bfd/elf-nacl.c

   1 /* Native Client support for ELF
   2    Copyright (C) 2012-2023 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 bool
  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 bool
  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 bool
  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           bool 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
 160                     = bfd_alloc (abfd, (sizeof (*newseg)
 161                                         + seg->count * sizeof (asection *)));
 162                   if (newseg == NULL)
 163                     return false;
 164                   memcpy (newseg, seg, (sizeof (*newseg) - sizeof (asection *)
 165                                         + 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 bool
 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 bool
 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 }