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b710ec85 | 1 | /* Copyright (C) 2012, 2013 |
2 | Free Software Foundation, Inc. | |
3 | ||
4 | This file is part of GCC. | |
5 | ||
6 | GCC is free software; you can redistribute it and/or modify it under | |
7 | the terms of the GNU General Public License as published by the Free | |
8 | Software Foundation; either version 3, or (at your option) any later | |
9 | version. | |
10 | ||
11 | GCC is distributed in the hope that it will be useful, but WITHOUT ANY | |
12 | WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
13 | FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
14 | for more details. | |
15 | ||
16 | Under Section 7 of GPL version 3, you are granted additional | |
17 | permissions described in the GCC Runtime Library Exception, version | |
18 | 3.1, as published by the Free Software Foundation. | |
19 | ||
20 | You should have received a copy of the GNU General Public License and | |
21 | a copy of the GCC Runtime Library Exception along with this program; | |
22 | see the files COPYING3 and COPYING.RUNTIME respectively. If not, see | |
23 | <http://www.gnu.org/licenses/>. */ | |
24 | ||
25 | /* This file is part of the vtable verification feature (for a | |
26 | detailed description of the feature, see comments in | |
27 | vtable-verify.c). The vtable verification feature creates | |
28 | certain global symbols that need to be read-write sometimes during | |
29 | program execution, and read-only at others. It uses 'mprotect' to | |
30 | change the memory protections of the pages on which these variables | |
31 | are stored. In order to not affect the protections of other | |
32 | program variables, these variables are put into a special named | |
33 | section, ".vtable_map_vars", which is page-aligned at the start, | |
34 | and which is padded with a page-sized amount of zeros at the end. | |
35 | To make this section page aligned, we create a special symbol, | |
36 | "_vtable_map_vars_start" which we make the very first thing that | |
37 | goes into the section. This file defines that symbol (and only | |
38 | that symbol). GCC compiles this file into vtv_start.o, and | |
39 | inserts vtv_start.o into the link line immediately after | |
40 | crtbegin.o, if the program is compiled with -fvtable.verify. | |
41 | ||
42 | In order to pad the ".vtable_map_vars" section with a page-sized | |
43 | amount of zeros at the end, there is a second symbol, | |
44 | _vtable_map_vars_end, which is defined in another file, vtv_end.c. | |
45 | This second symbol is a page-sized array of chars, zero-filled, and | |
46 | is the very last thing to go into the section. When the GCC driver | |
47 | inserts vtv_start.o into the link line (just after crtbegin.o) it | |
48 | also inserts vtv_end.o into the link line, just before crtend.o. | |
49 | This has the desired effect of making our section page-aligned and | |
50 | page-size paded, ensuring that no other program data lands on our | |
51 | pages. */ | |
52 | ||
53 | #include "vtv-change-permission.h" | |
54 | ||
55 | void | |
56 | __VLTUnprotectPreinit (void) | |
57 | { | |
58 | __VLTChangePermission (__VLTP_READ_WRITE); | |
59 | } | |
60 | ||
61 | /* Page-aligned symbol to mark beginning of .vtable_map_vars section. */ | |
62 | char _vtable_map_vars_start [] | |
63 | __attribute__ ((__visibility__ ("protected"), used, aligned(VTV_PAGE_SIZE), | |
64 | section(".vtable_map_vars"))) | |
65 | = { }; | |
66 | ||
67 | ||
68 | /* Put the function __VLTUnprotectPreinit into the .preinit_array | |
69 | section. */ | |
70 | ||
71 | __attribute__ ((section (".preinit_array"))) | |
72 | typeof (__VLTUnprotectPreinit) *__preinit = __VLTUnprotectPreinit; | |
73 |