blob: b33a5f64e2cd913bc32a7661c9294ccf7ea28226 [file] [log] [blame]
/* Copyright (c) 2011 The Chromium Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
* This is a custom linker script used to build nacl_helper_bootstrap.
* It has a very special layout. This script will only work with input
* that is kept extremely minimal. If there are unexpected input sections
* not named here, the result will not be correct.
* We need to use a standalone loader program rather than just using a
* dynamically-linked program here because its entire address space will be
* taken over for the NaCl untrusted address space. A normal program would
* cause dynamic linker data structures to point to its .dynamic section,
* which is no longer available after startup.
* We need this special layout (and the nacl_helper_bootstrap_munge_phdr
* step) because simply having bss space large enough to reserve the
* address space would cause the kernel loader to think we're using that
* much anonymous memory and refuse to execute the program on a machine
* with not much memory available.
* Set the entry point to the symbol called _start, which we define in assembly.
* This is the address where the program text starts.
* We set this as low as we think we can get away with.
* The common settings for sysctl vm.mmap_min_addr range from 4k to 64k.
TEXT_START = 0x10000;
* This is the top of the range we are trying to reserve, which is 1G
* for x86-32 and ARM. For an x86-64 zero-based sandbox, this really
* needs to be 36G.
RESERVE_TOP = 1 << 30;
* We specify the program headers we want explicitly, to get the layout
* exactly right and to give the "reserve" segment p_flags of zero, so
* that it gets mapped as PROT_NONE.
data PT_LOAD;
reserve PT_LOAD FLAGS(0);
note PT_NOTE;
stack PT_GNU_STACK FLAGS(6); /* RW, no E */
* Now we lay out the sections across those segments.
* The build ID note usually comes first.
* It's both part of the text PT_LOAD segment (like other rodata) and
* it's what the PT_NOTE header points to.
*/ : {
} :text :note
* Here is the program itself.
.text : {
} :text
.rodata : {
etext = .;
* Adjust the address for the data segment. We want to adjust up to
* the same address within the page on the next page up.
.data : {
} :data
.bss : {
* Now we move up to the next p_align increment, and place the dummy
* segment there. The linker emits this segment with the p_vaddr and
* p_memsz we want, which reserves the address space. But the linker
* gives it a p_filesz of zero. We have to edit the phdr after link
* time to give it a p_filesz matching its p_memsz. That way, the
* kernel doesn't think we are preallocating a huge amount of memory.
* It just maps it from the file, i.e. way off the end of the file,
* which is perfect for reserving the address space.
.reserve : {
} :reserve
* These are empty input sections the linker generates.
* If we don't discard them, they pollute the flags in the output segment.