bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2019 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
 136      If STT_GNU_IFUNC symbol is defined in position-dependent executable,
 137      backend should change it to the normal function and set its address
 138      to its PLT entry which should be resolved by R_*_IRELATIVE at
 139      run-time.  All external references should be resolved to its PLT in
 140      executable.  */
 141   if (!need_dynreloc
 142       && !(bfd_link_pde (info) && h->def_regular)
 143       && (h->dynindx != -1
 144           || info->export_dynamic)
 145       && h->pointer_equality_needed)
 146     {
 147       info->callbacks->einfo
 148         /* xgettext:c-format */
 149         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 150            "equality in `%pB' can not be used when making an "
 151            "executable; recompile with -fPIE and relink with -pie\n"),
 152          h->root.root.string,
 153          h->root.u.def.section->owner);
 154       bfd_set_error (bfd_error_bad_value);
 155       return FALSE;
 156     }
 157
 158   htab = elf_hash_table (info);
 159
 160   /* When the symbol is marked with regular reference, if PLT isn't used
 161      or we are building a PIC object, we must keep dynamic relocation
 162      if there is non-GOT reference and use PLT if there is PC-relative
 163      reference.  */
 164   if (need_dynreloc && h->ref_regular)
 165     {
 166       bfd_boolean keep = FALSE;
 167       for (p = *head; p != NULL; p = p->next)
 168         if (p->count)
 169           {
 170             h->non_got_ref = 1;
 171             /* Need dynamic relocations for non-GOT reference.  */
 172             keep = TRUE;
 173             if (p->pc_count)
 174               {
 175                 /* Must use PLT for PC-relative reference.  */
 176                 use_plt = TRUE;
 177                 need_dynreloc = bfd_link_pic (info);
 178                 break;
 179               }
 180           }
 181       if (keep)
 182         goto keep;
 183     }
 184
 185   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 186   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 187     {
 188       h->got = htab->init_got_offset;
 189       h->plt = htab->init_plt_offset;
 190       *head = NULL;
 191       return TRUE;
 192     }
 193
 194   /* Return and discard space for dynamic relocations against it if
 195      it is never referenced.  */
 196   if (!h->ref_regular)
 197     {
 198       if (h->plt.refcount > 0
 199           || h->got.refcount > 0)
 200         abort ();
 201       h->got = htab->init_got_offset;
 202       h->plt = htab->init_plt_offset;
 203       *head = NULL;
 204       return TRUE;
 205     }
 206
 207 keep:
 208   bed = get_elf_backend_data (info->output_bfd);
 209   if (bed->rela_plts_and_copies_p)
 210     sizeof_reloc = bed->s->sizeof_rela;
 211   else
 212     sizeof_reloc = bed->s->sizeof_rel;
 213
 214   /* When building a static executable, use .iplt, .igot.plt and
 215      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 216   if (htab->splt != NULL)
 217     {
 218       plt = htab->splt;
 219       gotplt = htab->sgotplt;
 220       relplt = htab->srelplt;
 221
 222       /* If this is the first .plt entry and PLT is used, make room for
 223          the special first entry.  */
 224       if (plt->size == 0 && use_plt)
 225         plt->size += plt_header_size;
 226     }
 227   else
 228     {
 229       plt = htab->iplt;
 230       gotplt = htab->igotplt;
 231       relplt = htab->irelplt;
 232     }
 233
 234   if (use_plt)
 235     {
 236       /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 237          the original value for R_*_IRELATIVE.  */
 238       h->plt.offset = plt->size;
 239
 240       /* Make room for this entry in the .plt/.iplt section.  */
 241       plt->size += plt_entry_size;
 242
 243       /* We also need to make an entry in the .got.plt/.got.iplt section,
 244          which will be placed in the .got section by the linker script.  */
 245       gotplt->size += got_entry_size;
 246     }
 247
 248   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 249      section for GOTPLT relocation if PLT is used.  */
 250   if (use_plt)
 251     {
 252       relplt->size += sizeof_reloc;
 253       relplt->reloc_count++;
 254     }
 255
 256   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 257      there is a non-GOT reference in a PIC object or PLT isn't used.  */
 258   if (!need_dynreloc || !h->non_got_ref)
 259     *head = NULL;
 260
 261   readonly_dynrelocs_against_ifunc = FALSE;
 262
 263   /* Finally, allocate space.  */
 264   p = *head;
 265   if (p != NULL)
 266     {
 267       bfd_size_type count = 0;
 268       do
 269         {
 270           if (!readonly_dynrelocs_against_ifunc)
 271             {
 272               asection *s = p->sec->output_section;
 273               if (s != NULL && (s->flags & SEC_READONLY) != 0)
 274                 readonly_dynrelocs_against_ifunc = TRUE;
 275             }
 276           count += p->count;
 277           p = p->next;
 278         }
 279       while (p != NULL);
 280
 281       /* Dynamic relocations are stored in
 282          1. .rel[a].ifunc section in PIC object.
 283          2. .rel[a].got section in dynamic executable.
 284          3. .rel[a].iplt section in static executable.  */
 285       if (bfd_link_pic (info))
 286         htab->irelifunc->size += count * sizeof_reloc;
 287       else if (htab->splt != NULL)
 288         htab->srelgot->size += count * sizeof_reloc;
 289       else
 290         {
 291           relplt->size += count * sizeof_reloc;
 292           relplt->reloc_count += count;
 293         }
 294     }
 295
 296   if (readonly_dynrelocs_against_ifunc_p)
 297     *readonly_dynrelocs_against_ifunc_p = readonly_dynrelocs_against_ifunc;
 298
 299   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 300      and .got has the PLT entry adddress.  We will load the GOT entry
 301      with the PLT entry in finish_dynamic_symbol if it is used.  For
 302      branch, it uses .got.plt.  For symbol value, if PLT is used,
 303      1. Use .got.plt in a PIC object if it is forced local or not
 304      dynamic.
 305      2. Use .got.plt in a non-PIC object if pointer equality isn't
 306      needed.
 307      3. Use .got.plt in PIE.
 308      4. Use .got.plt if .got isn't used.
 309      5. Otherwise use .got so that it can be shared among different
 310      objects at run-time.
 311      If PLT isn't used, always use .got for symbol value.
 312      We only need to relocate .got entry in PIC object or in dynamic
 313      executable without PLT.  */
 314   if (use_plt
 315       && (h->got.refcount <= 0
 316           || (bfd_link_pic (info)
 317               && (h->dynindx == -1
 318                   || h->forced_local))
 319           || (!bfd_link_pic (info)
 320               && !h->pointer_equality_needed)
 321           || bfd_link_pie (info)
 322           || htab->sgot == NULL))
 323     {
 324       /* Use .got.plt.  */
 325       h->got.offset = (bfd_vma) -1;
 326     }
 327   else
 328     {
 329       if (!use_plt)
 330         {
 331           /* PLT isn't used.  */
 332           h->plt.offset = (bfd_vma) -1;
 333         }
 334       if (h->got.refcount <= 0)
 335         {
 336           /* GOT isn't need when there are only relocations for static
 337              pointers.  */
 338           h->got.offset = (bfd_vma) -1;
 339         }
 340       else
 341         {
 342           h->got.offset = htab->sgot->size;
 343           htab->sgot->size += got_entry_size;
 344           /* Need to relocate the GOT entry in a PIC object or PLT isn't
 345              used.  Otherwise, the GOT entry will be filled with the PLT
 346              entry and dynamic GOT relocation isn't needed.  */
 347           if (need_dynreloc)
 348             {
 349               /* For non-static executable, dynamic GOT relocation is in
 350                  .rel[a].got section, but for static executable, it is
 351                  in .rel[a].iplt section.  */
 352               if (htab->splt != NULL)
 353                 htab->srelgot->size += sizeof_reloc;
 354               else
 355                 {
 356                   relplt->size += sizeof_reloc;
 357                   relplt->reloc_count++;
 358                 }
 359             }
 360         }
 361     }
 362
 363   return TRUE;
 364 }