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66 lines
2.9 KiB
C
66 lines
2.9 KiB
C
/* Copyright (C) 2012-2016 Free Software Foundation, Inc.
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This file is part of GCC.
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GCC is free software; you can redistribute it and/or modify it under
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the terms of the GNU General Public License as published by the Free
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Software Foundation; either version 3, or (at your option) any later
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version.
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GCC is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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/* This file is part of the vtable verification feature (for a
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detailed description of the feature, see comments in
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vtable-verify.c). The vtable verification feature creates
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certain global symbols that need to be read-write sometimes during
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program execution, and read-only at others. It uses 'mprotect' to
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change the memory protections of the pages on which these variables
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are stored. In order to not affect the protections of other
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program variables, these variables are put into a special named
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section, ".vtable_map_vars", which is page-aligned at the start,
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and which is padded with a page-sized amount of zeros at the end.
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To make this section page aligned, we create a special symbol,
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"_vtable_map_vars_start" which we make the very first thing that
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goes into the section. That is defined in vtv_start.c (which
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contains nothing else). vtv_start.c gest compiled into
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vtv_start.o, and vtv_start.o gets inserted into the link line
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immediately after crtbegin.o, if the program is compiled with
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-fvtable.verify.
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In order to pad the ".vtable_map_vars" section with a page-sized
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amount of zeros at the end, there is a second symbol,
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_vtable_map_vars_end. This file defines that symbol (and only this
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symbol). This second symbol is a page-sized array of chars,
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zero-filled, and is the very last thing to go into the section.
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When the GCC driver inserts vtv_start.o into the link line (just
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after crtbegin.o) it also inserts vtv_end.o into the link line,
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just before crtend.o. This has the desired effect of making our
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section page-aligned and page-size paded, ensuring that no other
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program data lands on our pages. */
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#include "vtv-change-permission.h"
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__attribute__ ((constructor(100))) void
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__VLTprotect (void)
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{
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__VLTChangePermission (__VLTP_READ_ONLY);
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}
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/* Page-sized variable to mark end of .vtable_map_vars section. */
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char _vtable_map_vars_end[VTV_PAGE_SIZE]
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__attribute__ ((__visibility__ ("protected"), used,
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section(".vtable_map_vars")));
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