1 /* Native Client support for ELF 2 Copyright (C) 2012-2020 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, see <http://www.gnu.org/licenses/>. */ 18 19 #include "sysdep.h" 20 #include "bfd.h" 21 #include "libbfd.h" 22 #include "elf-bfd.h" 23 #include "elf-nacl.h" 24 #include "elf/common.h" 25 #include "elf/internal.h" 26 27 static bfd_boolean 28 segment_executable (struct elf_segment_map *seg) 29 { 30 if (seg->p_flags_valid) 31 return (seg->p_flags & PF_X) != 0; 32 else 33 { 34 /* The p_flags value has not been computed yet, 35 so we have to look through the sections. */ 36 unsigned int i; 37 for (i = 0; i < seg->count; ++i) 38 if (seg->sections[i]->flags & SEC_CODE) 39 return TRUE; 40 } 41 return FALSE; 42 } 43 44 /* Determine if this segment is eligible to receive the file and program 45 headers. It must be read-only and non-executable. 46 Its first section must start far enough past the page boundary to 47 allow space for the headers. */ 48 static bfd_boolean 49 segment_eligible_for_headers (struct elf_segment_map *seg, 50 bfd_vma minpagesize, bfd_vma sizeof_headers) 51 { 52 unsigned int i; 53 if (seg->count == 0 || seg->sections[0]->lma % minpagesize < sizeof_headers) 54 return FALSE; 55 for (i = 0; i < seg->count; ++i) 56 { 57 if ((seg->sections[i]->flags & (SEC_CODE|SEC_READONLY)) != SEC_READONLY) 58 return FALSE; 59 } 60 return TRUE; 61 } 62 63 64 /* We permute the segment_map to get BFD to do the file layout we want: 65 The first non-executable PT_LOAD segment appears first in the file 66 and contains the ELF file header and phdrs. */ 67 bfd_boolean 68 nacl_modify_segment_map (bfd *abfd, struct bfd_link_info *info) 69 { 70 const struct elf_backend_data *const bed = get_elf_backend_data (abfd); 71 struct elf_segment_map **m = &elf_seg_map (abfd); 72 struct elf_segment_map **first_load = NULL; 73 struct elf_segment_map **headers = NULL; 74 int sizeof_headers; 75 76 if (info != NULL && info->user_phdrs) 77 /* The linker script used PHDRS explicitly, so don't change what the 78 user asked for. */ 79 return TRUE; 80 81 if (info != NULL) 82 /* We're doing linking, so evalute SIZEOF_HEADERS as in a linker script. */ 83 sizeof_headers = bfd_sizeof_headers (abfd, info); 84 else 85 { 86 /* We're not doing linking, so this is objcopy or suchlike. 87 We just need to collect the size of the existing headers. */ 88 struct elf_segment_map *seg; 89 sizeof_headers = bed->s->sizeof_ehdr; 90 for (seg = *m; seg != NULL; seg = seg->next) 91 sizeof_headers += bed->s->sizeof_phdr; 92 } 93 94 while (*m != NULL) 95 { 96 struct elf_segment_map *seg = *m; 97 98 if (seg->p_type == PT_LOAD) 99 { 100 bfd_boolean executable = segment_executable (seg); 101 102 if (executable 103 && seg->count > 0 104 && seg->sections[0]->vma % bed->minpagesize == 0) 105 { 106 asection *lastsec = seg->sections[seg->count - 1]; 107 bfd_vma end = lastsec->vma + lastsec->size; 108 if (end % bed->minpagesize != 0) 109 { 110 /* This is an executable segment that starts on a page 111 boundary but does not end on a page boundary. Fill 112 it out to a whole page with code fill (the tail of 113 the segment will not be within any section). Thus 114 the entire code segment can be mapped from the file 115 as whole pages and that mapping will contain only 116 valid instructions. 117 118 To accomplish this, we must fake out the code in 119 assign_file_positions_for_load_sections (elf.c) so 120 that it advances past the rest of the final page, 121 rather than trying to put the next (unaligned, or 122 unallocated) section. We do this by appending a 123 dummy section record to this element in the segment 124 map. No such output section ever actually exists, 125 but this gets the layout logic to advance the file 126 positions past this partial page. Since we are 127 lying to BFD like this, nothing will ever know to 128 write the section contents. So we do that by hand 129 after the fact, in nacl_final_write_processing, below. */ 130 131 struct elf_segment_map *newseg; 132 asection *sec; 133 struct bfd_elf_section_data *secdata; 134 135 BFD_ASSERT (!seg->p_size_valid); 136 137 secdata = bfd_zalloc (abfd, sizeof *secdata); 138 if (secdata == NULL) 139 return FALSE; 140 141 sec = bfd_zalloc (abfd, sizeof *sec); 142 if (sec == NULL) 143 return FALSE; 144 145 /* Fill in only the fields that actually affect the logic 146 in assign_file_positions_for_load_sections. */ 147 sec->vma = end; 148 sec->lma = lastsec->lma + lastsec->size; 149 sec->size = bed->minpagesize - (end % bed->minpagesize); 150 sec->flags = (SEC_ALLOC | SEC_LOAD 151 | SEC_READONLY | SEC_CODE | SEC_LINKER_CREATED); 152 sec->used_by_bfd = secdata; 153 154 secdata->this_hdr.sh_type = SHT_PROGBITS; 155 secdata->this_hdr.sh_flags = SHF_ALLOC | SHF_EXECINSTR; 156 secdata->this_hdr.sh_addr = sec->vma; 157 secdata->this_hdr.sh_size = sec->size; 158 159 newseg = bfd_alloc (abfd, 160 sizeof *newseg + ((seg->count + 1) 161 * sizeof (asection *))); 162 if (newseg == NULL) 163 return FALSE; 164 memcpy (newseg, seg, 165 sizeof *newseg + (seg->count * sizeof (asection *))); 166 newseg->sections[newseg->count++] = sec; 167 *m = seg = newseg; 168 } 169 } 170 171 /* First, we're just finding the earliest PT_LOAD. 172 By the normal rules, this will be the lowest-addressed one. */ 173 if (first_load == NULL) 174 first_load = m; 175 176 /* Now that we've noted the first PT_LOAD, we're looking for 177 the first non-executable PT_LOAD with a nonempty p_filesz. */ 178 else if (headers == NULL 179 && segment_eligible_for_headers (seg, bed->minpagesize, 180 sizeof_headers)) 181 headers = m; 182 } 183 m = &seg->next; 184 } 185 186 if (headers != NULL) 187 { 188 struct elf_segment_map **last_load = NULL; 189 struct elf_segment_map *seg; 190 191 m = first_load; 192 while ((seg = *m) != NULL) 193 { 194 if (seg->p_type == PT_LOAD) 195 { 196 /* Clear the flags on any previous segment that 197 included the file header and phdrs. */ 198 seg->includes_filehdr = 0; 199 seg->includes_phdrs = 0; 200 seg->no_sort_lma = 1; 201 /* Also strip out empty segments. */ 202 if (seg->count == 0) 203 { 204 if (headers == &seg->next) 205 headers = m; 206 *m = seg->next; 207 continue; 208 } 209 last_load = m; 210 } 211 m = &seg->next; 212 } 213 214 /* This segment will include those headers instead. */ 215 seg = *headers; 216 seg->includes_filehdr = 1; 217 seg->includes_phdrs = 1; 218 219 if (last_load != NULL && first_load != last_load && first_load != headers) 220 { 221 /* Put the first PT_LOAD header last. */ 222 struct elf_segment_map *first = *first_load; 223 struct elf_segment_map *last = *last_load; 224 *first_load = first->next; 225 first->next = last->next; 226 last->next = first; 227 } 228 } 229 230 return TRUE; 231 } 232 233 /* After nacl_modify_segment_map has done its work, the file layout has 234 been done as we wanted. But the PT_LOAD phdrs are no longer in the 235 proper order for the ELF rule that they must appear in ascending address 236 order. So find the two segments we swapped before, and swap them back. */ 237 bfd_boolean 238 nacl_modify_headers (bfd *abfd, struct bfd_link_info *info) 239 { 240 if (info != NULL && info->user_phdrs) 241 /* The linker script used PHDRS explicitly, so don't change what the 242 user asked for. */ 243 ; 244 else 245 { 246 struct elf_segment_map **m = &elf_seg_map (abfd); 247 Elf_Internal_Phdr *phdr = elf_tdata (abfd)->phdr; 248 Elf_Internal_Phdr *p = phdr; 249 250 /* Find the PT_LOAD that contains the headers (should be the first). */ 251 while (*m != NULL) 252 { 253 if ((*m)->p_type == PT_LOAD && (*m)->includes_filehdr) 254 break; 255 256 m = &(*m)->next; 257 ++p; 258 } 259 260 if (*m != NULL) 261 { 262 struct elf_segment_map **first_load_seg = m; 263 Elf_Internal_Phdr *first_load_phdr = p; 264 struct elf_segment_map **next_load_seg = NULL; 265 Elf_Internal_Phdr *next_load_phdr = NULL; 266 267 /* Now move past that first one and find the PT_LOAD that should be 268 before it by address order. */ 269 270 m = &(*m)->next; 271 ++p; 272 273 while (*m != NULL) 274 { 275 if (p->p_type == PT_LOAD && p->p_vaddr < first_load_phdr->p_vaddr) 276 { 277 next_load_seg = m; 278 next_load_phdr = p; 279 break; 280 } 281 282 m = &(*m)->next; 283 ++p; 284 } 285 286 /* Swap their positions in the segment_map back to how they 287 used to be. The phdrs have already been set up by now, 288 so we have to slide up the earlier ones to insert the one 289 that should be first. */ 290 if (next_load_seg != NULL) 291 { 292 Elf_Internal_Phdr move_phdr; 293 struct elf_segment_map *first_seg = *first_load_seg; 294 struct elf_segment_map *next_seg = *next_load_seg; 295 struct elf_segment_map *first_next = first_seg->next; 296 struct elf_segment_map *next_next = next_seg->next; 297 298 if (next_load_seg == &first_seg->next) 299 { 300 *first_load_seg = next_seg; 301 next_seg->next = first_seg; 302 first_seg->next = next_next; 303 } 304 else 305 { 306 *first_load_seg = first_next; 307 *next_load_seg = next_next; 308 309 first_seg->next = *next_load_seg; 310 *next_load_seg = first_seg; 311 312 next_seg->next = *first_load_seg; 313 *first_load_seg = next_seg; 314 } 315 316 move_phdr = *next_load_phdr; 317 memmove (first_load_phdr + 1, first_load_phdr, 318 (next_load_phdr - first_load_phdr) * sizeof move_phdr); 319 *first_load_phdr = move_phdr; 320 } 321 } 322 } 323 324 return _bfd_elf_modify_headers (abfd, info); 325 } 326 327 bfd_boolean 328 nacl_final_write_processing (bfd *abfd) 329 { 330 struct elf_segment_map *seg; 331 for (seg = elf_seg_map (abfd); seg != NULL; seg = seg->next) 332 if (seg->p_type == PT_LOAD 333 && seg->count > 1 334 && seg->sections[seg->count - 1]->owner == NULL) 335 { 336 /* This is a fake section added in nacl_modify_segment_map, above. 337 It's not a real BFD section, so nothing wrote its contents. 338 Now write out its contents. */ 339 340 asection *sec = seg->sections[seg->count - 1]; 341 char *fill; 342 343 BFD_ASSERT (sec->flags & SEC_LINKER_CREATED); 344 BFD_ASSERT (sec->flags & SEC_CODE); 345 BFD_ASSERT (sec->size > 0); 346 347 fill = abfd->arch_info->fill (sec->size, bfd_big_endian (abfd), TRUE); 348 349 if (fill == NULL 350 || bfd_seek (abfd, sec->filepos, SEEK_SET) != 0 351 || bfd_bwrite (fill, sec->size, abfd) != sec->size) 352 { 353 /* We don't have a proper way to report an error here. So 354 instead fudge things so that elf_write_shdrs_and_ehdr will 355 fail. */ 356 elf_elfheader (abfd)->e_shoff = (file_ptr) -1; 357 } 358 359 free (fill); 360 } 361 return _bfd_elf_final_write_processing (abfd); 362 } 363