1 /* Copyright (C) 2012-2015 Free Software Foundation, Inc. 2 3 This file is part of GCC. 4 5 GCC is free software; you can redistribute it and/or modify it under 6 the terms of the GNU General Public License as published by the Free 7 Software Foundation; either version 3, or (at your option) any later 8 version. 9 10 GCC is distributed in the hope that it will be useful, but WITHOUT ANY 11 WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 for more details. 14 15 Under Section 7 of GPL version 3, you are granted additional 16 permissions described in the GCC Runtime Library Exception, version 17 3.1, as published by the Free Software Foundation. 18 19 You should have received a copy of the GNU General Public License and 20 a copy of the GCC Runtime Library Exception along with this program; 21 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see 22 <http://www.gnu.org/licenses/>. */ 23 24 /* This file is part of the vtable verification feature (for a 25 detailed description of the feature, see comments in 26 vtable-verify.c). The vtable verification feature creates 27 certain global symbols that need to be read-write sometimes during 28 program execution, and read-only at others. It uses 'mprotect' to 29 change the memory protections of the pages on which these variables 30 are stored. In order to not affect the protections of other 31 program variables, these variables are put into a special named 32 section, ".vtable_map_vars", which is page-aligned at the start, 33 and which is padded with a page-sized amount of zeros at the end. 34 To make this section page aligned, we create a special symbol, 35 "_vtable_map_vars_start" which we make the very first thing that 36 goes into the section. This file defines that symbol (and only 37 that symbol). GCC compiles this file into vtv_start.o, and 38 inserts vtv_start.o into the link line immediately after 39 crtbegin.o, if the program is compiled with -fvtable.verify. 40 41 In order to pad the ".vtable_map_vars" section with a page-sized 42 amount of zeros at the end, there is a second symbol, 43 _vtable_map_vars_end, which is defined in another file, vtv_end.c. 44 This second symbol is a page-sized array of chars, zero-filled, and 45 is the very last thing to go into the section. When the GCC driver 46 inserts vtv_start.o into the link line (just after crtbegin.o) it 47 also inserts vtv_end.o into the link line, just before crtend.o. 48 This has the desired effect of making our section page-aligned and 49 page-size paded, ensuring that no other program data lands on our 50 pages. */ 51 52 #include "vtv-change-permission.h" 53 54 __attribute__ ((constructor(98))) void 55 __VLTunprotect (void) 56 { 57 __VLTChangePermission (__VLTP_READ_WRITE); 58 } 59 60 /* Page-aligned symbol to mark beginning of .vtable_map_vars section. */ 61 char _vtable_map_vars_start [] 62 __attribute__ ((__visibility__ ("protected"), used, aligned(VTV_PAGE_SIZE), 63 section(".vtable_map_vars"))) 64 = { }; 65