bfd, include, ld, binutils, libctf: CTF should use the dynstr/sym

This is embarrassing.

The whole point of CTF is that it remains intact even after a binary is
stripped, providing a compact mapping from symbols to types for
everything in the externally-visible interface of an ELF object: it has
connections to the symbol table for that purpose, and to the string
table to avoid duplicating symbol names.  So it's a shame that the hooks
I implemented last year served to hook it up to the .symtab and .strtab,
which obviously disappear on strip, leaving any accompanying the CTF
dict containing references to strings (and, soon, symbols) which don't
exist any more because their containing strtab has been vaporized.  The
original Solaris design used .dynsym and .dynstr (well, actually,
.ldynsym, which has more symbols) which do not disappear. So should we.

Thankfully the work we did before serves as guide rails, and adjusting
things to use the .dynstr and .dynsym was fast and easy.  The only
annoyance is that the dynsym is assembled inside elflink.c in a fairly
piecemeal fashion, so that the easiest way to get the symbols out was to
hook in before every call to swap_symbol_out (we also leave in a hook in
front of symbol additions to the .symtab because it seems plausible that
we might want to hook them in future too: for now that hook is unused).
We adjust things so that rather than being offered a whole hash table of
symbols at once, libctf is now given symbols one at a time, with st_name
indexes already resolved and pointing at their final .dynstr offsets:
it's now up to libctf to resolve these to names as needed using the
strtab info we pass it separately.

Some bits might be contentious.  The ctf_new_dynstr callback takes an
elf_internal_sym, and this remains an elf_internal_sym right down
through the generic emulation layers into ldelfgen.  This is no worse
than the elf_sym_strtab we used to pass down, but in the future when we
gain non-ELF CTF symtab support we might want to lower the
elf_internal_sym to some other representation (perhaps a
ctf_link_symbol) in bfd or in ldlang_ctf_new_dynsym.  We rename the
'apply_strsym' hooks to 'acquire_strings' instead, becuse they no longer
have anything to do with symbols.

There are some API changes to pieces of API which are technically public
but actually totally unused by anything and/or unused by anything but ld
so they can change freely: the ctf_link_symbol gains new fields to allow
symbol names to be given as strtab offsets as well as strings, and a
symidx so that the symbol index can be passed in.  ctf_link_shuffle_syms
loses its callback parameter: the idea now is that linkers call the new
ctf_link_add_linker_symbol for every symbol in .dynsym, feed in all the
strtab entries with ctf_link_add_strtab, and then a call to
ctf_link_shuffle_syms will apply both and arrange to use them to reorder
the CTF symtab at CTF serialization time (which is coming in the next
commit).

Inside libctf we have a new preamble flag CTF_F_DYNSTR which is always
set in v3-format CTF dicts from this commit forwards: CTF dicts without
this flag are associated with .strtab like they used to be, so that old
dicts' external strings don't turn to garbage when loaded by new libctf.
Dicts with this flag are associated with .dynstr and .dynsym instead.
(The flag is not the next in sequence because this commit was written
quite late: the missing flags will be filled in by the next commit.)

Tests forthcoming in a later commit in this series.

bfd/ChangeLog
2020-11-20  Nick Alcock  <nick.alcock@oracle.com>

	* elflink.c (elf_finalize_dynstr): Call examine_strtab after
	dynstr finalization.
	(elf_link_swap_symbols_out): Don't call it here.  Call
	ctf_new_symbol before swap_symbol_out.
	(elf_link_output_extsym): Call ctf_new_dynsym before
	swap_symbol_out.
	(bfd_elf_final_link): Likewise.
	* elf.c (swap_out_syms): Pass in bfd_link_info.  Call
	ctf_new_symbol before swap_symbol_out.
	(_bfd_elf_compute_section_file_positions): Adjust.

binutils/ChangeLog
2020-11-20  Nick Alcock  <nick.alcock@oracle.com>

	* readelf.c (dump_section_as_ctf): Use .dynsym and .dynstr, not
	.symtab and .strtab.

include/ChangeLog
2020-11-20  Nick Alcock  <nick.alcock@oracle.com>

	* bfdlink.h (struct elf_sym_strtab): Replace with...
	(struct elf_internal_sym): ... this.
	(struct bfd_link_callbacks) <examine_strtab>: Take only a
	symstrtab argument.
	<ctf_new_symbol>: New.
	<ctf_new_dynsym>: Likewise.
	* ctf-api.h (struct ctf_link_sym) <st_symidx>: New.
	<st_nameidx>: Likewise.
	<st_nameidx_set>: Likewise.
	(ctf_link_iter_symbol_f): Removed.
	(ctf_link_shuffle_syms): Remove most parameters, just takes a
	ctf_dict_t now.
	(ctf_link_add_linker_symbol): New, split from
	ctf_link_shuffle_syms.
	* ctf.h (CTF_F_DYNSTR): New.
	(CTF_F_MAX): Adjust.

ld/ChangeLog
2020-11-20  Nick Alcock  <nick.alcock@oracle.com>

	* ldelfgen.c (struct ctf_strsym_iter_cb_arg): Rename to...
	(struct ctf_strtab_iter_cb_arg): ... this, changing fields:
	<syms>: Remove.
	<symcount>: Remove.
	<symstrtab>: Rename to...
	<strtab>: ... this.
	(ldelf_ctf_strtab_iter_cb): Adjust.
	(ldelf_ctf_symbols_iter_cb): Remove.
	(ldelf_new_dynsym_for_ctf): New, tell libctf about a single
	symbol.
	(ldelf_examine_strtab_for_ctf): Rename to...
	(ldelf_acquire_strings_for_ctf): ... this, only doing the strtab
	portion and not symbols.
	* ldelfgen.h: Adjust declarations accordingly.
	* ldemul.c (ldemul_examine_strtab_for_ctf): Rename to...
	(ldemul_acquire_strings_for_ctf): ... this.
	(ldemul_new_dynsym_for_ctf): New.
	* ldemul.h: Adjust declarations accordingly.
	* ldlang.c (ldlang_ctf_apply_strsym): Rename to...
	(ldlang_ctf_acquire_strings): ... this.
	(ldlang_ctf_new_dynsym): New.
	(lang_write_ctf): Call ldemul_new_dynsym_for_ctf with NULL to do
	the actual symbol shuffle.
	* ldlang.h (struct elf_strtab_hash): Adjust accordingly.
	* ldmain.c (bfd_link_callbacks): Wire up new/renamed callbacks.

libctf/ChangeLog
2020-11-20  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-link.c (ctf_link_shuffle_syms): Adjust.
	(ctf_link_add_linker_symbol): New, unimplemented stub.
	* libctf.ver: Add it.
	* ctf-create.c (ctf_serialize): Set CTF_F_DYNSTR on newly-serialized
	dicts.
	* ctf-open-bfd.c (ctf_bfdopen_ctfsect): Check for the flag: open the
	symtab/strtab if not present, dynsym/dynstr otherwise.
	* ctf-archive.c (ctf_arc_bufpreamble): New, get the preamble from
	some arbitrary member of a CTF archive.
	* ctf-impl.h (ctf_arc_bufpreamble): Declare it.

1 files changed, 57 insertions, 53 deletions

diff --git a/ld/ldelfgen.c b/ld/ldelfgen.c
index e9496f918f5..ca531ee7cfe 100644
--- a/ld/ldelfgen.c
+++ b/ld/ldelfgen.c
@@ -28,6 +28,7 @@
 #include "ldexp.h"
 #include "ldlang.h"
 #include "elf-bfd.h"
+#include "elf/internal.h"
 #include "ldelfgen.h"
 
 void
@@ -103,11 +104,9 @@ ldelf_emit_ctf_early (void)
 /* Callbacks used to map from bfd types to libctf types, under libctf's
    control.  */
 
-struct ctf_strsym_iter_cb_arg
+struct ctf_strtab_iter_cb_arg
 {
-  struct elf_sym_strtab *syms;
-  bfd_size_type symcount;
-  struct elf_strtab_hash *symstrtab;
+  struct elf_strtab_hash *strtab;
   size_t next_i;
   size_t next_idx;
 };
@@ -121,20 +120,20 @@ ldelf_ctf_strtab_iter_cb (uint32_t *offset, void *arg_)
   bfd_size_type off;
   const char *ret;
 
-  struct ctf_strsym_iter_cb_arg *arg =
-    (struct ctf_strsym_iter_cb_arg *) arg_;
+  struct ctf_strtab_iter_cb_arg *arg =
+    (struct ctf_strtab_iter_cb_arg *) arg_;
 
   /* There is no zeroth string.  */
   if (arg->next_i == 0)
     arg->next_i = 1;
 
-  if (arg->next_i >= _bfd_elf_strtab_len (arg->symstrtab))
+  if (arg->next_i >= _bfd_elf_strtab_len (arg->strtab))
     {
       arg->next_i = 0;
       return NULL;
     }
 
-  ret = _bfd_elf_strtab_str (arg->symstrtab, arg->next_i++, &off);
+  ret = _bfd_elf_strtab_str (arg->strtab, arg->next_i++, &off);
   *offset = off;
 
   /* If we've overflowed, we cannot share any further strings: the CTF
@@ -145,69 +144,74 @@ ldelf_ctf_strtab_iter_cb (uint32_t *offset, void *arg_)
   return ret;
 }
 
-/* Return symbols from the symbol table to libctf, one by one.  We assume (and
-   assert) that the symbols in the elf_link_hash_table are in strictly ascending
-   order, and that none will be added in between existing ones.  Returns NULL
-   when iteration is complete.  */
-
-static struct ctf_link_sym *
-ldelf_ctf_symbols_iter_cb (struct ctf_link_sym *dest,
-					   void *arg_)
+void
+ldelf_acquire_strings_for_ctf
+  (struct ctf_dict *ctf_output, struct elf_strtab_hash *strtab)
 {
-  struct ctf_strsym_iter_cb_arg *arg =
-    (struct ctf_strsym_iter_cb_arg *) arg_;
+  struct ctf_strtab_iter_cb_arg args = { strtab, 0, 0 };
+  if (!ctf_output)
+    return;
 
-  if (arg->next_i > arg->symcount)
+  if (bfd_get_flavour (link_info.output_bfd) == bfd_target_elf_flavour)
     {
-      arg->next_i = 0;
-      arg->next_idx = 0;
-      return NULL;
+      if (ctf_link_add_strtab (ctf_output, ldelf_ctf_strtab_iter_cb,
+			       &args) < 0)
+	einfo (_("%F%P: warning: CTF strtab association failed; strings will "
+		 "not be shared: %s\n"),
+	       ctf_errmsg (ctf_errno (ctf_output)));
     }
-
-  ASSERT (arg->syms[arg->next_i].dest_index == arg->next_idx);
-  dest->st_name = _bfd_elf_strtab_str (arg->symstrtab, arg->next_i, NULL);
-  dest->st_shndx = arg->syms[arg->next_i].sym.st_shndx;
-  dest->st_type = ELF_ST_TYPE (arg->syms[arg->next_i].sym.st_info);
-  dest->st_value = arg->syms[arg->next_i].sym.st_value;
-  arg->next_i++;
-  return dest;
 }
 
 void
-ldelf_examine_strtab_for_ctf
-  (struct ctf_dict *ctf_output, struct elf_sym_strtab *syms,
-   bfd_size_type symcount, struct elf_strtab_hash *symstrtab)
+ldelf_new_dynsym_for_ctf (struct ctf_dict *ctf_output, int symidx,
+			  struct elf_internal_sym *sym)
 {
-  struct ctf_strsym_iter_cb_arg args = { syms, symcount, symstrtab,
-					  0, 0 };
-   if (!ctf_output)
+  ctf_link_sym_t lsym;
+
+  if (!ctf_output)
      return;
 
-   if (bfd_get_flavour (link_info.output_bfd) == bfd_target_elf_flavour
-       && !bfd_link_relocatable (&link_info))
+  /* New symbol.  */
+  if (sym != NULL)
     {
-      if (ctf_link_add_strtab (ctf_output, ldelf_ctf_strtab_iter_cb,
-			       &args) < 0)
-	einfo (_("%F%P: warning: CTF strtab association failed; strings will "
-		 "not be shared: %s\n"),
-	       ctf_errmsg (ctf_errno (ctf_output)));
+      lsym.st_name = NULL;
+      lsym.st_nameidx = sym->st_name;
+      lsym.st_nameidx_set = 1;
+      lsym.st_symidx = symidx;
+      lsym.st_shndx = sym->st_shndx;
+      lsym.st_type = ELF_ST_TYPE (sym->st_info);
+      lsym.st_value = sym->st_value;
+      if (ctf_link_add_linker_symbol (ctf_output, &lsym) < 0)
+	{
+	  einfo (_("%F%P: warning: CTF symbol addition failed; CTF will "
+		   "not be tied to symbols: %s\n"),
+		 ctf_errmsg (ctf_errno (ctf_output)));
+	}
+    }
+  else
+    {
+      /* Shuffle all the symbols.  */
 
-      if (ctf_link_shuffle_syms (ctf_output, ldelf_ctf_symbols_iter_cb,
-				 &args) < 0)
-	einfo (_("%F%P: warning: CTF symbol shuffling failed; slight space "
-		 "cost: %s\n"), ctf_errmsg (ctf_errno (ctf_output)));
+      if (ctf_link_shuffle_syms (ctf_output) < 0)
+	einfo (_("%F%P: warning: CTF symbol shuffling failed; CTF will "
+		 "not be tied to symbols: %s\n"),
+	       ctf_errmsg (ctf_errno (ctf_output)));
     }
 }
 #else
-extern int ldelf_emit_ctf_early (void)
+int
+ldelf_emit_ctf_early (void)
 {
   return 0;
 }
 
-extern void ldelf_examine_strtab_for_ctf
-  (struct ctf_dict *ctf_output ATTRIBUTE_UNUSED,
-   struct elf_sym_strtab *syms ATTRIBUTE_UNUSED,
-   bfd_size_type symcount ATTRIBUTE_UNUSED,
-   struct elf_strtab_hash *symstrtab ATTRIBUTE_UNUSED)
+void
+ldelf_acquire_strings_for_ctf (struct ctf_dict *ctf_output ATTRIBUTE_UNUSED,
+			       struct elf_strtab_hash *strtab ATTRIBUTE_UNUSED)
+{}
+void
+ldelf_new_dynsym_for_ctf (struct ctf_dict *ctf_output ATTRIBUTE_UNUSED,
+			  int symidx ATTRIBUTE_UNUSED,
+			  struct elf_internal_sym *sym ATTRIBUTE_UNUSED)
 {}
 #endif