EXTERNAL_HEADERS/mach-o/reloc.h

   1 /*
   2  * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
   5  *
   6  * This file contains Original Code and/or Modifications of Original Code
   7  * as defined in and that are subject to the Apple Public Source License
   8  * Version 2.0 (the 'License'). You may not use this file except in
   9  * compliance with the License. The rights granted to you under the License
  10  * may not be used to create, or enable the creation or redistribution of,
  11  * unlawful or unlicensed copies of an Apple operating system, or to
  12  * circumvent, violate, or enable the circumvention or violation of, any
  13  * terms of an Apple operating system software license agreement.
  14  *
  15  * Please obtain a copy of the License at
  16  * http://www.opensource.apple.com/apsl/ and read it before using this file.
  17  *
  18  * The Original Code and all software distributed under the License are
  19  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  20  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  21  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
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  23  * Please see the License for the specific language governing rights and
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  27  */
  28 /*      $NetBSD: exec.h,v 1.6 1994/10/27 04:16:05 cgd Exp $     */
  29
  30 /*
  31  * Copyright (c) 1993 Christopher G. Demetriou
  32  * All rights reserved.
  33  *
  34  * Redistribution and use in source and binary forms, with or without
  35  * modification, are permitted provided that the following conditions
  36  * are met:
  37  * 1. Redistributions of source code must retain the above copyright
  38  *    notice, this list of conditions and the following disclaimer.
  39  * 2. Redistributions in binary form must reproduce the above copyright
  40  *    notice, this list of conditions and the following disclaimer in the
  41  *    documentation and/or other materials provided with the distribution.
  42  * 3. The name of the author may not be used to endorse or promote products
  43  *    derived from this software without specific prior written permission
  44  *
  45  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  46  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  47  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  48  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  49  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  50  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  51  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  52  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  53  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  54  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  55  */
  56
  57 #ifndef _MACHO_RELOC_H_
  58 #define _MACHO_RELOC_H_
  59
  60 /*
  61  * Format of a relocation entry of a Mach-O file.  Modified from the 4.3BSD
  62  * format.  The modifications from the original format were changing the value
  63  * of the r_symbolnum field for "local" (r_extern == 0) relocation entries.
  64  * This modification is required to support symbols in an arbitrary number of
  65  * sections not just the three sections (text, data and bss) in a 4.3BSD file.
  66  * Also the last 4 bits have had the r_type tag added to them.
  67  */
  68 struct relocation_info {
  69    long         r_address;      /* offset in the section to what is being
  70                                    relocated */
  71    unsigned int r_symbolnum:24, /* symbol index if r_extern == 1 or section
  72                                    ordinal if r_extern == 0 */
  73                 r_pcrel:1,      /* was relocated pc relative already */
  74                 r_length:2,     /* 0=byte, 1=word, 2=long */
  75                 r_extern:1,     /* does not include value of sym referenced */
  76                 r_type:4;       /* if not 0, machine specific relocation type */
  77 };
  78 #define R_ABS   0               /* absolute relocation type for Mach-O files */
  79
  80 /*
  81  * The r_address is not really the address as it's name indicates but an offset.
  82  * In 4.3BSD a.out objects this offset is from the start of the "segment" for
  83  * which relocation entry is for (text or data).  For Mach-O object files it is
  84  * also an offset but from the start of the "section" for which the relocation
  85  * entry is for.  See comments in <mach-o/loader.h> about the r_address feild
  86  * in images for used with the dynamic linker.
  87  *
  88  * In 4.3BSD a.out objects if r_extern is zero then r_symbolnum is an ordinal
  89  * for the segment the symbol being relocated is in.  These ordinals are the
  90  * symbol types N_TEXT, N_DATA, N_BSS or N_ABS.  In Mach-O object files these
  91  * ordinals refer to the sections in the object file in the order their section
  92  * structures appear in the headers of the object file they are in.  The first
  93  * section has the ordinal 1, the second 2, and so on.  This means that the
  94  * same ordinal in two different object files could refer to two different
  95  * sections.  And further could have still different ordinals when combined
  96  * by the link-editor.  The value R_ABS is used for relocation entries for
  97  * absolute symbols which need no further relocation.
  98  */
  99
 100 /*
 101  * For RISC machines some of the references are split across two instructions
 102  * and the instruction does not contain the complete value of the reference.
 103  * In these cases a second, or paired relocation entry, follows each of these
 104  * relocation entries, using a PAIR r_type, which contains the other part of the
 105  * reference not contained in the instruction.  This other part is stored in the
 106  * pair's r_address field.  The exact number of bits of the other part of the
 107  * reference store in the r_address field is dependent on the particular
 108  * relocation type for the particular architecture.
 109  */
 110
 111 /*
 112  * To make scattered loading by the link editor work correctly "local"
 113  * relocation entries can't be used when the item to be relocated is the value
 114  * of a symbol plus an offset (where the resulting expresion is outside the
 115  * block the link editor is moving, a blocks are divided at symbol addresses).
 116  * In this case. where the item is a symbol value plus offset, the link editor
 117  * needs to know more than just the section the symbol was defined.  What is
 118  * needed is the actual value of the symbol without the offset so it can do the
 119  * relocation correctly based on where the value of the symbol got relocated to
 120  * not the value of the expression (with the offset added to the symbol value).
 121  * So for the NeXT 2.0 release no "local" relocation entries are ever used when
 122  * there is a non-zero offset added to a symbol.  The "external" and "local"
 123  * relocation entries remain unchanged.
 124  *
 125  * The implemention is quite messy given the compatibility with the existing
 126  * relocation entry format.  The ASSUMPTION is that a section will never be
 127  * bigger than 2**24 - 1 (0x00ffffff or 16,777,215) bytes.  This assumption
 128  * allows the r_address (which is really an offset) to fit in 24 bits and high
 129  * bit of the r_address field in the relocation_info structure to indicate
 130  * it is really a scattered_relocation_info structure.  Since these are only
 131  * used in places where "local" relocation entries are used and not where
 132  * "external" relocation entries are used the r_extern field has been removed.
 133  *
 134  * For scattered loading to work on a RISC machine where some of the references
 135  * are split across two instructions the link editor needs to be assured that
 136  * each reference has a unique 32 bit reference (that more than one reference is
 137  * NOT sharing the same high 16 bits for example) so it move each referenced
 138  * item independent of each other.  Some compilers guarantees this but the
 139  * compilers don't so scattered loading can be done on those that do guarantee
 140  * this.
 141  */
 142 #if defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__)
 143 /*
 144  * The reason for the ifdef's of __BIG_ENDIAN__ and __LITTLE_ENDIAN__ are that
 145  * when stattered relocation entries were added the mistake of using a mask
 146  * against a structure that is made up of bit fields was used.  To make this
 147  * design work this structure must be laid out in memory the same way so the
 148  * mask can be applied can check the same bit each time (r_scattered).
 149  */
 150 #endif /* defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__) */
 151 #define R_SCATTERED 0x80000000  /* mask to be applied to the r_address field
 152                                    of a relocation_info structure to tell that
 153                                    is is really a scattered_relocation_info
 154                                    stucture */
 155 struct scattered_relocation_info {
 156 #ifdef __BIG_ENDIAN__
 157    unsigned int r_scattered:1,  /* 1=scattered, 0=non-scattered (see above) */
 158                 r_pcrel:1,      /* was relocated pc relative already */
 159                 r_length:2,     /* 0=byte, 1=word, 2=long */
 160                 r_type:4,       /* if not 0, machine specific relocation type */
 161                 r_address:24;   /* offset in the section to what is being
 162                                    relocated */
 163    long         r_value;        /* the value the item to be relocated is
 164                                    refering to (without any offset added) */
 165 #endif /* __BIG_ENDIAN__ */
 166 #ifdef __LITTLE_ENDIAN__
 167    unsigned int
 168                 r_address:24,   /* offset in the section to what is being
 169                                    relocated */
 170                 r_type:4,       /* if not 0, machine specific relocation type */
 171                 r_length:2,     /* 0=byte, 1=word, 2=long */
 172                 r_pcrel:1,      /* was relocated pc relative already */
 173                 r_scattered:1;  /* 1=scattered, 0=non-scattered (see above) */
 174    long         r_value;        /* the value the item to be relocated is
 175                                    refering to (without any offset added) */
 176 #endif /* __LITTLE_ENDIAN__ */
 177 };
 178
 179 /*
 180  * Relocation types used in a generic implementation.  Relocation entries for
 181  * nornal things use the generic relocation as discribed above and their r_type
 182  * is GENERIC_RELOC_VANILLA (a value of zero).
 183  *
 184  * Another type of generic relocation, GENERIC_RELOC_SECTDIFF, is to support
 185  * the difference of two symbols defined in different sections.  That is the
 186  * expression "symbol1 - symbol2 + constant" is a relocatable expression when
 187  * both symbols are defined in some section.  For this type of relocation the
 188  * both relocations entries are scattered relocation entries.  The value of
 189  * symbol1 is stored in the first relocation entry's r_value field and the
 190  * value of symbol2 is stored in the pair's r_value field.
 191  *
 192  * A special case for a prebound lazy pointer is needed to beable to set the
 193  * value of the lazy pointer back to its non-prebound state.  This is done
 194  * using the GENERIC_RELOC_PB_LA_PTR r_type.  This is a scattered relocation
 195  * entry where the r_value feild is the value of the lazy pointer not prebound.
 196  */
 197 enum reloc_type_generic
 198 {
 199     GENERIC_RELOC_VANILLA,      /* generic relocation as discribed above */
 200     GENERIC_RELOC_PAIR,         /* Only follows a GENRIC_RELOC_SECTDIFF */
 201     GENERIC_RELOC_SECTDIFF,
 202     GENERIC_RELOC_PB_LA_PTR     /* prebound lazy pointer */
 203 };
 204
 205 #endif /* _MACHO_RELOC_H_ */