
to get the size or alignment of forwards: we also return it when you try
to do things that require libctf itself to get the size or alignment of
a forward, notably using a forward as an array index type (which C
should never do in any case) or adding forwards to structures without
specifying their offset explicitly.

The dumper will not emit size or alignment info for forwards any more.

(This should not be an API break since ctf_type_size and ctf_type_align
could both return errors before now: any code that isn't expecting error
returns is already potentially broken.)

include/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-api.h (ECTF_INCOMPLETE): New.
	(ECTF_NERR): Adjust.

ld/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* testsuite/ld-ctf/conflicting-cycle-1.parent.d: Adjust for dumper
	changes.
	* testsuite/ld-ctf/cross-tu-cyclic-conflicting.d: Likewise.
	* testsuite/ld-ctf/forward.c: New test...
	* testsuite/ld-ctf/forward.d: ... and results.

libctf/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-types.c (ctf_type_resolve): Improve comment.
	(ctf_type_size): Yield ECTF_INCOMPLETE when applied to forwards.
	Emit errors into the right dict.
	(ctf_type_align): Likewise.
	* ctf-create.c (ctf_add_member_offset): Yield ECTF_INCOMPLETE
	when adding a member without explicit offset when this member, or
	the previous member, is incomplete.
	* ctf-dump.c (ctf_dump_format_type): Do not try to print the size of
	forwards.
	(ctf_dump_member): Do not try to print their alignment.

1 files changed, 43 insertions, 7 deletions

diff --git a/libctf/ctf-create.c b/libctf/ctf-create.c
index 0af46d3..6fe7461 100644
--- a/libctf/ctf-create.c
+++ b/libctf/ctf-create.c
@@ -1690,6 +1690,14 @@ ctf_add_array (ctf_dict_t *fp, uint32_t flag, const ctf_arinfo_t *arp)
   if (ctf_lookup_by_id (&tmp, arp->ctr_index) == NULL)
     return CTF_ERR;		/* errno is set for us.  */
 
+  if (ctf_type_kind (fp, arp->ctr_index) == CTF_K_FORWARD)
+    {
+      ctf_err_warn (fp, 1, ECTF_INCOMPLETE,
+		    _("ctf_add_array: index type %lx is incomplete"),
+		    arp->ctr_contents);
+      return (ctf_set_errno (fp, ECTF_INCOMPLETE));
+    }
+
   if ((type = ctf_add_generic (fp, flag, NULL, CTF_K_ARRAY, &dtd)) == CTF_ERR)
     return CTF_ERR;		/* errno is set for us.  */
 
@@ -2040,6 +2048,7 @@ ctf_add_member_offset (ctf_dict_t *fp, ctf_id_t souid, const char *name,
   ssize_t msize, malign, ssize;
   uint32_t kind, vlen, root;
   char *s = NULL;
+  int is_incomplete = 0;
 
   if (!(fp->ctf_flags & LCTF_RDWR))
     return (ctf_set_errno (fp, ECTF_RDONLY));
@@ -2075,14 +2084,19 @@ ctf_add_member_offset (ctf_dict_t *fp, ctf_id_t souid, const char *name,
     {
       /* The unimplemented type, and any type that resolves to it, has no size
 	 and no alignment: it can correspond to any number of compiler-inserted
-	 types.  */
-
+	 types.  We allow incomplete types through since they are routinely
+	 added to the ends of structures, and can even be added elsewhere in
+	 structures by the deduplicator.  They are assumed to be zero-size with
+	 no alignment: this is often wrong, but problems can be avoided in this
+	 case by explicitly specifying the size of the structure via the _sized
+	 functions.  The deduplicator always does this.  */
+
+      msize = 0;
+      malign = 0;
       if (ctf_errno (fp) == ECTF_NONREPRESENTABLE)
-	{
-	  msize = 0;
-	  malign = 0;
-	  ctf_set_errno (fp, 0);
-	}
+	ctf_set_errno (fp, 0);
+      else if (ctf_errno (fp) == ECTF_INCOMPLETE)
+	is_incomplete = 1;
       else
 	return -1;		/* errno is set for us.  */
     }
@@ -2123,10 +2137,32 @@ ctf_add_member_offset (ctf_dict_t *fp, ctf_id_t souid, const char *name,
 	      return -1;	/* errno is set for us.  */
 	    }
 
+	  if (is_incomplete)
+	    {
+	      ctf_err_warn (fp, 1, ECTF_INCOMPLETE,
+			    _("ctf_add_member_offset: cannot add member %s of "
+			      "incomplete type %lx to struct %lx without "
+			      "specifying explicit offset\n"),
+			    name ? name : _("(unnamed member)"), type, souid);
+	      return (ctf_set_errno (fp, ECTF_INCOMPLETE));
+	    }
+
 	  if (ctf_type_encoding (fp, ltype, &linfo) == 0)
 	    off += linfo.cte_bits;
 	  else if ((lsize = ctf_type_size (fp, ltype)) > 0)
 	    off += lsize * CHAR_BIT;
+	  else if (lsize == -1 && ctf_errno (fp) == ECTF_INCOMPLETE)
+	    {
+	      ctf_err_warn (fp, 1, ECTF_INCOMPLETE,
+			    _("ctf_add_member_offset: cannot add member %s of "
+			      "type %lx to struct %lx without specifying "
+			      "explicit offset after member %s of type %lx, "
+			      "which is an incomplete type\n"),
+			    name ? name : _("(unnamed member)"), type, souid,
+			    lmd->dmd_name ? lmd->dmd_name
+			    : _("(unnamed member)"), ltype);
+	      return -1;			/* errno is set for us.  */
+	    }
 
 	  /* Round up the offset of the end of the last member to
 	     the next byte boundary, convert 'off' to bytes, and