Mercurial > hg > truffle
diff src/share/vm/utilities/elfFuncDescTable.hpp @ 14460:8a9bb7821e28
Merge
author | kvn |
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date | Wed, 19 Feb 2014 12:08:49 -0800 |
parents | e7cbc95179c4 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/share/vm/utilities/elfFuncDescTable.hpp Wed Feb 19 12:08:49 2014 -0800 @@ -0,0 +1,149 @@ +/* + * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. + * Copyright 2012, 2013 SAP AG. 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 + * or visit www.oracle.com if you need additional information or have any + * questions. + * + */ + +#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