EXTERNAL_HEADERS/mach-o/reloc.h

   1 /*
   2  * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * Copyright (c) 1999-2003 Apple Computer, Inc.  All Rights Reserved.
   7  *
   8  * This file contains Original Code and/or Modifications of Original Code
   9  * as defined in and that are subject to the Apple Public Source License
  10  * Version 2.0 (the 'License'). You may not use this file except in
  11  * compliance with the License. Please obtain a copy of the License at
  12  * http://www.opensource.apple.com/apsl/ and read it before using this
  13  * file.
  14  *
  15  * The Original Code and all software distributed under the License are
  16  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  17  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  18  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  19  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  20  * Please see the License for the specific language governing rights and
  21  * limitations under the License.
  22  *
  23  * @APPLE_LICENSE_HEADER_END@
  24  */
  25 /*      $NetBSD: exec.h,v 1.6 1994/10/27 04:16:05 cgd Exp $     */
  26
  27 /*
  28  * Copyright (c) 1993 Christopher G. Demetriou
  29  * All rights reserved.
  30  *
  31  * Redistribution and use in source and binary forms, with or without
  32  * modification, are permitted provided that the following conditions
  33  * are met:
  34  * 1. Redistributions of source code must retain the above copyright
  35  *    notice, this list of conditions and the following disclaimer.
  36  * 2. Redistributions in binary form must reproduce the above copyright
  37  *    notice, this list of conditions and the following disclaimer in the
  38  *    documentation and/or other materials provided with the distribution.
  39  * 3. The name of the author may not be used to endorse or promote products
  40  *    derived from this software without specific prior written permission
  41  *
  42  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  43  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  44  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  45  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  46  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  47  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  48  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  49  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  50  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  51  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  52  */
  53
  54 #ifndef _MACHO_RELOC_H_
  55 #define _MACHO_RELOC_H_
  56
  57 /*
  58  * Format of a relocation entry of a Mach-O file.  Modified from the 4.3BSD
  59  * format.  The modifications from the original format were changing the value
  60  * of the r_symbolnum field for "local" (r_extern == 0) relocation entries.
  61  * This modification is required to support symbols in an arbitrary number of
  62  * sections not just the three sections (text, data and bss) in a 4.3BSD file.
  63  * Also the last 4 bits have had the r_type tag added to them.
  64  */
  65 struct relocation_info {
  66    long         r_address;      /* offset in the section to what is being
  67                                    relocated */
  68    unsigned int r_symbolnum:24, /* symbol index if r_extern == 1 or section
  69                                    ordinal if r_extern == 0 */
  70                 r_pcrel:1,      /* was relocated pc relative already */
  71                 r_length:2,     /* 0=byte, 1=word, 2=long */
  72                 r_extern:1,     /* does not include value of sym referenced */
  73                 r_type:4;       /* if not 0, machine specific relocation type */
  74 };
  75 #define R_ABS   0               /* absolute relocation type for Mach-O files */
  76
  77 /*
  78  * The r_address is not really the address as it's name indicates but an offset.
  79  * In 4.3BSD a.out objects this offset is from the start of the "segment" for
  80  * which relocation entry is for (text or data).  For Mach-O object files it is
  81  * also an offset but from the start of the "section" for which the relocation
  82  * entry is for.  See comments in <mach-o/loader.h> about the r_address feild
  83  * in images for used with the dynamic linker.
  84  *
  85  * In 4.3BSD a.out objects if r_extern is zero then r_symbolnum is an ordinal
  86  * for the segment the symbol being relocated is in.  These ordinals are the
  87  * symbol types N_TEXT, N_DATA, N_BSS or N_ABS.  In Mach-O object files these
  88  * ordinals refer to the sections in the object file in the order their section
  89  * structures appear in the headers of the object file they are in.  The first
  90  * section has the ordinal 1, the second 2, and so on.  This means that the
  91  * same ordinal in two different object files could refer to two different
  92  * sections.  And further could have still different ordinals when combined
  93  * by the link-editor.  The value R_ABS is used for relocation entries for
  94  * absolute symbols which need no further relocation.
  95  */
  96
  97 /*
  98  * For RISC machines some of the references are split across two instructions
  99  * and the instruction does not contain the complete value of the reference.
 100  * In these cases a second, or paired relocation entry, follows each of these
 101  * relocation entries, using a PAIR r_type, which contains the other part of the
 102  * reference not contained in the instruction.  This other part is stored in the
 103  * pair's r_address field.  The exact number of bits of the other part of the
 104  * reference store in the r_address field is dependent on the particular
 105  * relocation type for the particular architecture.
 106  */
 107
 108 /*
 109  * To make scattered loading by the link editor work correctly "local"
 110  * relocation entries can't be used when the item to be relocated is the value
 111  * of a symbol plus an offset (where the resulting expresion is outside the
 112  * block the link editor is moving, a blocks are divided at symbol addresses).
 113  * In this case. where the item is a symbol value plus offset, the link editor
 114  * needs to know more than just the section the symbol was defined.  What is
 115  * needed is the actual value of the symbol without the offset so it can do the
 116  * relocation correctly based on where the value of the symbol got relocated to
 117  * not the value of the expression (with the offset added to the symbol value).
 118  * So for the NeXT 2.0 release no "local" relocation entries are ever used when
 119  * there is a non-zero offset added to a symbol.  The "external" and "local"
 120  * relocation entries remain unchanged.
 121  *
 122  * The implemention is quite messy given the compatibility with the existing
 123  * relocation entry format.  The ASSUMPTION is that a section will never be
 124  * bigger than 2**24 - 1 (0x00ffffff or 16,777,215) bytes.  This assumption
 125  * allows the r_address (which is really an offset) to fit in 24 bits and high
 126  * bit of the r_address field in the relocation_info structure to indicate
 127  * it is really a scattered_relocation_info structure.  Since these are only
 128  * used in places where "local" relocation entries are used and not where
 129  * "external" relocation entries are used the r_extern field has been removed.
 130  *
 131  * For scattered loading to work on a RISC machine where some of the references
 132  * are split across two instructions the link editor needs to be assured that
 133  * each reference has a unique 32 bit reference (that more than one reference is
 134  * NOT sharing the same high 16 bits for example) so it move each referenced
 135  * item independent of each other.  Some compilers guarantees this but the
 136  * compilers don't so scattered loading can be done on those that do guarantee
 137  * this.
 138  */
 139 #if defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__)
 140 /*
 141  * The reason for the ifdef's of __BIG_ENDIAN__ and __LITTLE_ENDIAN__ are that
 142  * when stattered relocation entries were added the mistake of using a mask
 143  * against a structure that is made up of bit fields was used.  To make this
 144  * design work this structure must be laid out in memory the same way so the
 145  * mask can be applied can check the same bit each time (r_scattered).
 146  */
 147 #endif /* defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__) */
 148 #define R_SCATTERED 0x80000000  /* mask to be applied to the r_address field
 149                                    of a relocation_info structure to tell that
 150                                    is is really a scattered_relocation_info
 151                                    stucture */
 152 struct scattered_relocation_info {
 153 #ifdef __BIG_ENDIAN__
 154    unsigned int r_scattered:1,  /* 1=scattered, 0=non-scattered (see above) */
 155                 r_pcrel:1,      /* was relocated pc relative already */
 156                 r_length:2,     /* 0=byte, 1=word, 2=long */
 157                 r_type:4,       /* if not 0, machine specific relocation type */
 158                 r_address:24;   /* offset in the section to what is being
 159                                    relocated */
 160    long         r_value;        /* the value the item to be relocated is
 161                                    refering to (without any offset added) */
 162 #endif /* __BIG_ENDIAN__ */
 163 #ifdef __LITTLE_ENDIAN__
 164    unsigned int
 165                 r_address:24,   /* offset in the section to what is being
 166                                    relocated */
 167                 r_type:4,       /* if not 0, machine specific relocation type */
 168                 r_length:2,     /* 0=byte, 1=word, 2=long */
 169                 r_pcrel:1,      /* was relocated pc relative already */
 170                 r_scattered:1;  /* 1=scattered, 0=non-scattered (see above) */
 171    long         r_value;        /* the value the item to be relocated is
 172                                    refering to (without any offset added) */
 173 #endif /* __LITTLE_ENDIAN__ */
 174 };
 175
 176 /*
 177  * Relocation types used in a generic implementation.  Relocation entries for
 178  * nornal things use the generic relocation as discribed above and their r_type
 179  * is GENERIC_RELOC_VANILLA (a value of zero).
 180  *
 181  * Another type of generic relocation, GENERIC_RELOC_SECTDIFF, is to support
 182  * the difference of two symbols defined in different sections.  That is the
 183  * expression "symbol1 - symbol2 + constant" is a relocatable expression when
 184  * both symbols are defined in some section.  For this type of relocation the
 185  * both relocations entries are scattered relocation entries.  The value of
 186  * symbol1 is stored in the first relocation entry's r_value field and the
 187  * value of symbol2 is stored in the pair's r_value field.
 188  *
 189  * A special case for a prebound lazy pointer is needed to beable to set the
 190  * value of the lazy pointer back to its non-prebound state.  This is done
 191  * using the GENERIC_RELOC_PB_LA_PTR r_type.  This is a scattered relocation
 192  * entry where the r_value feild is the value of the lazy pointer not prebound.
 193  */
 194 enum reloc_type_generic
 195 {
 196     GENERIC_RELOC_VANILLA,      /* generic relocation as discribed above */
 197     GENERIC_RELOC_PAIR,         /* Only follows a GENRIC_RELOC_SECTDIFF */
 198     GENERIC_RELOC_SECTDIFF,
 199     GENERIC_RELOC_PB_LA_PTR     /* prebound lazy pointer */
 200 };
 201
 202 #endif /* _MACHO_RELOC_H_ */