bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2017 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         /* xgettext:c-format */
 142         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 143            "equality in `%B' can not be used when making an "
 144            "executable; recompile with -fPIE and relink with -pie\n"),
 145          h->root.root.string,
 146          h->root.u.def.section->owner);
 147       bfd_set_error (bfd_error_bad_value);
 148       return FALSE;
 149     }
 150
 151   htab = elf_hash_table (info);
 152
 153   /* When the symbol is marked with regular reference, if PLT isn't used
 154      or we are building a PIC object, we must keep dynamic relocation
 155      if there is non-GOT reference and use PLT if there is PC-relative
 156      reference.  */
 157   if (need_dynreloc && h->ref_regular)
 158     {
 159       bfd_boolean keep = FALSE;
 160       for (p = *head; p != NULL; p = p->next)
 161         if (p->count)
 162           {
 163             h->non_got_ref = 1;
 164             /* Need dynamic relocations for non-GOT reference.  */
 165             keep = TRUE;
 166             if (p->pc_count)
 167               {
 168                 /* Must use PLT for PC-relative reference.  */
 169                 use_plt = TRUE;
 170                 need_dynreloc = bfd_link_pic (info);
 171                 break;
 172               }
 173           }
 174       if (keep)
 175         goto keep;
 176     }
 177
 178   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 179   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 180     {
 181       h->got = htab->init_got_offset;
 182       h->plt = htab->init_plt_offset;
 183       *head = NULL;
 184       return TRUE;
 185     }
 186
 187   /* Return and discard space for dynamic relocations against it if
 188      it is never referenced.  */
 189   if (!h->ref_regular)
 190     {
 191       if (h->plt.refcount > 0
 192           || h->got.refcount > 0)
 193         abort ();
 194       h->got = htab->init_got_offset;
 195       h->plt = htab->init_plt_offset;
 196       *head = NULL;
 197       return TRUE;
 198     }
 199
 200 keep:
 201   bed = get_elf_backend_data (info->output_bfd);
 202   if (bed->rela_plts_and_copies_p)
 203     sizeof_reloc = bed->s->sizeof_rela;
 204   else
 205     sizeof_reloc = bed->s->sizeof_rel;
 206
 207   /* When building a static executable, use .iplt, .igot.plt and
 208      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 209   if (htab->splt != NULL)
 210     {
 211       plt = htab->splt;
 212       gotplt = htab->sgotplt;
 213       relplt = htab->srelplt;
 214
 215       /* If this is the first .plt entry and PLT is used, make room for
 216          the special first entry.  */
 217       if (plt->size == 0 && use_plt)
 218         plt->size += plt_header_size;
 219     }
 220   else
 221     {
 222       plt = htab->iplt;
 223       gotplt = htab->igotplt;
 224       relplt = htab->irelplt;
 225     }
 226
 227   if (use_plt)
 228     {
 229       /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 230          the original value for R_*_IRELATIVE.  */
 231       h->plt.offset = plt->size;
 232
 233       /* Make room for this entry in the .plt/.iplt section.  */
 234       plt->size += plt_entry_size;
 235
 236       /* We also need to make an entry in the .got.plt/.got.iplt section,
 237          which will be placed in the .got section by the linker script.  */
 238       gotplt->size += got_entry_size;
 239     }
 240
 241   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 242      section for GOTPLT relocation if PLT is used.  */
 243   if (use_plt)
 244     {
 245       relplt->size += sizeof_reloc;
 246       relplt->reloc_count++;
 247     }
 248
 249   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 250      there is a non-GOT reference in a PIC object or PLT isn't used.  */
 251   if (!need_dynreloc || !h->non_got_ref)
 252     *head = NULL;
 253
 254   readonly_dynrelocs_against_ifunc = FALSE;
 255
 256   /* Finally, allocate space.  */
 257   p = *head;
 258   if (p != NULL)
 259     {
 260       bfd_size_type count = 0;
 261       do
 262         {
 263           if (!readonly_dynrelocs_against_ifunc)
 264             {
 265               asection *s = p->sec->output_section;
 266               if (s != NULL && (s->flags & SEC_READONLY) != 0)
 267                 readonly_dynrelocs_against_ifunc = TRUE;
 268             }
 269           count += p->count;
 270           p = p->next;
 271         }
 272       while (p != NULL);
 273
 274       /* Dynamic relocations are stored in
 275          1. .rel[a].ifunc section in PIC object.
 276          2. .rel[a].got section in dynamic executable.
 277          3. .rel[a].iplt section in static executable.  */
 278       if (bfd_link_pic (info))
 279         htab->irelifunc->size += count * sizeof_reloc;
 280       else if (htab->splt != NULL)
 281         htab->srelgot->size += count * sizeof_reloc;
 282       else
 283         {
 284           relplt->size += count * sizeof_reloc;
 285           relplt->reloc_count += count;
 286         }
 287     }
 288
 289   if (readonly_dynrelocs_against_ifunc_p)
 290     *readonly_dynrelocs_against_ifunc_p = readonly_dynrelocs_against_ifunc;
 291
 292   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 293      and .got has the PLT entry adddress.  We will load the GOT entry
 294      with the PLT entry in finish_dynamic_symbol if it is used.  For
 295      branch, it uses .got.plt.  For symbol value, if PLT is used,
 296      1. Use .got.plt in a PIC object if it is forced local or not
 297      dynamic.
 298      2. Use .got.plt in a non-PIC object if pointer equality isn't
 299      needed.
 300      3. Use .got.plt in PIE.
 301      4. Use .got.plt if .got isn't used.
 302      5. Otherwise use .got so that it can be shared among different
 303      objects at run-time.
 304      If PLT isn't used, always use .got for symbol value.
 305      We only need to relocate .got entry in PIC object or in dynamic
 306      executable without PLT.  */
 307   if (use_plt
 308       && (h->got.refcount <= 0
 309           || (bfd_link_pic (info)
 310               && (h->dynindx == -1
 311                   || h->forced_local))
 312           || (!bfd_link_pic (info)
 313               && !h->pointer_equality_needed)
 314           || bfd_link_pie (info)
 315           || htab->sgot == NULL))
 316     {
 317       /* Use .got.plt.  */
 318       h->got.offset = (bfd_vma) -1;
 319     }
 320   else
 321     {
 322       if (!use_plt)
 323         {
 324           /* PLT isn't used.  */
 325           h->plt.offset = (bfd_vma) -1;
 326         }
 327       if (h->got.refcount <= 0)
 328         {
 329           /* GOT isn't need when there are only relocations for static
 330              pointers.  */
 331           h->got.offset = (bfd_vma) -1;
 332         }
 333       else
 334         {
 335           h->got.offset = htab->sgot->size;
 336           htab->sgot->size += got_entry_size;
 337           /* Need to relocate the GOT entry in a PIC object or PLT isn't
 338              used.  Otherwise, the GOT entry will be filled with the PLT
 339              entry and dynamic GOT relocation isn't needed.  */
 340           if (need_dynreloc)
 341             {
 342               /* For non-static executable, dynamic GOT relocation is in
 343                  .rel[a].got section, but for static executable, it is
 344                  in .rel[a].iplt section.  */
 345               if (htab->splt != NULL)
 346                 htab->srelgot->size += sizeof_reloc;
 347               else
 348                 {
 349                   relplt->size += sizeof_reloc;
 350                   relplt->reloc_count++;
 351                 }
 352             }
 353         }
 354     }
 355
 356   return TRUE;
 357 }