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| 1 /* Copyright (c) 2011 The Chromium Authors. All rights reserved. | |
| 2 * Use of this source code is governed by a BSD-style license that can be | |
| 3 * found in the LICENSE file. | |
| 4 * | |
| 5 * This is a custom linker script used to build nacl_helper_bootstrap. | |
| 6 * It has a very special layout. This script will only work with input | |
| 7 * that is kept extremely minimal. If there are unexpected input sections | |
| 8 * not named here, the result will not be correct. | |
| 9 * | |
| 10 * We need to use a standalone loader program rather than just using a | |
| 11 * dynamically-linked program here because its entire address space will be | |
| 12 * taken over for the NaCl untrusted address space. A normal program would | |
| 13 * cause dynamic linker data structures to point to its .dynamic section, | |
| 14 * which is no longer available after startup. | |
| 15 * | |
| 16 * We need this special layout (and the nacl_helper_bootstrap_munge_phdr | |
| 17 * step) because simply having bss space large enough to reserve the | |
| 18 * address space would cause the kernel loader to think we're using that | |
| 19 * much anonymous memory and refuse to execute the program on a machine | |
| 20 * with not much memory available. | |
| 21 */ | |
| 22 | |
| 23 /* | |
| 24 * Set the entry point to the symbol called _start, which we define in assembly. | |
| 25 */ | |
| 26 ENTRY(_start) | |
| 27 | |
| 28 /* | |
| 29 * This is the address where the program text starts. | |
| 30 * We set this as low as we think we can get away with. | |
| 31 * The common settings for sysctl vm.mmap_min_addr range from 4k to 64k. | |
| 32 */ | |
| 33 TEXT_START = 0x10000; | |
| 34 | |
| 35 /* | |
| 36 * The symbol RESERVE_TOP is the top of the range we are trying to reserve. | |
| 37 * This is set via --defsym on the linker command line, because the correct | |
| 38 * value differs for each machine. It's not defined at all if we do not | |
| 39 * actually need any space reserved for this configuration. | |
| 40 */ | |
| 41 | |
| 42 /* | |
| 43 * We specify the program headers we want explicitly, to get the layout | |
| 44 * exactly right and to give the "reserve" segment p_flags of zero, so | |
| 45 * that it gets mapped as PROT_NONE. | |
| 46 */ | |
| 47 PHDRS { | |
| 48 text PT_LOAD FILEHDR PHDRS; | |
| 49 data PT_LOAD; | |
| 50 reserve PT_LOAD FLAGS(0); | |
| 51 r_debug PT_LOAD; | |
| 52 note PT_NOTE; | |
| 53 stack PT_GNU_STACK FLAGS(6); /* RW, no E */ | |
| 54 } | |
| 55 | |
| 56 /* | |
| 57 * Now we lay out the sections across those segments. | |
| 58 */ | |
| 59 SECTIONS { | |
| 60 . = TEXT_START + SIZEOF_HEADERS; | |
| 61 | |
| 62 /* | |
| 63 * The build ID note usually comes first. | |
| 64 * It's both part of the text PT_LOAD segment (like other rodata) and | |
| 65 * it's what the PT_NOTE header points to. | |
| 66 */ | |
| 67 .note.gnu.build-id : { | |
| 68 *(.note.gnu.build-id) | |
| 69 } :text :note | |
| 70 | |
| 71 /* | |
| 72 * Here is the program itself. | |
| 73 */ | |
| 74 .text : { | |
| 75 *(.text*) | |
| 76 } :text | |
| 77 .rodata : { | |
| 78 *(.rodata*) | |
| 79 *(.eh_frame*) | |
| 80 } | |
| 81 | |
| 82 etext = .; | |
| 83 | |
| 84 /* | |
| 85 * Adjust the address for the data segment. We want to adjust up to | |
| 86 * the same address within the page on the next page up. | |
| 87 */ | |
| 88 . = (ALIGN(CONSTANT(MAXPAGESIZE)) - | |
| 89 ((CONSTANT(MAXPAGESIZE) - .) & (CONSTANT(MAXPAGESIZE) - 1))); | |
| 90 . = DATA_SEGMENT_ALIGN(CONSTANT(MAXPAGESIZE), CONSTANT(COMMONPAGESIZE)); | |
| 91 | |
| 92 .data : { | |
| 93 *(.data*) | |
| 94 } :data | |
| 95 .bss : { | |
| 96 *(.bss*) | |
| 97 } | |
| 98 | |
| 99 /* | |
| 100 * Now we move up to the next p_align increment, and place the dummy | |
| 101 * segment there. The linker emits this segment with the p_vaddr and | |
| 102 * p_memsz we want, which reserves the address space. But the linker | |
| 103 * gives it a p_filesz of zero. We have to edit the phdr after link | |
| 104 * time to give it a p_filesz matching its p_memsz. That way, the | |
| 105 * kernel doesn't think we are preallocating a huge amount of memory. | |
| 106 * It just maps it from the file, i.e. way off the end of the file, | |
| 107 * which is perfect for reserving the address space. | |
| 108 */ | |
| 109 . = ALIGN(CONSTANT(COMMONPAGESIZE)); | |
| 110 RESERVE_START = .; | |
| 111 .reserve : { | |
| 112 . += DEFINED(RESERVE_TOP) ? (RESERVE_TOP - RESERVE_START) : 0; | |
| 113 } :reserve | |
| 114 | |
| 115 /* | |
| 116 * This must be placed above the reserved address space, so it won't | |
| 117 * be clobbered by NaCl. We want this to be visible at its fixed address | |
| 118 * in the memory image so the debugger can make sense of things. | |
| 119 */ | |
| 120 .r_debug : { | |
| 121 *(.r_debug) | |
| 122 } :r_debug | |
| 123 | |
| 124 /* | |
| 125 * These are empty input sections the linker generates. | |
| 126 * If we don't discard them, they pollute the flags in the output segment. | |
| 127 */ | |
| 128 /DISCARD/ : { | |
| 129 *(.iplt) | |
| 130 *(.rel*) | |
| 131 *(.igot.plt) | |
| 132 } | |
| 133 } | |
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Chromium Code Reviews
Issue 8800034:
Use nacl_helper_bootstrap from native_client repository (Closed)
Base URL: svn://svn.chromium.org/chrome/trunk/src