bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2014 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 (info->shared)
  57     {
  58       /* We need to create .rel[a].ifunc for shared 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                                     unsigned int plt_entry_size,
 114                                     unsigned int plt_header_size,
 115                                     unsigned int got_entry_size)
 116 {
 117   asection *plt, *gotplt, *relplt;
 118   struct elf_dyn_relocs *p;
 119   unsigned int sizeof_reloc;
 120   const struct elf_backend_data *bed;
 121   struct elf_link_hash_table *htab;
 122
 123   /* When a shared library references a STT_GNU_IFUNC symbol defined
 124      in executable, the address of the resolved function may be used.
 125      But in non-shared executable, the address of its .plt slot may
 126      be used.  Pointer equality may not work correctly.  PIE should
 127      be used if pointer equality is required here.  */
 128   if (!info->shared
 129       && (h->dynindx != -1
 130           || info->export_dynamic)
 131       && h->pointer_equality_needed)
 132     {
 133       info->callbacks->einfo
 134         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 135            "equality in `%B' can not be used when making an "
 136            "executable; recompile with -fPIE and relink with -pie\n"),
 137          h->root.root.string,
 138          h->root.u.def.section->owner);
 139       bfd_set_error (bfd_error_bad_value);
 140       return FALSE;
 141     }
 142
 143   htab = elf_hash_table (info);
 144
 145   /* When building shared library, we need to handle the case where it is
 146      marked with regular reference, but not non-GOT reference since the
 147      non-GOT reference bit may not be set here.  */
 148   if (info->shared && !h->non_got_ref && h->ref_regular)
 149     for (p = *head; p != NULL; p = p->next)
 150       if (p->count)
 151         {
 152           h->non_got_ref = 1;
 153           goto keep;
 154         }
 155
 156   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 157   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 158     {
 159       h->got = htab->init_got_offset;
 160       h->plt = htab->init_plt_offset;
 161       *head = NULL;
 162       return TRUE;
 163     }
 164
 165   /* Return and discard space for dynamic relocations against it if
 166      it is never referenced in a non-shared object.  */
 167   if (!h->ref_regular)
 168     {
 169       if (h->plt.refcount > 0
 170           || h->got.refcount > 0)
 171         abort ();
 172       h->got = htab->init_got_offset;
 173       h->plt = htab->init_plt_offset;
 174       *head = NULL;
 175       return TRUE;
 176     }
 177
 178 keep:
 179   bed = get_elf_backend_data (info->output_bfd);
 180   if (bed->rela_plts_and_copies_p)
 181     sizeof_reloc = bed->s->sizeof_rela;
 182   else
 183     sizeof_reloc = bed->s->sizeof_rel;
 184
 185   /* When building a static executable, use .iplt, .igot.plt and
 186      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 187   if (htab->splt != NULL)
 188     {
 189       plt = htab->splt;
 190       gotplt = htab->sgotplt;
 191       relplt = htab->srelplt;
 192
 193       /* If this is the first .plt entry, make room for the special
 194          first entry.  */
 195       if (plt->size == 0)
 196         plt->size += plt_header_size;
 197     }
 198   else
 199     {
 200       plt = htab->iplt;
 201       gotplt = htab->igotplt;
 202       relplt = htab->irelplt;
 203     }
 204
 205   /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 206      the original value for R_*_IRELATIVE.  */
 207   h->plt.offset = plt->size;
 208
 209   /* Make room for this entry in the .plt/.iplt section.  */
 210   plt->size += plt_entry_size;
 211
 212   /* We also need to make an entry in the .got.plt/.got.iplt section,
 213      which will be placed in the .got section by the linker script.  */
 214   gotplt->size += got_entry_size;
 215
 216   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 217      section.  */
 218   relplt->size += sizeof_reloc;
 219   relplt->reloc_count++;
 220
 221   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 222      there is a non-GOT reference in a shared object.  */
 223   if (!info->shared
 224       || !h->non_got_ref)
 225     *head = NULL;
 226
 227   /* Finally, allocate space.  */
 228   p = *head;
 229   if (p != NULL)
 230     {
 231       bfd_size_type count = 0;
 232       do
 233         {
 234           count += p->count;
 235           p = p->next;
 236         }
 237       while (p != NULL);
 238       htab->irelifunc->size += count * sizeof_reloc;
 239     }
 240
 241   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 242      and .got has the PLT entry adddress.  We will load the GOT entry
 243      with the PLT entry in finish_dynamic_symbol if it is used.  For
 244      branch, it uses .got.plt.  For symbol value,
 245      1. Use .got.plt in a shared object if it is forced local or not
 246      dynamic.
 247      2. Use .got.plt in a non-shared object if pointer equality isn't
 248      needed.
 249      3. Use .got.plt in PIE.
 250      4. Use .got.plt if .got isn't used.
 251      5. Otherwise use .got so that it can be shared among different
 252      objects at run-time.
 253      We only need to relocate .got entry in shared object.  */
 254   if (h->got.refcount <= 0
 255       || (info->shared
 256           && (h->dynindx == -1
 257               || h->forced_local))
 258       || (!info->shared
 259           && !h->pointer_equality_needed)
 260       || (info->executable && info->shared)
 261       || htab->sgot == NULL)
 262     {
 263       /* Use .got.plt.  */
 264       h->got.offset = (bfd_vma) -1;
 265     }
 266   else
 267     {
 268       h->got.offset = htab->sgot->size;
 269       htab->sgot->size += got_entry_size;
 270       if (info->shared)
 271         htab->srelgot->size += sizeof_reloc;
 272     }
 273
 274   return TRUE;
 275 }