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1 /* Copyright (C) 2012-2020 Free Software Foundation, Inc.
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111
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2
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3 This file is part of GCC.
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4
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5 GCC is free software; you can redistribute it and/or modify it under
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6 the terms of the GNU General Public License as published by the Free
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7 Software Foundation; either version 3, or (at your option) any later
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8 version.
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9
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10 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
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11 WARRANTY; without even the implied warranty of MERCHANTABILITY or
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12 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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13 for more details.
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14
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15 Under Section 7 of GPL version 3, you are granted additional
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16 permissions described in the GCC Runtime Library Exception, version
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17 3.1, as published by the Free Software Foundation.
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18
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19 You should have received a copy of the GNU General Public License and
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20 a copy of the GCC Runtime Library Exception along with this program;
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21 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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22 <http://www.gnu.org/licenses/>. */
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23
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24 /* This file is part of the vtable verification feature (for a
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25 detailed description of the feature, see comments in
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26 vtable-verify.c). The vtable verification feature creates
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27 certain global symbols that need to be read-write sometimes during
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28 program execution, and read-only at others. It uses 'mprotect' to
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29 change the memory protections of the pages on which these variables
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30 are stored. In order to not affect the protections of other
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31 program variables, these variables are put into a special named
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32 section, ".vtable_map_vars", which is page-aligned at the start,
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33 and which is padded with a page-sized amount of zeros at the end.
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34 To make this section page aligned, we create a special symbol,
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35 "_vtable_map_vars_start" which we make the very first thing that
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36 goes into the section. This file defines that symbol (and only
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37 that symbol). GCC compiles this file into vtv_start.o, and
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38 inserts vtv_start.o into the link line immediately after
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39 crtbegin.o, if the program is compiled with -fvtable.verify.
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40
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41 In order to pad the ".vtable_map_vars" section with a page-sized
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42 amount of zeros at the end, there is a second symbol,
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43 _vtable_map_vars_end, which is defined in another file, vtv_end.c.
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44 This second symbol is a page-sized array of chars, zero-filled, and
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45 is the very last thing to go into the section. When the GCC driver
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46 inserts vtv_start.o into the link line (just after crtbegin.o) it
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47 also inserts vtv_end.o into the link line, just before crtend.o.
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48 This has the desired effect of making our section page-aligned and
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49 page-size paded, ensuring that no other program data lands on our
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50 pages. */
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51
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52 #include "vtv-change-permission.h"
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53
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54 __attribute__ ((constructor(98))) void
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55 __VLTunprotect (void)
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56 {
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57 __VLTChangePermission (__VLTP_READ_WRITE);
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58 }
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59
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60 /* Page-aligned symbol to mark beginning of .vtable_map_vars section. */
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61 char _vtable_map_vars_start []
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62 __attribute__ ((__visibility__ ("protected"), used, aligned(VTV_PAGE_SIZE),
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63 section(".vtable_map_vars")))
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64 = { };
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