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1 /* |
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2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. |
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3 * Copyright 2012, 2013 SAP AG. All rights reserved. |
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4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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5 * |
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6 * This code is free software; you can redistribute it and/or modify it |
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7 * under the terms of the GNU General Public License version 2 only, as |
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8 * published by the Free Software Foundation. |
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9 * |
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10 * This code is distributed in the hope that it will be useful, but WITHOUT |
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11 * ANY 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 * version 2 for more details (a copy is included in the LICENSE file that |
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14 * accompanied this code). |
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15 * |
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16 * You should have received a copy of the GNU General Public License version |
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17 * 2 along with this work; if not, write to the Free Software Foundation, |
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18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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19 * |
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20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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21 * or visit www.oracle.com if you need additional information or have any |
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22 * questions. |
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23 * |
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24 */ |
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25 |
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26 #ifndef SHARE_VM_UTILITIES_ELF_FUNC_DESC_TABLE_HPP |
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27 #define SHARE_VM_UTILITIES_ELF_FUNC_DESC_TABLE_HPP |
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28 |
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29 #if !defined(_WINDOWS) && !defined(__APPLE__) |
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30 |
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31 |
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32 #include "memory/allocation.hpp" |
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33 #include "utilities/decoder.hpp" |
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34 #include "utilities/elfFile.hpp" |
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35 |
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36 /* |
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37 |
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38 On PowerPC-64 (and other architectures like for example IA64) a pointer to a |
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39 function is not just a plain code address, but instead a pointer to a so called |
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40 function descriptor (which is simply a structure containing 3 pointers). |
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41 This fact is also reflected in the ELF ABI for PowerPC-64. |
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42 |
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43 On architectures like x86 or SPARC, the ELF symbol table contains the start |
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44 address and size of an object. So for example for a function object (i.e. type |
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45 'STT_FUNC') the symbol table's 'st_value' and 'st_size' fields directly |
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46 represent the starting address and size of that function. On PPC64 however, the |
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47 symbol table's 'st_value' field only contains an index into another, PPC64 |
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48 specific '.opd' (official procedure descriptors) section, while the 'st_size' |
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49 field still holds the size of the corresponding function. In order to get the |
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50 actual start address of a function, it is necessary to read the corresponding |
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51 function descriptor entry in the '.opd' section at the corresponding index and |
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52 extract the start address from there. |
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53 |
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54 That's exactly what this 'ElfFuncDescTable' class is used for. If the HotSpot |
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55 runs on a PPC64 machine, and the corresponding ELF files contains an '.opd' |
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56 section (which is actually mandatory on PPC64) it will be read into an object |
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57 of type 'ElfFuncDescTable' just like the string and symbol table sections. |
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58 Later on, during symbol lookup in 'ElfSymbolTable::lookup()' this function |
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59 descriptor table will be used if available to find the real function address. |
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60 |
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61 All this is how things work today (2013) on contemporary Linux distributions |
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62 (i.e. SLES 10) and new version of GCC (i.e. > 4.0). However there is a history, |
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63 and it goes like this: |
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64 |
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65 In SLES 9 times (sometimes before GCC 3.4) gcc/ld on PPC64 generated two |
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66 entries in the symbol table for every function. The value of the symbol with |
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67 the name of the function was the address of the function descriptor while the |
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68 dot '.' prefixed name was reserved to hold the actual address of that function |
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69 (http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#FUNC-DES). |
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70 |
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71 For a C-function 'foo' this resulted in two symbol table entries like this |
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72 (extracted from the output of 'readelf -a <lib.so>'): |
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73 |
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74 Section Headers: |
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75 [ 9] .text PROGBITS 0000000000000a20 00000a20 |
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76 00000000000005a0 0000000000000000 AX 0 0 16 |
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77 [21] .opd PROGBITS 00000000000113b8 000013b8 |
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78 0000000000000138 0000000000000000 WA 0 0 8 |
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79 |
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80 Symbol table '.symtab' contains 86 entries: |
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81 Num: Value Size Type Bind Vis Ndx Name |
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82 76: 00000000000114c0 24 FUNC GLOBAL DEFAULT 21 foo |
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83 78: 0000000000000bb0 76 FUNC GLOBAL DEFAULT 9 .foo |
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84 |
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85 You can see now that the '.foo' entry actually points into the '.text' segment |
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86 ('Ndx'=9) and its value and size fields represent the functions actual address |
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87 and size. On the other hand, the entry for plain 'foo' points into the '.opd' |
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88 section ('Ndx'=21) and its value and size fields are the index into the '.opd' |
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89 section and the size of the corresponding '.opd' section entry (3 pointers on |
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90 PPC64). |
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91 |
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92 These so called 'dot symbols' were dropped around gcc 3.4 from GCC and BINUTILS, |
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93 see http://gcc.gnu.org/ml/gcc-patches/2004-08/msg00557.html. |
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94 But nevertheless it may still be necessary to support both formats because we |
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95 either run on an old system or because it is possible at any time that functions |
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96 appear in the stack trace which come from old-style libraries. |
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97 |
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98 Therefore we not only have to check for the presence of the function descriptor |
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99 table during symbol lookup in 'ElfSymbolTable::lookup()'. We additionally have |
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100 to check that the symbol table entry references the '.opd' section. Only in |
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101 that case we can resolve the actual function address from there. Otherwise we |
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102 use the plain 'st_value' field from the symbol table as function address. This |
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103 way we can also lookup the symbols in old-style ELF libraries (although we get |
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104 the 'dotted' versions in that case). However, if present, the 'dot' will be |
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105 conditionally removed on PPC64 from the symbol in 'ElfDecoder::demangle()' in |
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106 decoder_linux.cpp. |
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107 |
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108 Notice that we can not reliably get the function address from old-style |
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109 libraries because the 'st_value' field of the symbol table entries which point |
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110 into the '.opd' section denote the size of the corresponding '.opd' entry and |
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111 not that of the corresponding function. This has changed for the symbol table |
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112 entries in new-style libraries as described at the beginning of this |
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113 documentation. |
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114 |
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115 */ |
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116 |
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117 class ElfFuncDescTable: public CHeapObj<mtInternal> { |
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118 friend class ElfFile; |
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119 public: |
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120 ElfFuncDescTable(FILE* file, Elf_Shdr shdr, int index); |
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121 ~ElfFuncDescTable(); |
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122 |
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123 // return the function address for the function descriptor at 'index' or NULL on error |
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124 address lookup(Elf_Word index); |
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125 |
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126 int get_index() { return m_index; }; |
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127 |
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128 NullDecoder::decoder_status get_status() { return m_status; }; |
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129 |
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130 protected: |
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131 // holds the complete function descriptor section if |
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132 // we can allocate enough memory |
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133 address* m_funcDescs; |
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134 |
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135 // file contains string table |
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136 FILE* m_file; |
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137 |
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138 // section header |
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139 Elf_Shdr m_shdr; |
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140 |
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141 // The section index of this function descriptor (i.e. '.opd') section in the ELF file |
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142 int m_index; |
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143 |
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144 NullDecoder::decoder_status m_status; |
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145 }; |
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146 |
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147 #endif // !_WINDOWS && !__APPLE__ |
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148 |
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149 #endif // SHARE_VM_UTILITIES_ELF_FUNC_DESC_TABLE_HPP |