bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright 2009
   3    Free Software Foundation, Inc.
   4
   5    This file is part of BFD, the Binary File Descriptor library.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program; if not, write to the Free Software
  19    Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
  20    MA 02110-1301, USA.  */
  21
  22 #include "sysdep.h"
  23 #include "bfd.h"
  24 #include "bfdlink.h"
  25 #include "libbfd.h"
  26 #define ARCH_SIZE 0
  27 #include "elf-bfd.h"
  28 #include "safe-ctype.h"
  29 #include "libiberty.h"
  30 #include "objalloc.h"
  31
  32 /* Create sections needed by STT_GNU_IFUNC symbol.  */
  33
  34 bfd_boolean
  35 _bfd_elf_create_ifunc_sections (bfd *abfd, struct bfd_link_info *info)
  36 {
  37   flagword flags, pltflags;
  38   asection *s;
  39   const struct elf_backend_data *bed = get_elf_backend_data (abfd);
  40   struct elf_link_hash_table *htab = elf_hash_table (info);
  41
  42   if (htab->irelifunc != NULL || htab->iplt != NULL)
  43     return TRUE;
  44
  45   flags = bed->dynamic_sec_flags;
  46   pltflags = flags;
  47   if (bed->plt_not_loaded)
  48     /* We do not clear SEC_ALLOC here because we still want the OS to
  49        allocate space for the section; it's just that there's nothing
  50        to read in from the object file.  */
  51     pltflags &= ~ (SEC_CODE | SEC_LOAD | SEC_HAS_CONTENTS);
  52   else
  53     pltflags |= SEC_ALLOC | SEC_CODE | SEC_LOAD;
  54   if (bed->plt_readonly)
  55     pltflags |= SEC_READONLY;
  56
  57   if (info->shared)
  58     {
  59       /* We need to create .rel[a].ifunc for shared objects.  */
  60       const char *rel_sec = (bed->rela_plts_and_copies_p
  61                              ? ".rela.ifunc" : ".rel.ifunc");
  62
  63       s = bfd_make_section_with_flags (abfd, rel_sec,
  64                                        flags | SEC_READONLY);
  65       if (s == NULL
  66           || ! bfd_set_section_alignment (abfd, s,
  67                                           bed->s->log_file_align))
  68         return FALSE;
  69       htab->irelifunc = s;
  70     }
  71   else
  72     {
  73       /* We need to create .iplt, .rel[a].iplt, .igot and .igot.plt
  74          for static executables.   */
  75       s = bfd_make_section_with_flags (abfd, ".iplt", pltflags);
  76       if (s == NULL
  77           || ! bfd_set_section_alignment (abfd, s, bed->plt_alignment))
  78         return FALSE;
  79       htab->iplt = s;
  80
  81       s = bfd_make_section_with_flags (abfd,
  82                                        (bed->rela_plts_and_copies_p
  83                                         ? ".rela.iplt" : ".rel.iplt"),
  84                                        flags | SEC_READONLY);
  85       if (s == NULL
  86           || ! bfd_set_section_alignment (abfd, s,
  87                                           bed->s->log_file_align))
  88         return FALSE;
  89       htab->irelplt = s;
  90
  91       /* We don't need the .igot section if we have the .igot.plt
  92          section.  */
  93       if (bed->want_got_plt)
  94         s = bfd_make_section_with_flags (abfd, ".igot.plt", flags);
  95       else
  96         s = bfd_make_section_with_flags (abfd, ".igot", flags);
  97       if (s == NULL
  98           || !bfd_set_section_alignment (abfd, s,
  99                                          bed->s->log_file_align))
 100         return FALSE;
 101       htab->igotplt = s;
 102     }
 103
 104   return TRUE;
 105 }
 106
 107 /* For a STT_GNU_IFUNC symbol, create a dynamic reloc section, SRELOC,
 108    for the input section, SEC, and append this reloc to HEAD.  */
 109
 110 asection *
 111 _bfd_elf_create_ifunc_dyn_reloc (bfd *abfd, struct bfd_link_info *info,
 112                                  asection *sec, asection *sreloc,
 113                                  struct elf_dyn_relocs **head)
 114 {
 115   struct elf_dyn_relocs *p;
 116   struct elf_link_hash_table *htab = elf_hash_table (info);
 117
 118   if (sreloc == NULL)
 119     {
 120       const struct elf_backend_data *bed = get_elf_backend_data (abfd);
 121
 122       if (htab->dynobj == NULL)
 123         htab->dynobj = abfd;
 124
 125       sreloc = _bfd_elf_make_dynamic_reloc_section (sec, htab->dynobj,
 126                                                     bed->s->log_file_align,
 127                                                     abfd,
 128                                                     bed->rela_plts_and_copies_p);
 129       if (sreloc == NULL)
 130         return NULL;
 131     }
 132
 133   p = *head;
 134   if (p == NULL || p->sec != sec)
 135     {
 136       bfd_size_type amt = sizeof *p;
 137
 138       p = ((struct elf_dyn_relocs *) bfd_alloc (htab->dynobj, amt));
 139       if (p == NULL)
 140         return NULL;
 141       p->next = *head;
 142       *head = p;
 143       p->sec = sec;
 144       p->count = 0;
 145       p->pc_count = 0;
 146     }
 147   p->count += 1;
 148
 149   return sreloc;
 150 }
 151
 152 /* Allocate space in .plt, .got and associated reloc sections for
 153    dynamic relocs against a STT_GNU_IFUNC symbol definition.  */
 154
 155 bfd_boolean
 156 _bfd_elf_allocate_ifunc_dyn_relocs (struct bfd_link_info *info,
 157                                     struct elf_link_hash_entry *h,
 158                                     struct elf_dyn_relocs **head,
 159                                     unsigned int plt_entry_size,
 160                                     unsigned int got_entry_size)
 161 {
 162   asection *plt, *gotplt, *relplt;
 163   struct elf_dyn_relocs *p;
 164   unsigned int sizeof_reloc;
 165   const struct elf_backend_data *bed;
 166   struct elf_link_hash_table *htab;
 167
 168   /* When a shared library references a STT_GNU_IFUNC symbol defined
 169      in executable, the address of the resolved function may be used.
 170      But in non-shared executable, the address of its .plt slot may
 171      be used.  Pointer equality may not work correctly.  PIE should
 172      be used if pointer equality is required here.  */
 173   if (!info->shared
 174       && (h->dynindx != -1
 175           || info->export_dynamic)
 176       && h->pointer_equality_needed)
 177     {
 178       info->callbacks->einfo
 179         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 180            "equality in `%B' can not be used when making an "
 181            "executable; recompile with -fPIE and relink with -pie\n"),
 182          h->root.root.string,
 183          h->root.u.def.section->owner);
 184       bfd_set_error (bfd_error_bad_value);
 185       return FALSE;
 186     }
 187
 188   htab = elf_hash_table (info);
 189
 190   /* When building shared library, we need to handle the case where it is
 191      marked with regular reference, but not non-GOT reference since the
 192      non-GOT reference bit may not be set here.  */
 193   if (info->shared && !h->non_got_ref && h->ref_regular)
 194     for (p = *head; p != NULL; p = p->next)
 195       if (p->count)
 196         {
 197           h->non_got_ref = 1;
 198           goto keep;
 199         }
 200
 201   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 202   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 203     {
 204       h->got = htab->init_got_offset;
 205       h->plt = htab->init_plt_offset;
 206       *head = NULL;
 207       return TRUE;
 208     }
 209
 210   /* Return and discard space for dynamic relocations against it if
 211      it is never referenced in a non-shared object.  */
 212   if (!h->ref_regular)
 213     {
 214       if (h->plt.refcount > 0
 215           || h->got.refcount > 0)
 216         abort ();
 217       h->got = htab->init_got_offset;
 218       h->plt = htab->init_plt_offset;
 219       *head = NULL;
 220       return TRUE;
 221     }
 222
 223 keep:
 224   bed = get_elf_backend_data (info->output_bfd);
 225   if (bed->rela_plts_and_copies_p)
 226     sizeof_reloc = bed->s->sizeof_rela;
 227   else
 228     sizeof_reloc = bed->s->sizeof_rel;
 229
 230   /* When building a static executable, use .iplt, .igot.plt and
 231      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 232   if (htab->splt != NULL)
 233     {
 234       plt = htab->splt;
 235       gotplt = htab->sgotplt;
 236       relplt = htab->srelplt;
 237
 238       /* If this is the first .plt entry, make room for the special
 239          first entry.  */
 240       if (plt->size == 0)
 241         plt->size += plt_entry_size;
 242     }
 243   else
 244     {
 245       plt = htab->iplt;
 246       gotplt = htab->igotplt;
 247       relplt = htab->irelplt;
 248     }
 249
 250   /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 251      the original value for R_*_IRELATIVE.  */
 252   h->plt.offset = plt->size;
 253
 254   /* Make room for this entry in the .plt/.iplt section.  */
 255   plt->size += plt_entry_size;
 256
 257   /* We also need to make an entry in the .got.plt/.got.iplt section,
 258      which will be placed in the .got section by the linker script.  */
 259   gotplt->size += got_entry_size;
 260
 261   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 262      section.  */
 263   relplt->size += sizeof_reloc;
 264   relplt->reloc_count++;
 265
 266   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 267      there is a non-GOT reference in a shared object.  */
 268   if (!info->shared
 269       || !h->non_got_ref)
 270     *head = NULL;
 271
 272   /* Finally, allocate space.  */
 273   p = *head;
 274   if (p != NULL)
 275     {
 276       bfd_size_type count = 0;
 277       do
 278         {
 279           count += p->count;
 280           p = p->next;
 281         }
 282       while (p != NULL);
 283       htab->irelifunc->size += count * sizeof_reloc;
 284     }
 285
 286   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 287      and .got has the PLT entry adddress.  We will load the GOT entry
 288      with the PLT entry in finish_dynamic_symbol if it is used.  For
 289      branch, it uses .got.plt.  For symbol value,
 290      1. Use .got.plt in a shared object if it is forced local or not
 291      dynamic.
 292      2. Use .got.plt in a non-shared object if pointer equality isn't
 293      needed.
 294      3. Use .got.plt in PIE.
 295      4. Use .got.plt if .got isn't used.
 296      5. Otherwise use .got so that it can be shared among different
 297      objects at run-time.
 298      We only need to relocate .got entry in shared object.  */
 299   if (h->got.refcount <= 0
 300       || (info->shared
 301           && (h->dynindx == -1
 302               || h->forced_local))
 303       || (!info->shared
 304           && !h->pointer_equality_needed)
 305       || (info->executable && info->shared)
 306       || htab->sgot == NULL)
 307     {
 308       /* Use .got.plt.  */
 309       h->got.offset = (bfd_vma) -1;
 310     }
 311   else
 312     {
 313       h->got.offset = htab->sgot->size;
 314       htab->sgot->size += got_entry_size;
 315       if (info->shared)
 316         htab->srelgot->size += sizeof_reloc;
 317     }
 318
 319   return TRUE;
 320 }