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   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 191   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 192     {
 193       h->got = htab->init_got_offset;
 194       h->plt = htab->init_plt_offset;
 195       *head = NULL;
 196       return TRUE;
 197     }
 198
 199   /* Return and discard space for dynamic relocations against it if
 200      it is never referenced in a non-shared object.  */
 201   if (!h->ref_regular)
 202     {
 203       if (h->plt.refcount > 0
 204           || h->got.refcount > 0)
 205         abort ();
 206       h->got = htab->init_got_offset;
 207       h->plt = htab->init_plt_offset;
 208       *head = NULL;
 209       return TRUE;
 210     }
 211
 212   bed = get_elf_backend_data (info->output_bfd);
 213   if (bed->rela_plts_and_copies_p)
 214     sizeof_reloc = bed->s->sizeof_rela;
 215   else
 216     sizeof_reloc = bed->s->sizeof_rel;
 217
 218   /* When building a static executable, use .iplt, .igot.plt and
 219      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 220   if (htab->splt != NULL)
 221     {
 222       plt = htab->splt;
 223       gotplt = htab->sgotplt;
 224       relplt = htab->srelplt;
 225
 226       /* If this is the first .plt entry, make room for the special
 227          first entry.  */
 228       if (plt->size == 0)
 229         plt->size += plt_entry_size;
 230     }
 231   else
 232     {
 233       plt = htab->iplt;
 234       gotplt = htab->igotplt;
 235       relplt = htab->irelplt;
 236     }
 237
 238   /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 239      the original value for R_*_IRELATIVE.  */
 240   h->plt.offset = plt->size;
 241
 242   /* Make room for this entry in the .plt/.iplt section.  */
 243   plt->size += plt_entry_size;
 244
 245   /* We also need to make an entry in the .got.plt/.got.iplt section,
 246      which will be placed in the .got section by the linker script.  */
 247   gotplt->size += got_entry_size;
 248
 249   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 250      section.  */
 251   relplt->size += sizeof_reloc;
 252   relplt->reloc_count++;
 253
 254   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 255      there is a non-GOT reference in a shared object.  */
 256   if (!info->shared
 257       || !h->non_got_ref)
 258     *head = NULL;
 259
 260   /* Finally, allocate space.  */
 261   for (p = *head; p != NULL; p = p->next)
 262     htab->irelifunc->size += p->count * sizeof_reloc;
 263
 264   /* For STT_GNU_IFUNC symbol, .got.plt has the real function addres
 265      and .got has the PLT entry adddress.  We will load the GOT entry
 266      with the PLT entry in finish_dynamic_symbol if it is used.  For
 267      branch, it uses .got.plt.  For symbol value,
 268      1. Use .got.plt in a shared object if it is forced local or not
 269      dynamic.
 270      2. Use .got.plt in a non-shared object if pointer equality isn't
 271      needed.
 272      3. Use .got.plt in PIE.
 273      4. Use .got.plt if .got isn't used.
 274      5. Otherwise use .got so that it can be shared among different
 275      objects at run-time.
 276      We only need to relocate .got entry in shared object.  */
 277   if ((info->shared
 278        && (h->dynindx == -1
 279            || h->forced_local))
 280       || (!info->shared
 281           && !h->pointer_equality_needed)
 282       || (info->executable && info->shared)
 283       || htab->sgot == NULL)
 284     {
 285       /* Use .got.plt.  */
 286       h->got.offset = (bfd_vma) -1;
 287     }
 288   else
 289     {
 290       h->got.offset = htab->sgot->size;
 291       htab->sgot->size += got_entry_size;
 292       if (info->shared)
 293         htab->srelgot->size += sizeof_reloc;
 294     }
 295
 296   return TRUE;
 297 }