1 /* ELF STT_GNU_IFUNC support. 2 Copyright 2009 3 Free Software Foundation, Inc. 4 5 This file is part of BFD, the Binary File Descriptor library. 6 7 This program is free software; you can redistribute it and/or modify 8 it under the terms of the GNU General Public License as published by 9 the Free Software Foundation; either version 3 of the License, or 10 (at your option) any later version. 11 12 This program is distributed in the hope that it will be useful, 13 but WITHOUT ANY WARRANTY; without even the implied warranty of 14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 15 GNU General Public License for more details. 16 17 You should have received a copy of the GNU General Public License 18 along with this program; if not, write to the Free Software 19 Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, 20 MA 02110-1301, USA. */ 21 22 #include "sysdep.h" 23 #include "bfd.h" 24 #include "bfdlink.h" 25 #include "libbfd.h" 26 #define ARCH_SIZE 0 27 #include "elf-bfd.h" 28 #include "safe-ctype.h" 29 #include "libiberty.h" 30 #include "objalloc.h" 31 32 /* Create sections needed by STT_GNU_IFUNC symbol. */ 33 34 bfd_boolean 35 _bfd_elf_create_ifunc_sections (bfd *abfd, struct bfd_link_info *info) 36 { 37 flagword flags, pltflags; 38 asection *s; 39 const struct elf_backend_data *bed = get_elf_backend_data (abfd); 40 struct elf_link_hash_table *htab = elf_hash_table (info); 41 42 if (htab->irelifunc != NULL || htab->iplt != NULL) 43 return TRUE; 44 45 flags = bed->dynamic_sec_flags; 46 pltflags = flags; 47 if (bed->plt_not_loaded) 48 /* We do not clear SEC_ALLOC here because we still want the OS to 49 allocate space for the section; it's just that there's nothing 50 to read in from the object file. */ 51 pltflags &= ~ (SEC_CODE | SEC_LOAD | SEC_HAS_CONTENTS); 52 else 53 pltflags |= SEC_ALLOC | SEC_CODE | SEC_LOAD; 54 if (bed->plt_readonly) 55 pltflags |= SEC_READONLY; 56 57 if (info->shared) 58 { 59 /* We need to create .rel[a].ifunc for shared objects. */ 60 const char *rel_sec = (bed->rela_plts_and_copies_p 61 ? ".rela.ifunc" : ".rel.ifunc"); 62 63 s = bfd_make_section_with_flags (abfd, rel_sec, 64 flags | SEC_READONLY); 65 if (s == NULL 66 || ! bfd_set_section_alignment (abfd, s, 67 bed->s->log_file_align)) 68 return FALSE; 69 htab->irelifunc = s; 70 } 71 else 72 { 73 /* We need to create .iplt, .rel[a].iplt, .igot and .igot.plt 74 for static executables. */ 75 s = bfd_make_section_with_flags (abfd, ".iplt", pltflags); 76 if (s == NULL 77 || ! bfd_set_section_alignment (abfd, s, bed->plt_alignment)) 78 return FALSE; 79 htab->iplt = s; 80 81 s = bfd_make_section_with_flags (abfd, 82 (bed->rela_plts_and_copies_p 83 ? ".rela.iplt" : ".rel.iplt"), 84 flags | SEC_READONLY); 85 if (s == NULL 86 || ! bfd_set_section_alignment (abfd, s, 87 bed->s->log_file_align)) 88 return FALSE; 89 htab->irelplt = s; 90 91 /* We don't need the .igot section if we have the .igot.plt 92 section. */ 93 if (bed->want_got_plt) 94 s = bfd_make_section_with_flags (abfd, ".igot.plt", flags); 95 else 96 s = bfd_make_section_with_flags (abfd, ".igot", flags); 97 if (s == NULL 98 || !bfd_set_section_alignment (abfd, s, 99 bed->s->log_file_align)) 100 return FALSE; 101 htab->igotplt = s; 102 } 103 104 return TRUE; 105 } 106 107 /* For a STT_GNU_IFUNC symbol, create a dynamic reloc section, SRELOC, 108 for the input section, SEC, and append this reloc to HEAD. */ 109 110 asection * 111 _bfd_elf_create_ifunc_dyn_reloc (bfd *abfd, struct bfd_link_info *info, 112 asection *sec, asection *sreloc, 113 struct elf_dyn_relocs **head) 114 { 115 struct elf_dyn_relocs *p; 116 struct elf_link_hash_table *htab = elf_hash_table (info); 117 118 if (sreloc == NULL) 119 { 120 const struct elf_backend_data *bed = get_elf_backend_data (abfd); 121 122 if (htab->dynobj == NULL) 123 htab->dynobj = abfd; 124 125 sreloc = _bfd_elf_make_dynamic_reloc_section (sec, htab->dynobj, 126 bed->s->log_file_align, 127 abfd, 128 bed->rela_plts_and_copies_p); 129 if (sreloc == NULL) 130 return NULL; 131 } 132 133 p = *head; 134 if (p == NULL || p->sec != sec) 135 { 136 bfd_size_type amt = sizeof *p; 137 138 p = ((struct elf_dyn_relocs *) bfd_alloc (htab->dynobj, amt)); 139 if (p == NULL) 140 return NULL; 141 p->next = *head; 142 *head = p; 143 p->sec = sec; 144 p->count = 0; 145 p->pc_count = 0; 146 } 147 p->count += 1; 148 149 return sreloc; 150 } 151 152 /* Allocate space in .plt, .got and associated reloc sections for 153 dynamic relocs against a STT_GNU_IFUNC symbol definition. */ 154 155 bfd_boolean 156 _bfd_elf_allocate_ifunc_dyn_relocs (struct bfd_link_info *info, 157 struct elf_link_hash_entry *h, 158 struct elf_dyn_relocs **head, 159 unsigned int plt_entry_size, 160 unsigned int got_entry_size) 161 { 162 asection *plt, *gotplt, *relplt; 163 struct elf_dyn_relocs *p; 164 unsigned int sizeof_reloc; 165 const struct elf_backend_data *bed; 166 struct elf_link_hash_table *htab; 167 168 /* When a shared library references a STT_GNU_IFUNC symbol defined 169 in executable, the address of the resolved function may be used. 170 But in non-shared executable, the address of its .plt slot may 171 be used. Pointer equality may not work correctly. PIE should 172 be used if pointer equality is required here. */ 173 if (!info->shared 174 && (h->dynindx != -1 175 || info->export_dynamic) 176 && h->pointer_equality_needed) 177 { 178 info->callbacks->einfo 179 (_("%F%P: dynamic STT_GNU_IFUNC symbol `%s' with pointer " 180 "equality in `%B' can not be used when making an " 181 "executable; recompile with -fPIE and relink with -pie\n"), 182 h->root.root.string, 183 h->root.u.def.section->owner); 184 bfd_set_error (bfd_error_bad_value); 185 return FALSE; 186 } 187 188 htab = elf_hash_table (info); 189 190 /* Support garbage collection against STT_GNU_IFUNC symbols. */ 191 if (h->plt.refcount <= 0 && h->got.refcount <= 0) 192 { 193 /* When building shared library, we need to handle the case 194 where it is marked with regular reference, but not non-GOT 195 reference. It may happen if we didn't see STT_GNU_IFUNC 196 symbol at the time when checking relocations. */ 197 bfd_size_type count = 0; 198 199 if (info->shared 200 && !h->non_got_ref 201 && h->ref_regular) 202 { 203 for (p = *head; p != NULL; p = p->next) 204 count += p->count; 205 if (count != 0) 206 h->non_got_ref = 1; 207 } 208 209 if (count == 0) 210 { 211 h->got = htab->init_got_offset; 212 h->plt = htab->init_plt_offset; 213 *head = NULL; 214 return TRUE; 215 } 216 } 217 218 /* Return and discard space for dynamic relocations against it if 219 it is never referenced in a non-shared object. */ 220 if (!h->ref_regular) 221 { 222 if (h->plt.refcount > 0 223 || h->got.refcount > 0) 224 abort (); 225 h->got = htab->init_got_offset; 226 h->plt = htab->init_plt_offset; 227 *head = NULL; 228 return TRUE; 229 } 230 231 bed = get_elf_backend_data (info->output_bfd); 232 if (bed->rela_plts_and_copies_p) 233 sizeof_reloc = bed->s->sizeof_rela; 234 else 235 sizeof_reloc = bed->s->sizeof_rel; 236 237 /* When building a static executable, use .iplt, .igot.plt and 238 .rel[a].iplt sections for STT_GNU_IFUNC symbols. */ 239 if (htab->splt != NULL) 240 { 241 plt = htab->splt; 242 gotplt = htab->sgotplt; 243 relplt = htab->srelplt; 244 245 /* If this is the first .plt entry, make room for the special 246 first entry. */ 247 if (plt->size == 0) 248 plt->size += plt_entry_size; 249 } 250 else 251 { 252 plt = htab->iplt; 253 gotplt = htab->igotplt; 254 relplt = htab->irelplt; 255 } 256 257 /* Don't update value of STT_GNU_IFUNC symbol to PLT. We need 258 the original value for R_*_IRELATIVE. */ 259 h->plt.offset = plt->size; 260 261 /* Make room for this entry in the .plt/.iplt section. */ 262 plt->size += plt_entry_size; 263 264 /* We also need to make an entry in the .got.plt/.got.iplt section, 265 which will be placed in the .got section by the linker script. */ 266 gotplt->size += got_entry_size; 267 268 /* We also need to make an entry in the .rel[a].plt/.rel[a].iplt 269 section. */ 270 relplt->size += sizeof_reloc; 271 relplt->reloc_count++; 272 273 /* We need dynamic relocation for STT_GNU_IFUNC symbol only when 274 there is a non-GOT reference in a shared object. */ 275 if (!info->shared 276 || !h->non_got_ref) 277 *head = NULL; 278 279 /* Finally, allocate space. */ 280 for (p = *head; p != NULL; p = p->next) 281 htab->irelifunc->size += p->count * sizeof_reloc; 282 283 /* For STT_GNU_IFUNC symbol, .got.plt has the real function addres 284 and .got has the PLT entry adddress. We will load the GOT entry 285 with the PLT entry in finish_dynamic_symbol if it is used. For 286 branch, it uses .got.plt. For symbol value, 287 1. Use .got.plt in a shared object if it is forced local or not 288 dynamic. 289 2. Use .got.plt in a non-shared object if pointer equality isn't 290 needed. 291 3. Use .got.plt in PIE. 292 4. Use .got.plt if .got isn't used. 293 5. Otherwise use .got so that it can be shared among different 294 objects at run-time. 295 We only need to relocate .got entry in shared object. */ 296 if ((info->shared 297 && (h->dynindx == -1 298 || h->forced_local)) 299 || (!info->shared 300 && !h->pointer_equality_needed) 301 || (info->executable && info->shared) 302 || htab->sgot == NULL) 303 { 304 /* Use .got.plt. */ 305 h->got.offset = (bfd_vma) -1; 306 } 307 else 308 { 309 h->got.offset = htab->sgot->size; 310 htab->sgot->size += got_entry_size; 311 if (info->shared) 312 htab->srelgot->size += sizeof_reloc; 313 } 314 315 return TRUE; 316 } 317