bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2021 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                                     unsigned int plt_entry_size,
 111                                     unsigned int plt_header_size,
 112                                     unsigned int got_entry_size,
 113                                     bfd_boolean avoid_plt)
 114 {
 115   asection *plt, *gotplt, *relplt;
 116   struct elf_dyn_relocs *p;
 117   unsigned int sizeof_reloc;
 118   const struct elf_backend_data *bed;
 119   struct elf_link_hash_table *htab;
 120   /* If AVOID_PLT is TRUE, don't use PLT if possible.  */
 121   bfd_boolean use_plt = !avoid_plt || h->plt.refcount > 0;
 122   bfd_boolean need_dynreloc = !use_plt || bfd_link_pic (info);
 123
 124   /* When a PIC object references a STT_GNU_IFUNC symbol defined
 125      in executable or it isn't referenced via PLT, the address of
 126      the resolved function may be used.  But in non-PIC executable,
 127      the address of its .plt slot may be used.  Pointer equality may
 128      not work correctly.  PIE or non-PLT reference should be used if
 129      pointer equality is required here.
 130
 131      If STT_GNU_IFUNC symbol is defined in position-dependent executable,
 132      backend should change it to the normal function and set its address
 133      to its PLT entry which should be resolved by R_*_IRELATIVE at
 134      run-time.  All external references should be resolved to its PLT in
 135      executable.  */
 136   if (!need_dynreloc
 137       && !(bfd_link_pde (info) && h->def_regular)
 138       && (h->dynindx != -1
 139           || info->export_dynamic)
 140       && h->pointer_equality_needed)
 141     {
 142       info->callbacks->einfo
 143         /* xgettext:c-format */
 144         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 145            "equality in `%pB' can not be used when making an "
 146            "executable; recompile with -fPIE and relink with -pie\n"),
 147          h->root.root.string,
 148          h->root.u.def.section->owner);
 149       bfd_set_error (bfd_error_bad_value);
 150       return FALSE;
 151     }
 152
 153   htab = elf_hash_table (info);
 154
 155   /* When the symbol is marked with regular reference, if PLT isn't used
 156      or we are building a PIC object, we must keep dynamic relocation
 157      if there is non-GOT reference and use PLT if there is PC-relative
 158      reference.  */
 159   if (need_dynreloc && h->ref_regular)
 160     {
 161       bfd_boolean keep = FALSE;
 162       for (p = *head; p != NULL; p = p->next)
 163         if (p->count)
 164           {
 165             h->non_got_ref = 1;
 166             /* Need dynamic relocations for non-GOT reference.  */
 167             keep = TRUE;
 168             if (p->pc_count)
 169               {
 170                 /* Must use PLT for PC-relative reference.  */
 171                 use_plt = TRUE;
 172                 need_dynreloc = bfd_link_pic (info);
 173                 break;
 174               }
 175           }
 176       if (keep)
 177         goto keep;
 178     }
 179
 180   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 181   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 182     {
 183       h->got = htab->init_got_offset;
 184       h->plt = htab->init_plt_offset;
 185       *head = NULL;
 186       return TRUE;
 187     }
 188
 189   /* Return and discard space for dynamic relocations against it if
 190      it is never referenced.  */
 191   if (!h->ref_regular)
 192     {
 193       if (h->plt.refcount > 0
 194           || h->got.refcount > 0)
 195         abort ();
 196       h->got = htab->init_got_offset;
 197       h->plt = htab->init_plt_offset;
 198       *head = NULL;
 199       return TRUE;
 200     }
 201
 202  keep:
 203   bed = get_elf_backend_data (info->output_bfd);
 204   if (bed->rela_plts_and_copies_p)
 205     sizeof_reloc = bed->s->sizeof_rela;
 206   else
 207     sizeof_reloc = bed->s->sizeof_rel;
 208
 209   /* When building a static executable, use .iplt, .igot.plt and
 210      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 211   if (htab->splt != NULL)
 212     {
 213       plt = htab->splt;
 214       gotplt = htab->sgotplt;
 215       relplt = htab->srelplt;
 216
 217       /* If this is the first .plt entry and PLT is used, make room for
 218          the special first entry.  */
 219       if (plt->size == 0 && use_plt)
 220         plt->size += plt_header_size;
 221     }
 222   else
 223     {
 224       plt = htab->iplt;
 225       gotplt = htab->igotplt;
 226       relplt = htab->irelplt;
 227     }
 228
 229   if (use_plt)
 230     {
 231       /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 232          the original value for R_*_IRELATIVE.  */
 233       h->plt.offset = plt->size;
 234
 235       /* Make room for this entry in the .plt/.iplt section.  */
 236       plt->size += plt_entry_size;
 237
 238       /* We also need to make an entry in the .got.plt/.got.iplt section,
 239          which will be placed in the .got section by the linker script.  */
 240       gotplt->size += got_entry_size;
 241     }
 242
 243   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 244      section for GOTPLT relocation if PLT is used.  */
 245   if (use_plt)
 246     {
 247       relplt->size += sizeof_reloc;
 248       relplt->reloc_count++;
 249     }
 250
 251   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 252      there is a non-GOT reference in a PIC object or PLT isn't used.  */
 253   if (!need_dynreloc || !h->non_got_ref)
 254     *head = NULL;
 255
 256   /* Finally, allocate space.  */
 257   p = *head;
 258   if (p != NULL)
 259     {
 260       bfd_size_type count = 0;
 261       do
 262         {
 263           count += p->count;
 264           p = p->next;
 265         }
 266       while (p != NULL);
 267
 268       htab->ifunc_resolvers = count != 0;
 269
 270       /* Dynamic relocations are stored in
 271          1. .rel[a].ifunc section in PIC object.
 272          2. .rel[a].got section in dynamic executable.
 273          3. .rel[a].iplt section in static executable.  */
 274       if (bfd_link_pic (info))
 275         htab->irelifunc->size += count * sizeof_reloc;
 276       else if (htab->splt != NULL)
 277         htab->srelgot->size += count * sizeof_reloc;
 278       else
 279         {
 280           relplt->size += count * sizeof_reloc;
 281           relplt->reloc_count += count;
 282         }
 283     }
 284
 285   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 286      and .got has the PLT entry adddress.  We will load the GOT entry
 287      with the PLT entry in finish_dynamic_symbol if it is used.  For
 288      branch, it uses .got.plt.  For symbol value, if PLT is used,
 289      1. Use .got.plt in a PIC object if it is forced local or not
 290      dynamic.
 291      2. Use .got.plt in a non-PIC object if pointer equality isn't
 292      needed.
 293      3. Use .got.plt in PIE.
 294      4. Use .got.plt if .got isn't used.
 295      5. Otherwise use .got so that it can be shared among different
 296      objects at run-time.
 297      If PLT isn't used, always use .got for symbol value.
 298      We only need to relocate .got entry in PIC object or in dynamic
 299      executable without PLT.  */
 300   if (use_plt
 301       && (h->got.refcount <= 0
 302           || (bfd_link_pic (info)
 303               && (h->dynindx == -1
 304                   || h->forced_local))
 305           || (!bfd_link_pic (info)
 306               && !h->pointer_equality_needed)
 307           || bfd_link_pie (info)
 308           || htab->sgot == NULL))
 309     {
 310       /* Use .got.plt.  */
 311       h->got.offset = (bfd_vma) -1;
 312     }
 313   else
 314     {
 315       if (!use_plt)
 316         {
 317           /* PLT isn't used.  */
 318           h->plt.offset = (bfd_vma) -1;
 319         }
 320       if (h->got.refcount <= 0)
 321         {
 322           /* GOT isn't need when there are only relocations for static
 323              pointers.  */
 324           h->got.offset = (bfd_vma) -1;
 325         }
 326       else
 327         {
 328           h->got.offset = htab->sgot->size;
 329           htab->sgot->size += got_entry_size;
 330           /* Need to relocate the GOT entry in a PIC object or PLT isn't
 331              used.  Otherwise, the GOT entry will be filled with the PLT
 332              entry and dynamic GOT relocation isn't needed.  */
 333           if (need_dynreloc)
 334             {
 335               /* For non-static executable, dynamic GOT relocation is in
 336                  .rel[a].got section, but for static executable, it is
 337                  in .rel[a].iplt section.  */
 338               if (htab->splt != NULL)
 339                 htab->srelgot->size += sizeof_reloc;
 340               else
 341                 {
 342                   relplt->size += sizeof_reloc;
 343                   relplt->reloc_count++;
 344                 }
 345             }
 346         }
 347     }
 348
 349   return TRUE;
 350 }