bfd/elf-nacl.c

   1 /* Native Client support for ELF
   2    Copyright (C) 2012-2019 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               /* Also strip out empty segments.  */
 201               if (seg->count == 0)
 202                 {
 203                   if (headers == &seg->next)
 204                     headers = m;
 205                   *m = seg->next;
 206                   continue;
 207                 }
 208               last_load = m;
 209             }
 210           m = &seg->next;
 211         }
 212
 213       /* This segment will include those headers instead.  */
 214       seg = *headers;
 215       seg->includes_filehdr = 1;
 216       seg->includes_phdrs = 1;
 217
 218       if (last_load != NULL && first_load != last_load && first_load != headers)
 219         {
 220           /* Put the first PT_LOAD header last.  */
 221           struct elf_segment_map *first = *first_load;
 222           struct elf_segment_map *last = *last_load;
 223           *first_load = first->next;
 224           first->next = last->next;
 225           last->next = first;
 226         }
 227     }
 228
 229   return TRUE;
 230 }
 231
 232 /* After nacl_modify_segment_map has done its work, the file layout has
 233    been done as we wanted.  But the PT_LOAD phdrs are no longer in the
 234    proper order for the ELF rule that they must appear in ascending address
 235    order.  So find the two segments we swapped before, and swap them back.  */
 236 bfd_boolean
 237 nacl_modify_program_headers (bfd *abfd, struct bfd_link_info *info)
 238 {
 239   struct elf_segment_map **m = &elf_seg_map (abfd);
 240   Elf_Internal_Phdr *phdr = elf_tdata (abfd)->phdr;
 241   Elf_Internal_Phdr *p = phdr;
 242
 243   if (info != NULL && info->user_phdrs)
 244     /* The linker script used PHDRS explicitly, so don't change what the
 245        user asked for.  */
 246     return TRUE;
 247
 248   /* Find the PT_LOAD that contains the headers (should be the first).  */
 249   while (*m != NULL)
 250     {
 251       if ((*m)->p_type == PT_LOAD && (*m)->includes_filehdr)
 252         break;
 253
 254       m = &(*m)->next;
 255       ++p;
 256     }
 257
 258   if (*m != NULL)
 259     {
 260       struct elf_segment_map **first_load_seg = m;
 261       Elf_Internal_Phdr *first_load_phdr = p;
 262       struct elf_segment_map **next_load_seg = NULL;
 263       Elf_Internal_Phdr *next_load_phdr = NULL;
 264
 265       /* Now move past that first one and find the PT_LOAD that should be
 266          before it by address order.  */
 267
 268       m = &(*m)->next;
 269       ++p;
 270
 271       while (*m != NULL)
 272         {
 273           if (p->p_type == PT_LOAD && p->p_vaddr < first_load_phdr->p_vaddr)
 274             {
 275               next_load_seg = m;
 276               next_load_phdr = p;
 277               break;
 278             }
 279
 280           m = &(*m)->next;
 281           ++p;
 282         }
 283
 284       /* Swap their positions in the segment_map back to how they used to be.
 285          The phdrs have already been set up by now, so we have to slide up
 286          the earlier ones to insert the one that should be first.  */
 287       if (next_load_seg != NULL)
 288         {
 289           Elf_Internal_Phdr move_phdr;
 290           struct elf_segment_map *first_seg = *first_load_seg;
 291           struct elf_segment_map *next_seg = *next_load_seg;
 292           struct elf_segment_map *first_next = first_seg->next;
 293           struct elf_segment_map *next_next = next_seg->next;
 294
 295           if (next_load_seg == &first_seg->next)
 296             {
 297               *first_load_seg = next_seg;
 298               next_seg->next = first_seg;
 299               first_seg->next = next_next;
 300             }
 301           else
 302             {
 303               *first_load_seg = first_next;
 304               *next_load_seg = next_next;
 305
 306               first_seg->next = *next_load_seg;
 307               *next_load_seg = first_seg;
 308
 309               next_seg->next = *first_load_seg;
 310               *first_load_seg = next_seg;
 311             }
 312
 313           move_phdr = *next_load_phdr;
 314           memmove (first_load_phdr + 1, first_load_phdr,
 315                    (next_load_phdr - first_load_phdr) * sizeof move_phdr);
 316           *first_load_phdr = move_phdr;
 317         }
 318     }
 319
 320   return TRUE;
 321 }
 322
 323 void
 324 nacl_final_write_processing (bfd *abfd, bfd_boolean linker)
 325 {
 326   struct elf_segment_map *seg;
 327   for (seg = elf_seg_map (abfd); seg != NULL; seg = seg->next)
 328     if (seg->p_type == PT_LOAD
 329         && seg->count > 1
 330         && seg->sections[seg->count - 1]->owner == NULL)
 331       {
 332         /* This is a fake section added in nacl_modify_segment_map, above.
 333            It's not a real BFD section, so nothing wrote its contents.
 334            Now write out its contents.  */
 335
 336         asection *sec = seg->sections[seg->count - 1];
 337         char *fill;
 338
 339         BFD_ASSERT (sec->flags & SEC_LINKER_CREATED);
 340         BFD_ASSERT (sec->flags & SEC_CODE);
 341         BFD_ASSERT (sec->size > 0);
 342
 343         fill = abfd->arch_info->fill (sec->size, bfd_big_endian (abfd), TRUE);
 344
 345         if (fill == NULL
 346             || bfd_seek (abfd, sec->filepos, SEEK_SET) != 0
 347             || bfd_bwrite (fill, sec->size, abfd) != sec->size)
 348           {
 349             /* We don't have a proper way to report an error here.  So
 350                instead fudge things so that elf_write_shdrs_and_ehdr will
 351                fail.  */
 352             elf_elfheader (abfd)->e_shoff = (file_ptr) -1;
 353           }
 354
 355         free (fill);
 356       }
 357   _bfd_elf_final_write_processing (abfd, linker);
 358 }