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       /* When building shared library, we need to handle the case
 194          where it is marked with regular reference, but not non-GOT
 195          reference.  It may happen if we didn't see STT_GNU_IFUNC
 196          symbol at the time when checking relocations.  */
 197       if (info->shared
 198           && !h->non_got_ref
 199           && h->ref_regular)
 200         for (p = *head; p != NULL; p = p->next)
 201           if (p->count)
 202             {
 203               h->non_got_ref = 1;
 204               goto keep;
 205             }
 206
 207       h->got = htab->init_got_offset;
 208       h->plt = htab->init_plt_offset;
 209       *head = NULL;
 210       return TRUE;
 211     }
 212
 213   /* Return and discard space for dynamic relocations against it if
 214      it is never referenced in a non-shared object.  */
 215   if (!h->ref_regular)
 216     {
 217       if (h->plt.refcount > 0
 218           || h->got.refcount > 0)
 219         abort ();
 220       h->got = htab->init_got_offset;
 221       h->plt = htab->init_plt_offset;
 222       *head = NULL;
 223       return TRUE;
 224     }
 225
 226 keep:
 227   bed = get_elf_backend_data (info->output_bfd);
 228   if (bed->rela_plts_and_copies_p)
 229     sizeof_reloc = bed->s->sizeof_rela;
 230   else
 231     sizeof_reloc = bed->s->sizeof_rel;
 232
 233   /* When building a static executable, use .iplt, .igot.plt and
 234      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 235   if (htab->splt != NULL)
 236     {
 237       plt = htab->splt;
 238       gotplt = htab->sgotplt;
 239       relplt = htab->srelplt;
 240
 241       /* If this is the first .plt entry, make room for the special
 242          first entry.  */
 243       if (plt->size == 0)
 244         plt->size += plt_entry_size;
 245     }
 246   else
 247     {
 248       plt = htab->iplt;
 249       gotplt = htab->igotplt;
 250       relplt = htab->irelplt;
 251     }
 252
 253   /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 254      the original value for R_*_IRELATIVE.  */
 255   h->plt.offset = plt->size;
 256
 257   /* Make room for this entry in the .plt/.iplt section.  */
 258   plt->size += plt_entry_size;
 259
 260   /* We also need to make an entry in the .got.plt/.got.iplt section,
 261      which will be placed in the .got section by the linker script.  */
 262   gotplt->size += got_entry_size;
 263
 264   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 265      section.  */
 266   relplt->size += sizeof_reloc;
 267   relplt->reloc_count++;
 268
 269   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 270      there is a non-GOT reference in a shared object.  */
 271   if (!info->shared
 272       || !h->non_got_ref)
 273     *head = NULL;
 274
 275   /* Finally, allocate space.  */
 276   p = *head;
 277   if (p != NULL)
 278     {
 279       bfd_size_type count = 0;
 280       do
 281         {
 282           count += p->count;
 283           p = p->next;
 284         }
 285       while (p != NULL);
 286       htab->irelifunc->size += count * sizeof_reloc;
 287     }
 288
 289   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 290      and .got has the PLT entry adddress.  We will load the GOT entry
 291      with the PLT entry in finish_dynamic_symbol if it is used.  For
 292      branch, it uses .got.plt.  For symbol value,
 293      1. Use .got.plt in a shared object if it is forced local or not
 294      dynamic.
 295      2. Use .got.plt in a non-shared object if pointer equality isn't
 296      needed.
 297      3. Use .got.plt in PIE.
 298      4. Use .got.plt if .got isn't used.
 299      5. Otherwise use .got so that it can be shared among different
 300      objects at run-time.
 301      We only need to relocate .got entry in shared object.  */
 302   if (h->got.refcount <= 0
 303       || (info->shared
 304           && (h->dynindx == -1
 305               || h->forced_local))
 306       || (!info->shared
 307           && !h->pointer_equality_needed)
 308       || (info->executable && info->shared)
 309       || htab->sgot == NULL)
 310     {
 311       /* Use .got.plt.  */
 312       h->got.offset = (bfd_vma) -1;
 313     }
 314   else
 315     {
 316       h->got.offset = htab->sgot->size;
 317       htab->sgot->size += got_entry_size;
 318       if (info->shared)
 319         htab->srelgot->size += sizeof_reloc;
 320     }
 321
 322   return TRUE;
 323 }