| /* |
| * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. |
| * Copyright (c) 2012, 2013 SAP SE. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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| */ |
| |
| #ifndef SHARE_VM_UTILITIES_ELF_FUNC_DESC_TABLE_HPP |
| #define SHARE_VM_UTILITIES_ELF_FUNC_DESC_TABLE_HPP |
| |
| #if !defined(_WINDOWS) && !defined(__APPLE__) |
| |
| |
| #include "memory/allocation.hpp" |
| #include "utilities/decoder.hpp" |
| #include "utilities/elfFile.hpp" |
| |
| /* |
| |
| On PowerPC-64 (and other architectures like for example IA64) a pointer to a |
| function is not just a plain code address, but instead a pointer to a so called |
| function descriptor (which is simply a structure containing 3 pointers). |
| This fact is also reflected in the ELF ABI for PowerPC-64. |
| |
| On architectures like x86 or SPARC, the ELF symbol table contains the start |
| address and size of an object. So for example for a function object (i.e. type |
| 'STT_FUNC') the symbol table's 'st_value' and 'st_size' fields directly |
| represent the starting address and size of that function. On PPC64 however, the |
| symbol table's 'st_value' field only contains an index into another, PPC64 |
| specific '.opd' (official procedure descriptors) section, while the 'st_size' |
| field still holds the size of the corresponding function. In order to get the |
| actual start address of a function, it is necessary to read the corresponding |
| function descriptor entry in the '.opd' section at the corresponding index and |
| extract the start address from there. |
| |
| That's exactly what this 'ElfFuncDescTable' class is used for. If the HotSpot |
| runs on a PPC64 machine, and the corresponding ELF files contains an '.opd' |
| section (which is actually mandatory on PPC64) it will be read into an object |
| of type 'ElfFuncDescTable' just like the string and symbol table sections. |
| Later on, during symbol lookup in 'ElfSymbolTable::lookup()' this function |
| descriptor table will be used if available to find the real function address. |
| |
| All this is how things work today (2013) on contemporary Linux distributions |
| (i.e. SLES 10) and new version of GCC (i.e. > 4.0). However there is a history, |
| and it goes like this: |
| |
| In SLES 9 times (sometimes before GCC 3.4) gcc/ld on PPC64 generated two |
| entries in the symbol table for every function. The value of the symbol with |
| the name of the function was the address of the function descriptor while the |
| dot '.' prefixed name was reserved to hold the actual address of that function |
| (http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#FUNC-DES). |
| |
| For a C-function 'foo' this resulted in two symbol table entries like this |
| (extracted from the output of 'readelf -a <lib.so>'): |
| |
| Section Headers: |
| [ 9] .text PROGBITS 0000000000000a20 00000a20 |
| 00000000000005a0 0000000000000000 AX 0 0 16 |
| [21] .opd PROGBITS 00000000000113b8 000013b8 |
| 0000000000000138 0000000000000000 WA 0 0 8 |
| |
| Symbol table '.symtab' contains 86 entries: |
| Num: Value Size Type Bind Vis Ndx Name |
| 76: 00000000000114c0 24 FUNC GLOBAL DEFAULT 21 foo |
| 78: 0000000000000bb0 76 FUNC GLOBAL DEFAULT 9 .foo |
| |
| You can see now that the '.foo' entry actually points into the '.text' segment |
| ('Ndx'=9) and its value and size fields represent the functions actual address |
| and size. On the other hand, the entry for plain 'foo' points into the '.opd' |
| section ('Ndx'=21) and its value and size fields are the index into the '.opd' |
| section and the size of the corresponding '.opd' section entry (3 pointers on |
| PPC64). |
| |
| These so called 'dot symbols' were dropped around gcc 3.4 from GCC and BINUTILS, |
| see http://gcc.gnu.org/ml/gcc-patches/2004-08/msg00557.html. |
| But nevertheless it may still be necessary to support both formats because we |
| either run on an old system or because it is possible at any time that functions |
| appear in the stack trace which come from old-style libraries. |
| |
| Therefore we not only have to check for the presence of the function descriptor |
| table during symbol lookup in 'ElfSymbolTable::lookup()'. We additionally have |
| to check that the symbol table entry references the '.opd' section. Only in |
| that case we can resolve the actual function address from there. Otherwise we |
| use the plain 'st_value' field from the symbol table as function address. This |
| way we can also lookup the symbols in old-style ELF libraries (although we get |
| the 'dotted' versions in that case). However, if present, the 'dot' will be |
| conditionally removed on PPC64 from the symbol in 'ElfDecoder::demangle()' in |
| decoder_linux.cpp. |
| |
| Notice that we can not reliably get the function address from old-style |
| libraries because the 'st_value' field of the symbol table entries which point |
| into the '.opd' section denote the size of the corresponding '.opd' entry and |
| not that of the corresponding function. This has changed for the symbol table |
| entries in new-style libraries as described at the beginning of this |
| documentation. |
| |
| */ |
| |
| class ElfFuncDescTable: public CHeapObj<mtInternal> { |
| friend class ElfFile; |
| public: |
| ElfFuncDescTable(FILE* file, Elf_Shdr shdr, int index); |
| ~ElfFuncDescTable(); |
| |
| // return the function address for the function descriptor at 'index' or NULL on error |
| address lookup(Elf_Word index); |
| |
| int get_index() { return m_index; }; |
| |
| NullDecoder::decoder_status get_status() { return m_status; }; |
| |
| protected: |
| // holds the complete function descriptor section if |
| // we can allocate enough memory |
| address* m_funcDescs; |
| |
| // file contains string table |
| FILE* m_file; |
| |
| // section header |
| Elf_Shdr m_shdr; |
| |
| // The section index of this function descriptor (i.e. '.opd') section in the ELF file |
| int m_index; |
| |
| NullDecoder::decoder_status m_status; |
| }; |
| |
| #endif // !_WINDOWS && !__APPLE__ |
| |
| #endif // SHARE_VM_UTILITIES_ELF_FUNC_DESC_TABLE_HPP |