bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2020 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 (s, bed->s->log_file_align))
  66         return FALSE;
  67       htab->irelifunc = s;
  68     }
  69   else
  70     {
  71       /* We need to create .iplt, .rel[a].iplt, .igot and .igot.plt
  72          for static executables.   */
  73       s = bfd_make_section_with_flags (abfd, ".iplt", pltflags);
  74       if (s == NULL
  75           || !bfd_set_section_alignment (s, bed->plt_alignment))
  76         return FALSE;
  77       htab->iplt = s;
  78
  79       s = bfd_make_section_with_flags (abfd,
  80                                        (bed->rela_plts_and_copies_p
  81                                         ? ".rela.iplt" : ".rel.iplt"),
  82                                        flags | SEC_READONLY);
  83       if (s == NULL
  84           || !bfd_set_section_alignment (s, bed->s->log_file_align))
  85         return FALSE;
  86       htab->irelplt = s;
  87
  88       /* We don't need the .igot section if we have the .igot.plt
  89          section.  */
  90       if (bed->want_got_plt)
  91         s = bfd_make_section_with_flags (abfd, ".igot.plt", flags);
  92       else
  93         s = bfd_make_section_with_flags (abfd, ".igot", flags);
  94       if (s == NULL
  95           || !bfd_set_section_alignment (s, bed->s->log_file_align))
  96         return FALSE;
  97       htab->igotplt = s;
  98     }
  99
 100   return TRUE;
 101 }
 102
 103 /* Allocate space in .plt, .got and associated reloc sections for
 104    dynamic relocs against a STT_GNU_IFUNC symbol definition.  */
 105
 106 bfd_boolean
 107 _bfd_elf_allocate_ifunc_dyn_relocs (struct bfd_link_info *info,
 108                                     struct elf_link_hash_entry *h,
 109                                     struct elf_dyn_relocs **head,
 110                                     bfd_boolean *readonly_dynrelocs_against_ifunc_p,
 111                                     unsigned int plt_entry_size,
 112                                     unsigned int plt_header_size,
 113                                     unsigned int got_entry_size,
 114                                     bfd_boolean avoid_plt)
 115 {
 116   asection *plt, *gotplt, *relplt;
 117   struct elf_dyn_relocs *p;
 118   unsigned int sizeof_reloc;
 119   const struct elf_backend_data *bed;
 120   struct elf_link_hash_table *htab;
 121   bfd_boolean readonly_dynrelocs_against_ifunc;
 122   /* If AVOID_PLT is TRUE, don't use PLT if possible.  */
 123   bfd_boolean use_plt = !avoid_plt || h->plt.refcount > 0;
 124   bfd_boolean need_dynreloc = !use_plt || bfd_link_pic (info);
 125
 126   /* When a PIC object references a STT_GNU_IFUNC symbol defined
 127      in executable or it isn't referenced via PLT, the address of
 128      the resolved function may be used.  But in non-PIC executable,
 129      the address of its .plt slot may be used.  Pointer equality may
 130      not work correctly.  PIE or non-PLT reference should be used if
 131      pointer equality is required here.
 132
 133      If STT_GNU_IFUNC symbol is defined in position-dependent executable,
 134      backend should change it to the normal function and set its address
 135      to its PLT entry which should be resolved by R_*_IRELATIVE at
 136      run-time.  All external references should be resolved to its PLT in
 137      executable.  */
 138   if (!need_dynreloc
 139       && !(bfd_link_pde (info) && h->def_regular)
 140       && (h->dynindx != -1
 141           || info->export_dynamic)
 142       && h->pointer_equality_needed)
 143     {
 144       info->callbacks->einfo
 145         /* xgettext:c-format */
 146         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 147            "equality in `%pB' can not be used when making an "
 148            "executable; recompile with -fPIE and relink with -pie\n"),
 149          h->root.root.string,
 150          h->root.u.def.section->owner);
 151       bfd_set_error (bfd_error_bad_value);
 152       return FALSE;
 153     }
 154
 155   htab = elf_hash_table (info);
 156
 157   /* When the symbol is marked with regular reference, if PLT isn't used
 158      or we are building a PIC object, we must keep dynamic relocation
 159      if there is non-GOT reference and use PLT if there is PC-relative
 160      reference.  */
 161   if (need_dynreloc && h->ref_regular)
 162     {
 163       bfd_boolean keep = FALSE;
 164       for (p = *head; p != NULL; p = p->next)
 165         if (p->count)
 166           {
 167             h->non_got_ref = 1;
 168             /* Need dynamic relocations for non-GOT reference.  */
 169             keep = TRUE;
 170             if (p->pc_count)
 171               {
 172                 /* Must use PLT for PC-relative reference.  */
 173                 use_plt = TRUE;
 174                 need_dynreloc = bfd_link_pic (info);
 175                 break;
 176               }
 177           }
 178       if (keep)
 179         goto keep;
 180     }
 181
 182   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 183   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 184     {
 185       h->got = htab->init_got_offset;
 186       h->plt = htab->init_plt_offset;
 187       *head = NULL;
 188       return TRUE;
 189     }
 190
 191   /* Return and discard space for dynamic relocations against it if
 192      it is never referenced.  */
 193   if (!h->ref_regular)
 194     {
 195       if (h->plt.refcount > 0
 196           || h->got.refcount > 0)
 197         abort ();
 198       h->got = htab->init_got_offset;
 199       h->plt = htab->init_plt_offset;
 200       *head = NULL;
 201       return TRUE;
 202     }
 203
 204  keep:
 205   bed = get_elf_backend_data (info->output_bfd);
 206   if (bed->rela_plts_and_copies_p)
 207     sizeof_reloc = bed->s->sizeof_rela;
 208   else
 209     sizeof_reloc = bed->s->sizeof_rel;
 210
 211   /* When building a static executable, use .iplt, .igot.plt and
 212      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 213   if (htab->splt != NULL)
 214     {
 215       plt = htab->splt;
 216       gotplt = htab->sgotplt;
 217       relplt = htab->srelplt;
 218
 219       /* If this is the first .plt entry and PLT is used, make room for
 220          the special first entry.  */
 221       if (plt->size == 0 && use_plt)
 222         plt->size += plt_header_size;
 223     }
 224   else
 225     {
 226       plt = htab->iplt;
 227       gotplt = htab->igotplt;
 228       relplt = htab->irelplt;
 229     }
 230
 231   if (use_plt)
 232     {
 233       /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 234          the original value for R_*_IRELATIVE.  */
 235       h->plt.offset = plt->size;
 236
 237       /* Make room for this entry in the .plt/.iplt section.  */
 238       plt->size += plt_entry_size;
 239
 240       /* We also need to make an entry in the .got.plt/.got.iplt section,
 241          which will be placed in the .got section by the linker script.  */
 242       gotplt->size += got_entry_size;
 243     }
 244
 245   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 246      section for GOTPLT relocation if PLT is used.  */
 247   if (use_plt)
 248     {
 249       relplt->size += sizeof_reloc;
 250       relplt->reloc_count++;
 251     }
 252
 253   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 254      there is a non-GOT reference in a PIC object or PLT isn't used.  */
 255   if (!need_dynreloc || !h->non_got_ref)
 256     *head = NULL;
 257
 258   readonly_dynrelocs_against_ifunc = FALSE;
 259
 260   /* Finally, allocate space.  */
 261   p = *head;
 262   if (p != NULL)
 263     {
 264       bfd_size_type count = 0;
 265       do
 266         {
 267           if (!readonly_dynrelocs_against_ifunc)
 268             {
 269               asection *s = p->sec->output_section;
 270               if (s != NULL && (s->flags & SEC_READONLY) != 0)
 271                 readonly_dynrelocs_against_ifunc = TRUE;
 272             }
 273           count += p->count;
 274           p = p->next;
 275         }
 276       while (p != NULL);
 277
 278       /* Dynamic relocations are stored in
 279          1. .rel[a].ifunc section in PIC object.
 280          2. .rel[a].got section in dynamic executable.
 281          3. .rel[a].iplt section in static executable.  */
 282       if (bfd_link_pic (info))
 283         htab->irelifunc->size += count * sizeof_reloc;
 284       else if (htab->splt != NULL)
 285         htab->srelgot->size += count * sizeof_reloc;
 286       else
 287         {
 288           relplt->size += count * sizeof_reloc;
 289           relplt->reloc_count += count;
 290         }
 291     }
 292
 293   if (readonly_dynrelocs_against_ifunc_p)
 294     *readonly_dynrelocs_against_ifunc_p = readonly_dynrelocs_against_ifunc;
 295
 296   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 297      and .got has the PLT entry adddress.  We will load the GOT entry
 298      with the PLT entry in finish_dynamic_symbol if it is used.  For
 299      branch, it uses .got.plt.  For symbol value, if PLT is used,
 300      1. Use .got.plt in a PIC object if it is forced local or not
 301      dynamic.
 302      2. Use .got.plt in a non-PIC object if pointer equality isn't
 303      needed.
 304      3. Use .got.plt in PIE.
 305      4. Use .got.plt if .got isn't used.
 306      5. Otherwise use .got so that it can be shared among different
 307      objects at run-time.
 308      If PLT isn't used, always use .got for symbol value.
 309      We only need to relocate .got entry in PIC object or in dynamic
 310      executable without PLT.  */
 311   if (use_plt
 312       && (h->got.refcount <= 0
 313           || (bfd_link_pic (info)
 314               && (h->dynindx == -1
 315                   || h->forced_local))
 316           || (!bfd_link_pic (info)
 317               && !h->pointer_equality_needed)
 318           || bfd_link_pie (info)
 319           || htab->sgot == NULL))
 320     {
 321       /* Use .got.plt.  */
 322       h->got.offset = (bfd_vma) -1;
 323     }
 324   else
 325     {
 326       if (!use_plt)
 327         {
 328           /* PLT isn't used.  */
 329           h->plt.offset = (bfd_vma) -1;
 330         }
 331       if (h->got.refcount <= 0)
 332         {
 333           /* GOT isn't need when there are only relocations for static
 334              pointers.  */
 335           h->got.offset = (bfd_vma) -1;
 336         }
 337       else
 338         {
 339           h->got.offset = htab->sgot->size;
 340           htab->sgot->size += got_entry_size;
 341           /* Need to relocate the GOT entry in a PIC object or PLT isn't
 342              used.  Otherwise, the GOT entry will be filled with the PLT
 343              entry and dynamic GOT relocation isn't needed.  */
 344           if (need_dynreloc)
 345             {
 346               /* For non-static executable, dynamic GOT relocation is in
 347                  .rel[a].got section, but for static executable, it is
 348                  in .rel[a].iplt section.  */
 349               if (htab->splt != NULL)
 350                 htab->srelgot->size += sizeof_reloc;
 351               else
 352                 {
 353                   relplt->size += sizeof_reloc;
 354                   relplt->reloc_count++;
 355                 }
 356             }
 357         }
 358     }
 359
 360   return TRUE;
 361 }