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 }