bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2016 Free Software Foundation, Inc.
   3
   4    This file is part of BFD, the Binary File Descriptor library.
   5
   6    This program is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3 of the License, or
   9    (at your option) any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program; if not, write to the Free Software
  18    Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
  19    MA 02110-1301, USA.  */
  20
  21 #include "sysdep.h"
  22 #include "bfd.h"
  23 #include "bfdlink.h"
  24 #include "libbfd.h"
  25 #define ARCH_SIZE 0
  26 #include "elf-bfd.h"
  27 #include "safe-ctype.h"
  28 #include "libiberty.h"
  29 #include "objalloc.h"
  30
  31 /* Create sections needed by STT_GNU_IFUNC symbol.  */
  32
  33 bfd_boolean
  34 _bfd_elf_create_ifunc_sections (bfd *abfd, struct bfd_link_info *info)
  35 {
  36   flagword flags, pltflags;
  37   asection *s;
  38   const struct elf_backend_data *bed = get_elf_backend_data (abfd);
  39   struct elf_link_hash_table *htab = elf_hash_table (info);
  40
  41   if (htab->irelifunc != NULL || htab->iplt != NULL)
  42     return TRUE;
  43
  44   flags = bed->dynamic_sec_flags;
  45   pltflags = flags;
  46   if (bed->plt_not_loaded)
  47     /* We do not clear SEC_ALLOC here because we still want the OS to
  48        allocate space for the section; it's just that there's nothing
  49        to read in from the object file.  */
  50     pltflags &= ~ (SEC_CODE | SEC_LOAD | SEC_HAS_CONTENTS);
  51   else
  52     pltflags |= SEC_ALLOC | SEC_CODE | SEC_LOAD;
  53   if (bed->plt_readonly)
  54     pltflags |= SEC_READONLY;
  55
  56   if (bfd_link_pic (info))
  57     {
  58       /* We need to create .rel[a].ifunc for PIC objects.  */
  59       const char *rel_sec = (bed->rela_plts_and_copies_p
  60                              ? ".rela.ifunc" : ".rel.ifunc");
  61
  62       s = bfd_make_section_with_flags (abfd, rel_sec,
  63                                        flags | SEC_READONLY);
  64       if (s == NULL
  65           || ! bfd_set_section_alignment (abfd, s,
  66                                           bed->s->log_file_align))
  67         return FALSE;
  68       htab->irelifunc = s;
  69     }
  70   else
  71     {
  72       /* We need to create .iplt, .rel[a].iplt, .igot and .igot.plt
  73          for static executables.   */
  74       s = bfd_make_section_with_flags (abfd, ".iplt", pltflags);
  75       if (s == NULL
  76           || ! bfd_set_section_alignment (abfd, s, bed->plt_alignment))
  77         return FALSE;
  78       htab->iplt = s;
  79
  80       s = bfd_make_section_with_flags (abfd,
  81                                        (bed->rela_plts_and_copies_p
  82                                         ? ".rela.iplt" : ".rel.iplt"),
  83                                        flags | SEC_READONLY);
  84       if (s == NULL
  85           || ! bfd_set_section_alignment (abfd, s,
  86                                           bed->s->log_file_align))
  87         return FALSE;
  88       htab->irelplt = s;
  89
  90       /* We don't need the .igot section if we have the .igot.plt
  91          section.  */
  92       if (bed->want_got_plt)
  93         s = bfd_make_section_with_flags (abfd, ".igot.plt", flags);
  94       else
  95         s = bfd_make_section_with_flags (abfd, ".igot", flags);
  96       if (s == NULL
  97           || !bfd_set_section_alignment (abfd, s,
  98                                          bed->s->log_file_align))
  99         return FALSE;
 100       htab->igotplt = s;
 101     }
 102
 103   return TRUE;
 104 }
 105
 106 /* Allocate space in .plt, .got and associated reloc sections for
 107    dynamic relocs against a STT_GNU_IFUNC symbol definition.  */
 108
 109 bfd_boolean
 110 _bfd_elf_allocate_ifunc_dyn_relocs (struct bfd_link_info *info,
 111                                     struct elf_link_hash_entry *h,
 112                                     struct elf_dyn_relocs **head,
 113                                     bfd_boolean *readonly_dynrelocs_against_ifunc_p,
 114                                     unsigned int plt_entry_size,
 115                                     unsigned int plt_header_size,
 116                                     unsigned int got_entry_size,
 117                                     bfd_boolean avoid_plt)
 118 {
 119   asection *plt, *gotplt, *relplt;
 120   struct elf_dyn_relocs *p;
 121   unsigned int sizeof_reloc;
 122   const struct elf_backend_data *bed;
 123   struct elf_link_hash_table *htab;
 124   bfd_boolean readonly_dynrelocs_against_ifunc;
 125   /* If AVOID_PLT is TRUE, don't use PLT if possible.  */
 126   bfd_boolean use_plt = !avoid_plt || h->plt.refcount > 0;
 127   bfd_boolean need_dynreloc = !use_plt || bfd_link_pic (info);
 128
 129   /* When a PIC object references a STT_GNU_IFUNC symbol defined
 130      in executable or it isn't referenced via PLT, the address of
 131      the resolved function may be used.  But in non-PIC executable,
 132      the address of its .plt slot may be used.  Pointer equality may
 133      not work correctly.  PIE or non-PLT reference should be used if
 134      pointer equality is required here.  */
 135   if (!need_dynreloc
 136       && (h->dynindx != -1
 137           || info->export_dynamic)
 138       && h->pointer_equality_needed)
 139     {
 140       info->callbacks->einfo
 141         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 142            "equality in `%B' can not be used when making an "
 143            "executable; recompile with -fPIE and relink with -pie\n"),
 144          h->root.root.string,
 145          h->root.u.def.section->owner);
 146       bfd_set_error (bfd_error_bad_value);
 147       return FALSE;
 148     }
 149
 150   htab = elf_hash_table (info);
 151
 152   /* When the symbol is marked with regular reference, if PLT isn't used
 153      or we are building a PIC object, we must keep dynamic relocation
 154      if there is non-GOT reference and use PLT if there is PC-relative
 155      reference.  */
 156   if (need_dynreloc && h->ref_regular)
 157     {
 158       bfd_boolean keep = FALSE;
 159       for (p = *head; p != NULL; p = p->next)
 160         if (p->count)
 161           {
 162             h->non_got_ref = 1;
 163             /* Need dynamic relocations for non-GOT reference.  */
 164             keep = TRUE;
 165             if (p->pc_count)
 166               {
 167                 /* Must use PLT for PC-relative reference.  */
 168                 use_plt = TRUE;
 169                 need_dynreloc = bfd_link_pic (info);
 170                 break;
 171               }
 172           }
 173       if (keep)
 174         goto keep;
 175     }
 176
 177   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 178   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 179     {
 180       h->got = htab->init_got_offset;
 181       h->plt = htab->init_plt_offset;
 182       *head = NULL;
 183       return TRUE;
 184     }
 185
 186   /* Return and discard space for dynamic relocations against it if
 187      it is never referenced.  */
 188   if (!h->ref_regular)
 189     {
 190       if (h->plt.refcount > 0
 191           || h->got.refcount > 0)
 192         abort ();
 193       h->got = htab->init_got_offset;
 194       h->plt = htab->init_plt_offset;
 195       *head = NULL;
 196       return TRUE;
 197     }
 198
 199 keep:
 200   bed = get_elf_backend_data (info->output_bfd);
 201   if (bed->rela_plts_and_copies_p)
 202     sizeof_reloc = bed->s->sizeof_rela;
 203   else
 204     sizeof_reloc = bed->s->sizeof_rel;
 205
 206   /* When building a static executable, use .iplt, .igot.plt and
 207      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 208   if (htab->splt != NULL)
 209     {
 210       plt = htab->splt;
 211       gotplt = htab->sgotplt;
 212       relplt = htab->srelplt;
 213
 214       /* If this is the first .plt entry and PLT is used, make room for
 215          the special first entry.  */
 216       if (plt->size == 0 && use_plt)
 217         plt->size += plt_header_size;
 218     }
 219   else
 220     {
 221       plt = htab->iplt;
 222       gotplt = htab->igotplt;
 223       relplt = htab->irelplt;
 224     }
 225
 226   if (use_plt)
 227     {
 228       /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 229          the original value for R_*_IRELATIVE.  */
 230       h->plt.offset = plt->size;
 231
 232       /* Make room for this entry in the .plt/.iplt section.  */
 233       plt->size += plt_entry_size;
 234
 235       /* We also need to make an entry in the .got.plt/.got.iplt section,
 236          which will be placed in the .got section by the linker script.  */
 237       gotplt->size += got_entry_size;
 238     }
 239
 240   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 241      section for GOTPLT relocation if PLT is used.  */
 242   if (use_plt)
 243     {
 244       relplt->size += sizeof_reloc;
 245       relplt->reloc_count++;
 246     }
 247
 248   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 249      there is a non-GOT reference in a PIC object or PLT isn't used.  */
 250   if (!need_dynreloc || !h->non_got_ref)
 251     *head = NULL;
 252
 253   readonly_dynrelocs_against_ifunc = FALSE;
 254
 255   /* Finally, allocate space.  */
 256   p = *head;
 257   if (p != NULL)
 258     {
 259       bfd_size_type count = 0;
 260       do
 261         {
 262           if (!readonly_dynrelocs_against_ifunc)
 263             {
 264               asection *s = p->sec->output_section;
 265               if (s != NULL && (s->flags & SEC_READONLY) != 0)
 266                 readonly_dynrelocs_against_ifunc = TRUE;
 267             }
 268           count += p->count;
 269           p = p->next;
 270         }
 271       while (p != NULL);
 272
 273       /* Dynamic relocations are stored in
 274          1. .rel[a].ifunc section in PIC object.
 275          2. .rel[a].got section in dynamic executable.
 276          3. .rel[a].iplt section in static executable.  */
 277       if (bfd_link_pic (info))
 278         htab->irelifunc->size += count * sizeof_reloc;
 279       else if (htab->splt != NULL)
 280         htab->srelgot->size += count * sizeof_reloc;
 281       else
 282         {
 283           relplt->size += count * sizeof_reloc;
 284           relplt->reloc_count += count;
 285         }
 286     }
 287
 288   if (readonly_dynrelocs_against_ifunc_p)
 289     *readonly_dynrelocs_against_ifunc_p = readonly_dynrelocs_against_ifunc;
 290
 291   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 292      and .got has the PLT entry adddress.  We will load the GOT entry
 293      with the PLT entry in finish_dynamic_symbol if it is used.  For
 294      branch, it uses .got.plt.  For symbol value, if PLT is used,
 295      1. Use .got.plt in a PIC object if it is forced local or not
 296      dynamic.
 297      2. Use .got.plt in a non-PIC object if pointer equality isn't
 298      needed.
 299      3. Use .got.plt in PIE.
 300      4. Use .got.plt if .got isn't used.
 301      5. Otherwise use .got so that it can be shared among different
 302      objects at run-time.
 303      If PLT isn't used, always use .got for symbol value.
 304      We only need to relocate .got entry in PIC object or in dynamic
 305      executable without PLT.  */
 306   if (use_plt
 307       && (h->got.refcount <= 0
 308           || (bfd_link_pic (info)
 309               && (h->dynindx == -1
 310                   || h->forced_local))
 311           || (!bfd_link_pic (info)
 312               && !h->pointer_equality_needed)
 313           || bfd_link_pie (info)
 314           || htab->sgot == NULL))
 315     {
 316       /* Use .got.plt.  */
 317       h->got.offset = (bfd_vma) -1;
 318     }
 319   else
 320     {
 321       if (!use_plt)
 322         {
 323           /* PLT isn't used.  */
 324           h->plt.offset = (bfd_vma) -1;
 325         }
 326       if (h->got.refcount <= 0)
 327         {
 328           /* GOT isn't need when there are only relocations for static
 329              pointers.  */
 330           h->got.offset = (bfd_vma) -1;
 331         }
 332       else
 333         {
 334           h->got.offset = htab->sgot->size;
 335           htab->sgot->size += got_entry_size;
 336           /* Need to relocate the GOT entry in a PIC object or PLT isn't
 337              used.  Otherwise, the GOT entry will be filled with the PLT
 338              entry and dynamic GOT relocation isn't needed.  */
 339           if (need_dynreloc)
 340             {
 341               /* For non-static executable, dynamic GOT relocation is in
 342                  .rel[a].got section, but for static executable, it is
 343                  in .rel[a].iplt section.  */
 344               if (htab->splt != NULL)
 345                 htab->srelgot->size += sizeof_reloc;
 346               else
 347                 {
 348                   relplt->size += sizeof_reloc;
 349                   relplt->reloc_count++;
 350                 }
 351             }
 352         }
 353     }
 354
 355   return TRUE;
 356 }
 357
 358 /* Similar to _bfd_elf_get_synthetic_symtab, optimized for unsorted PLT
 359    entries.  PLT is the PLT section.  PLT_SYM_VAL is a function pointer
 360    which returns an array of PLT entry symbol values.  */
 361
 362 long
 363 _bfd_elf_ifunc_get_synthetic_symtab
 364   (bfd *abfd, long symcount ATTRIBUTE_UNUSED,
 365    asymbol **syms ATTRIBUTE_UNUSED, long dynsymcount, asymbol **dynsyms,
 366    asymbol **ret, asection *plt,
 367    bfd_vma *(*get_plt_sym_val) (bfd *, asymbol **, asection *, asection *))
 368 {
 369   const struct elf_backend_data *bed = get_elf_backend_data (abfd);
 370   asection *relplt;
 371   asymbol *s;
 372   const char *relplt_name;
 373   bfd_boolean (*slurp_relocs) (bfd *, asection *, asymbol **, bfd_boolean);
 374   arelent *p;
 375   long count, i, n;
 376   size_t size;
 377   Elf_Internal_Shdr *hdr;
 378   char *names;
 379   bfd_vma *plt_sym_val;
 380
 381   *ret = NULL;
 382
 383   if (plt == NULL)
 384     return 0;
 385
 386   if ((abfd->flags & (DYNAMIC | EXEC_P)) == 0)
 387     return 0;
 388
 389   if (dynsymcount <= 0)
 390     return 0;
 391
 392   relplt_name = bed->relplt_name;
 393   if (relplt_name == NULL)
 394     relplt_name = bed->rela_plts_and_copies_p ? ".rela.plt" : ".rel.plt";
 395   relplt = bfd_get_section_by_name (abfd, relplt_name);
 396   if (relplt == NULL)
 397     return 0;
 398
 399   hdr = &elf_section_data (relplt)->this_hdr;
 400   if (hdr->sh_link != elf_dynsymtab (abfd)
 401       || (hdr->sh_type != SHT_REL && hdr->sh_type != SHT_RELA))
 402     return 0;
 403
 404   slurp_relocs = get_elf_backend_data (abfd)->s->slurp_reloc_table;
 405   if (! (*slurp_relocs) (abfd, relplt, dynsyms, TRUE))
 406     return -1;
 407
 408   count = relplt->size / hdr->sh_entsize;
 409   size = count * sizeof (asymbol);
 410   p = relplt->relocation;
 411   for (i = 0; i < count; i++, p += bed->s->int_rels_per_ext_rel)
 412     {
 413       size += strlen ((*p->sym_ptr_ptr)->name) + sizeof ("@plt");
 414       if (p->addend != 0)
 415         {
 416 #ifdef BFD64
 417           size += sizeof ("+0x") - 1 + 8 + 8 * (bed->s->elfclass == ELFCLASS64);
 418 #else
 419           size += sizeof ("+0x") - 1 + 8;
 420 #endif
 421         }
 422     }
 423
 424   plt_sym_val = get_plt_sym_val (abfd, dynsyms, plt, relplt);
 425   if (plt_sym_val == NULL)
 426     return -1;
 427
 428   s = *ret = (asymbol *) bfd_malloc (size);
 429   if (s == NULL)
 430     {
 431       free (plt_sym_val);
 432       return -1;
 433     }
 434
 435   names = (char *) (s + count);
 436   p = relplt->relocation;
 437   n = 0;
 438   for (i = 0; i < count; i++, p += bed->s->int_rels_per_ext_rel)
 439     {
 440       size_t len;
 441       bfd_vma addr;
 442
 443       addr = plt_sym_val[i];
 444       if (addr == (bfd_vma) -1)
 445         continue;
 446
 447       *s = **p->sym_ptr_ptr;
 448       /* Undefined syms won't have BSF_LOCAL or BSF_GLOBAL set.  Since
 449          we are defining a symbol, ensure one of them is set.  */
 450       if ((s->flags & BSF_LOCAL) == 0)
 451         s->flags |= BSF_GLOBAL;
 452       s->flags |= BSF_SYNTHETIC;
 453       s->section = plt;
 454       s->value = addr - plt->vma;
 455       s->name = names;
 456       s->udata.p = NULL;
 457       len = strlen ((*p->sym_ptr_ptr)->name);
 458       memcpy (names, (*p->sym_ptr_ptr)->name, len);
 459       names += len;
 460       if (p->addend != 0)
 461         {
 462           char buf[30], *a;
 463
 464           memcpy (names, "+0x", sizeof ("+0x") - 1);
 465           names += sizeof ("+0x") - 1;
 466           bfd_sprintf_vma (abfd, buf, p->addend);
 467           for (a = buf; *a == '0'; ++a)
 468             ;
 469           len = strlen (a);
 470           memcpy (names, a, len);
 471           names += len;
 472         }
 473       memcpy (names, "@plt", sizeof ("@plt"));
 474       names += sizeof ("@plt");
 475       ++s, ++n;
 476     }
 477
 478   free (plt_sym_val);
 479
 480   return n;
 481 }