libctf: warn about information loss because of unreleased format changes

In the last cycle there have been various changes that have replaced
parts of the CTF format with other parts without format
compatibility.  This was not a compat break, because the old format was
never accepted by any version of libctf (the not-in-official-release CTF
compiler patch was emitting an invalid func info section), but
nonetheless it can confuse users using that patch if they link together
object files and find the func info sections in the inputs silently
disappearing.

Scan the linker inputs for this problem and emit a warning if any are
found.

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

	* ctf-link.c (ctf_link_warn_outdated_inputs): New.
	(ctf_link_write): Call it.

2 files changed, 35 insertions, 0 deletions

diff --git a/libctf/ChangeLog b/libctf/ChangeLog
index f11b511..db75fd5 100644
--- a/libctf/ChangeLog
+++ b/libctf/ChangeLog
@@ -1,5 +1,10 @@
 2021-01-05  Nick Alcock  <nick.alcock@oracle.com>
 
+	* ctf-link.c (ctf_link_warn_outdated_inputs): New.
+	(ctf_link_write): Call it.
+
+2021-01-05  Nick Alcock  <nick.alcock@oracle.com>
+
 	* testsuite/libctf-lookup/enum-symbol.lk: New symbol-lookup test.
 	* testsuite/libctf-lookup/enum-symbol-ctf.c: New CTF input.
 	* testsuite/libctf-lookup/enum-symbol.c: New lookup test.
diff --git a/libctf/ctf-link.c b/libctf/ctf-link.c
index 14ed322..5384b20 100644
--- a/libctf/ctf-link.c
+++ b/libctf/ctf-link.c
@@ -2004,6 +2004,34 @@ ctf_change_parent_name (void *key _libctf_unused_, void *value, void *arg)
   ctf_parent_name_set (fp, name);
 }
 
+/* Warn if we may suffer information loss because the CTF input files are too
+   old.  Usually we provide complete backward compatibility, but compiler
+   changes etc which never hit a release may have a flag in the header that
+   simply prevents those changes from being used.  */
+static void
+ctf_link_warn_outdated_inputs (ctf_dict_t *fp)
+{
+  ctf_next_t *i = NULL;
+  void *name_;
+  void *ifp_;
+  int err;
+
+  while ((err = ctf_dynhash_next (fp->ctf_link_inputs, &i, &name_, &ifp_)) == 0)
+    {
+      const char *name = (const char *) name_;
+      ctf_dict_t *ifp = (ctf_dict_t *) ifp_;
+
+      if (!(ifp->ctf_header->cth_flags & CTF_F_NEWFUNCINFO)
+	  && (ifp->ctf_header->cth_varoff - ifp->ctf_header->cth_funcoff) > 0)
+	ctf_err_warn (ifp, 1, 0, _("linker input %s has CTF func info but uses "
+				   "an old, unreleased func info format: "
+				   "this func info section will be dropped."),
+		      name);
+    }
+  if (err != ECTF_NEXT_END)
+    ctf_err_warn (fp, 0, err, _("error checking for outdated inputs"));
+}
+
 /* Write out a CTF archive (if there are per-CU CTF files) or a CTF file
    (otherwise) into a new dynamically-allocated string, and return it.
    Members with sizes above THRESHOLD are compressed.  */
@@ -2023,6 +2051,8 @@ ctf_link_write (ctf_dict_t *fp, size_t *size, size_t threshold)
   memset (&arg, 0, sizeof (ctf_name_list_accum_cb_arg_t));
   arg.fp = fp;
 
+  ctf_link_warn_outdated_inputs (fp);
+
   if (fp->ctf_link_outputs)
     {
       ctf_dynhash_iter (fp->ctf_link_outputs, ctf_accumulate_archive_names, &arg);