bfd/elf-ifunc.c

   1 /* ELF STT_GNU_IFUNC support.
   2    Copyright (C) 2009-2016 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 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                                     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 {
 118   asection *plt, *gotplt, *relplt;
 119   struct elf_dyn_relocs *p;
 120   unsigned int sizeof_reloc;
 121   const struct elf_backend_data *bed;
 122   struct elf_link_hash_table *htab;
 123   bfd_boolean readonly_dynrelocs_against_ifunc;
 124
 125   /* When a shared library references a STT_GNU_IFUNC symbol defined
 126      in executable, the address of the resolved function may be used.
 127      But in non-shared executable, the address of its .plt slot may
 128      be used.  Pointer equality may not work correctly.  PIE should
 129      be used if pointer equality is required here.  */
 130   if (!bfd_link_pic (info)
 131       && (h->dynindx != -1
 132           || info->export_dynamic)
 133       && h->pointer_equality_needed)
 134     {
 135       info->callbacks->einfo
 136         (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer "
 137            "equality in `%B' can not be used when making an "
 138            "executable; recompile with -fPIE and relink with -pie\n"),
 139          h->root.root.string,
 140          h->root.u.def.section->owner);
 141       bfd_set_error (bfd_error_bad_value);
 142       return FALSE;
 143     }
 144
 145   htab = elf_hash_table (info);
 146
 147   /* When building shared library, we need to handle the case where it is
 148      marked with regular reference, but not non-GOT reference since the
 149      non-GOT reference bit may not be set here.  */
 150   if (bfd_link_pic (info) && !h->non_got_ref && h->ref_regular)
 151     for (p = *head; p != NULL; p = p->next)
 152       if (p->count)
 153         {
 154           h->non_got_ref = 1;
 155           goto keep;
 156         }
 157
 158   /* Support garbage collection against STT_GNU_IFUNC symbols.  */
 159   if (h->plt.refcount <= 0 && h->got.refcount <= 0)
 160     {
 161       h->got = htab->init_got_offset;
 162       h->plt = htab->init_plt_offset;
 163       *head = NULL;
 164       return TRUE;
 165     }
 166
 167   /* Return and discard space for dynamic relocations against it if
 168      it is never referenced in a non-shared object.  */
 169   if (!h->ref_regular)
 170     {
 171       if (h->plt.refcount > 0
 172           || h->got.refcount > 0)
 173         abort ();
 174       h->got = htab->init_got_offset;
 175       h->plt = htab->init_plt_offset;
 176       *head = NULL;
 177       return TRUE;
 178     }
 179
 180 keep:
 181   bed = get_elf_backend_data (info->output_bfd);
 182   if (bed->rela_plts_and_copies_p)
 183     sizeof_reloc = bed->s->sizeof_rela;
 184   else
 185     sizeof_reloc = bed->s->sizeof_rel;
 186
 187   /* When building a static executable, use .iplt, .igot.plt and
 188      .rel[a].iplt sections for STT_GNU_IFUNC symbols.  */
 189   if (htab->splt != NULL)
 190     {
 191       plt = htab->splt;
 192       gotplt = htab->sgotplt;
 193       relplt = htab->srelplt;
 194
 195       /* If this is the first .plt entry, make room for the special
 196          first entry.  */
 197       if (plt->size == 0)
 198         plt->size += plt_header_size;
 199     }
 200   else
 201     {
 202       plt = htab->iplt;
 203       gotplt = htab->igotplt;
 204       relplt = htab->irelplt;
 205     }
 206
 207   /* Don't update value of STT_GNU_IFUNC symbol to PLT.  We need
 208      the original value for R_*_IRELATIVE.  */
 209   h->plt.offset = plt->size;
 210
 211   /* Make room for this entry in the .plt/.iplt section.  */
 212   plt->size += plt_entry_size;
 213
 214   /* We also need to make an entry in the .got.plt/.got.iplt section,
 215      which will be placed in the .got section by the linker script.  */
 216   gotplt->size += got_entry_size;
 217
 218   /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt
 219      section.  */
 220   relplt->size += sizeof_reloc;
 221   relplt->reloc_count++;
 222
 223   /* We need dynamic relocation for STT_GNU_IFUNC symbol only when
 224      there is a non-GOT reference in a shared object.  */
 225   if (!bfd_link_pic (info)
 226       || !h->non_got_ref)
 227     *head = NULL;
 228
 229   readonly_dynrelocs_against_ifunc = FALSE;
 230
 231   /* Finally, allocate space.  */
 232   p = *head;
 233   if (p != NULL)
 234     {
 235       bfd_size_type count = 0;
 236       do
 237         {
 238           if (!readonly_dynrelocs_against_ifunc)
 239             {
 240               asection *s = p->sec->output_section;
 241               if (s != NULL && (s->flags & SEC_READONLY) != 0)
 242                 readonly_dynrelocs_against_ifunc = TRUE;
 243             }
 244           count += p->count;
 245           p = p->next;
 246         }
 247       while (p != NULL);
 248       htab->irelifunc->size += count * sizeof_reloc;
 249     }
 250
 251   if (readonly_dynrelocs_against_ifunc_p)
 252     *readonly_dynrelocs_against_ifunc_p = readonly_dynrelocs_against_ifunc;
 253
 254   /* For STT_GNU_IFUNC symbol, .got.plt has the real function address
 255      and .got has the PLT entry adddress.  We will load the GOT entry
 256      with the PLT entry in finish_dynamic_symbol if it is used.  For
 257      branch, it uses .got.plt.  For symbol value,
 258      1. Use .got.plt in a shared object if it is forced local or not
 259      dynamic.
 260      2. Use .got.plt in a non-shared object if pointer equality isn't
 261      needed.
 262      3. Use .got.plt in PIE.
 263      4. Use .got.plt if .got isn't used.
 264      5. Otherwise use .got so that it can be shared among different
 265      objects at run-time.
 266      We only need to relocate .got entry in shared object.  */
 267   if (h->got.refcount <= 0
 268       || (bfd_link_pic (info)
 269           && (h->dynindx == -1
 270               || h->forced_local))
 271       || (!bfd_link_pic (info)
 272           && !h->pointer_equality_needed)
 273       || bfd_link_pie (info)
 274       || htab->sgot == NULL)
 275     {
 276       /* Use .got.plt.  */
 277       h->got.offset = (bfd_vma) -1;
 278     }
 279   else
 280     {
 281       h->got.offset = htab->sgot->size;
 282       htab->sgot->size += got_entry_size;
 283       if (bfd_link_pic (info))
 284         htab->srelgot->size += sizeof_reloc;
 285     }
 286
 287   return TRUE;
 288 }
 289
 290 /* Similar to _bfd_elf_get_synthetic_symtab, optimized for unsorted PLT
 291    entries.  PLT is the PLT section.  PLT_SYM_VAL is a function pointer
 292    which returns an array of PLT entry symbol values.  */
 293
 294 long
 295 _bfd_elf_ifunc_get_synthetic_symtab
 296   (bfd *abfd, long symcount ATTRIBUTE_UNUSED,
 297    asymbol **syms ATTRIBUTE_UNUSED, long dynsymcount, asymbol **dynsyms,
 298    asymbol **ret, asection *plt,
 299    bfd_vma *(*get_plt_sym_val) (bfd *, asymbol **, asection *, asection *))
 300 {
 301   const struct elf_backend_data *bed = get_elf_backend_data (abfd);
 302   asection *relplt;
 303   asymbol *s;
 304   const char *relplt_name;
 305   bfd_boolean (*slurp_relocs) (bfd *, asection *, asymbol **, bfd_boolean);
 306   arelent *p;
 307   long count, i, n;
 308   size_t size;
 309   Elf_Internal_Shdr *hdr;
 310   char *names;
 311   bfd_vma *plt_sym_val;
 312
 313   *ret = NULL;
 314
 315   if (plt == NULL)
 316     return 0;
 317
 318   if ((abfd->flags & (DYNAMIC | EXEC_P)) == 0)
 319     return 0;
 320
 321   if (dynsymcount <= 0)
 322     return 0;
 323
 324   relplt_name = bed->relplt_name;
 325   if (relplt_name == NULL)
 326     relplt_name = bed->rela_plts_and_copies_p ? ".rela.plt" : ".rel.plt";
 327   relplt = bfd_get_section_by_name (abfd, relplt_name);
 328   if (relplt == NULL)
 329     return 0;
 330
 331   hdr = &elf_section_data (relplt)->this_hdr;
 332   if (hdr->sh_link != elf_dynsymtab (abfd)
 333       || (hdr->sh_type != SHT_REL && hdr->sh_type != SHT_RELA))
 334     return 0;
 335
 336   slurp_relocs = get_elf_backend_data (abfd)->s->slurp_reloc_table;
 337   if (! (*slurp_relocs) (abfd, relplt, dynsyms, TRUE))
 338     return -1;
 339
 340   count = relplt->size / hdr->sh_entsize;
 341   size = count * sizeof (asymbol);
 342   p = relplt->relocation;
 343   for (i = 0; i < count; i++, p += bed->s->int_rels_per_ext_rel)
 344     {
 345       size += strlen ((*p->sym_ptr_ptr)->name) + sizeof ("@plt");
 346       if (p->addend != 0)
 347         {
 348 #ifdef BFD64
 349           size += sizeof ("+0x") - 1 + 8 + 8 * (bed->s->elfclass == ELFCLASS64);
 350 #else
 351           size += sizeof ("+0x") - 1 + 8;
 352 #endif
 353         }
 354     }
 355
 356   plt_sym_val = get_plt_sym_val (abfd, dynsyms, plt, relplt);
 357   if (plt_sym_val == NULL)
 358     return -1;
 359
 360   s = *ret = (asymbol *) bfd_malloc (size);
 361   if (s == NULL)
 362     {
 363       free (plt_sym_val);
 364       return -1;
 365     }
 366
 367   names = (char *) (s + count);
 368   p = relplt->relocation;
 369   n = 0;
 370   for (i = 0; i < count; i++, p += bed->s->int_rels_per_ext_rel)
 371     {
 372       size_t len;
 373       bfd_vma addr;
 374
 375       addr = plt_sym_val[i];
 376       if (addr == (bfd_vma) -1)
 377         continue;
 378
 379       *s = **p->sym_ptr_ptr;
 380       /* Undefined syms won't have BSF_LOCAL or BSF_GLOBAL set.  Since
 381          we are defining a symbol, ensure one of them is set.  */
 382       if ((s->flags & BSF_LOCAL) == 0)
 383         s->flags |= BSF_GLOBAL;
 384       s->flags |= BSF_SYNTHETIC;
 385       s->section = plt;
 386       s->value = addr - plt->vma;
 387       s->name = names;
 388       s->udata.p = NULL;
 389       len = strlen ((*p->sym_ptr_ptr)->name);
 390       memcpy (names, (*p->sym_ptr_ptr)->name, len);
 391       names += len;
 392       if (p->addend != 0)
 393         {
 394           char buf[30], *a;
 395
 396           memcpy (names, "+0x", sizeof ("+0x") - 1);
 397           names += sizeof ("+0x") - 1;
 398           bfd_sprintf_vma (abfd, buf, p->addend);
 399           for (a = buf; *a == '0'; ++a)
 400             ;
 401           len = strlen (a);
 402           memcpy (names, a, len);
 403           names += len;
 404         }
 405       memcpy (names, "@plt", sizeof ("@plt"));
 406       names += sizeof ("@plt");
 407       ++s, ++n;
 408     }
 409
 410   free (plt_sym_val);
 411
 412   return n;
 413 }