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  * 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. Please obtain a copy of the License at
  10  * http://www.opensource.apple.com/apsl/ and read it before using this
  11  * file.
  12  *
  13  * The Original Code and all software distributed under the License are
  14  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  15  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  16  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  18  * Please see the License for the specific language governing rights and
  19  * limitations under the License.
  20  *
  21  * @APPLE_LICENSE_HEADER_END@
  22  */
  23 /*      $NetBSD: exec.h,v 1.6 1994/10/27 04:16:05 cgd Exp $     */
  24
  25 /*
  26  * Copyright (c) 1993 Christopher G. Demetriou
  27  * All rights reserved.
  28  *
  29  * Redistribution and use in source and binary forms, with or without
  30  * modification, are permitted provided that the following conditions
  31  * are met:
  32  * 1. Redistributions of source code must retain the above copyright
  33  *    notice, this list of conditions and the following disclaimer.
  34  * 2. Redistributions in binary form must reproduce the above copyright
  35  *    notice, this list of conditions and the following disclaimer in the
  36  *    documentation and/or other materials provided with the distribution.
  37  * 3. The name of the author may not be used to endorse or promote products
  38  *    derived from this software without specific prior written permission
  39  *
  40  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  41  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  42  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  43  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  44  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  45  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  46  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  47  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  48  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  49  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  50  */
  51
  52 #ifndef _MACHO_RELOC_H_
  53 #define _MACHO_RELOC_H_
  54 #include <stdint.h>
  55
  56 /*
  57  * Format of a relocation entry of a Mach-O file.  Modified from the 4.3BSD
  58  * format.  The modifications from the original format were changing the value
  59  * of the r_symbolnum field for "local" (r_extern == 0) relocation entries.
  60  * This modification is required to support symbols in an arbitrary number of
  61  * sections not just the three sections (text, data and bss) in a 4.3BSD file.
  62  * Also the last 4 bits have had the r_type tag added to them.
  63  */
  64 struct relocation_info {
  65    int32_t      r_address;      /* offset in the section to what is being
  66                                    relocated */
  67    uint32_t     r_symbolnum:24, /* symbol index if r_extern == 1 or section
  68                                    ordinal if r_extern == 0 */
  69                 r_pcrel:1,      /* was relocated pc relative already */
  70                 r_length:2,     /* 0=byte, 1=word, 2=long, 3=quad */
  71                 r_extern:1,     /* does not include value of sym referenced */
  72                 r_type:4;       /* if not 0, machine specific relocation type */
  73 };
  74 #define R_ABS   0               /* absolute relocation type for Mach-O files */
  75
  76 /*
  77  * The r_address is not really the address as it's name indicates but an offset.
  78  * In 4.3BSD a.out objects this offset is from the start of the "segment" for
  79  * which relocation entry is for (text or data).  For Mach-O object files it is
  80  * also an offset but from the start of the "section" for which the relocation
  81  * entry is for.  See comments in <mach-o/loader.h> about the r_address feild
  82  * in images for used with the dynamic linker.
  83  *
  84  * In 4.3BSD a.out objects if r_extern is zero then r_symbolnum is an ordinal
  85  * for the segment the symbol being relocated is in.  These ordinals are the
  86  * symbol types N_TEXT, N_DATA, N_BSS or N_ABS.  In Mach-O object files these
  87  * ordinals refer to the sections in the object file in the order their section
  88  * structures appear in the headers of the object file they are in.  The first
  89  * section has the ordinal 1, the second 2, and so on.  This means that the
  90  * same ordinal in two different object files could refer to two different
  91  * sections.  And further could have still different ordinals when combined
  92  * by the link-editor.  The value R_ABS is used for relocation entries for
  93  * absolute symbols which need no further relocation.
  94  */
  95
  96 /*
  97  * For RISC machines some of the references are split across two instructions
  98  * and the instruction does not contain the complete value of the reference.
  99  * In these cases a second, or paired relocation entry, follows each of these
 100  * relocation entries, using a PAIR r_type, which contains the other part of the
 101  * reference not contained in the instruction.  This other part is stored in the
 102  * pair's r_address field.  The exact number of bits of the other part of the
 103  * reference store in the r_address field is dependent on the particular
 104  * relocation type for the particular architecture.
 105  */
 106
 107 /*
 108  * To make scattered loading by the link editor work correctly "local"
 109  * relocation entries can't be used when the item to be relocated is the value
 110  * of a symbol plus an offset (where the resulting expresion is outside the
 111  * block the link editor is moving, a blocks are divided at symbol addresses).
 112  * In this case. where the item is a symbol value plus offset, the link editor
 113  * needs to know more than just the section the symbol was defined.  What is
 114  * needed is the actual value of the symbol without the offset so it can do the
 115  * relocation correctly based on where the value of the symbol got relocated to
 116  * not the value of the expression (with the offset added to the symbol value).
 117  * So for the NeXT 2.0 release no "local" relocation entries are ever used when
 118  * there is a non-zero offset added to a symbol.  The "external" and "local"
 119  * relocation entries remain unchanged.
 120  *
 121  * The implemention is quite messy given the compatibility with the existing
 122  * relocation entry format.  The ASSUMPTION is that a section will never be
 123  * bigger than 2**24 - 1 (0x00ffffff or 16,777,215) bytes.  This assumption
 124  * allows the r_address (which is really an offset) to fit in 24 bits and high
 125  * bit of the r_address field in the relocation_info structure to indicate
 126  * it is really a scattered_relocation_info structure.  Since these are only
 127  * used in places where "local" relocation entries are used and not where
 128  * "external" relocation entries are used the r_extern field has been removed.
 129  *
 130  * For scattered loading to work on a RISC machine where some of the references
 131  * are split across two instructions the link editor needs to be assured that
 132  * each reference has a unique 32 bit reference (that more than one reference is
 133  * NOT sharing the same high 16 bits for example) so it move each referenced
 134  * item independent of each other.  Some compilers guarantees this but the
 135  * compilers don't so scattered loading can be done on those that do guarantee
 136  * this.
 137  */
 138 #if defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__)
 139 /*
 140  * The reason for the ifdef's of __BIG_ENDIAN__ and __LITTLE_ENDIAN__ are that
 141  * when stattered relocation entries were added the mistake of using a mask
 142  * against a structure that is made up of bit fields was used.  To make this
 143  * design work this structure must be laid out in memory the same way so the
 144  * mask can be applied can check the same bit each time (r_scattered).
 145  */
 146 #endif /* defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__) */
 147 #define R_SCATTERED 0x80000000  /* mask to be applied to the r_address field
 148                                    of a relocation_info structure to tell that
 149                                    is is really a scattered_relocation_info
 150                                    stucture */
 151 struct scattered_relocation_info {
 152 #ifdef __BIG_ENDIAN__
 153    uint32_t     r_scattered:1,  /* 1=scattered, 0=non-scattered (see above) */
 154                 r_pcrel:1,      /* was relocated pc relative already */
 155                 r_length:2,     /* 0=byte, 1=word, 2=long, 3=quad */
 156                 r_type:4,       /* if not 0, machine specific relocation type */
 157                 r_address:24;   /* offset in the section to what is being
 158                                    relocated */
 159    int32_t      r_value;        /* the value the item to be relocated is
 160                                    refering to (without any offset added) */
 161 #endif /* __BIG_ENDIAN__ */
 162 #ifdef __LITTLE_ENDIAN__
 163    uint32_t
 164                 r_address:24,   /* offset in the section to what is being
 165                                    relocated */
 166                 r_type:4,       /* if not 0, machine specific relocation type */
 167                 r_length:2,     /* 0=byte, 1=word, 2=long, 3=quad */
 168                 r_pcrel:1,      /* was relocated pc relative already */
 169                 r_scattered:1;  /* 1=scattered, 0=non-scattered (see above) */
 170    int32_t      r_value;        /* the value the item to be relocated is
 171                                    refering to (without any offset added) */
 172 #endif /* __LITTLE_ENDIAN__ */
 173 };
 174
 175 /*
 176  * Relocation types used in a generic implementation.  Relocation entries for
 177  * normal things use the generic relocation as discribed above and their r_type
 178  * is GENERIC_RELOC_VANILLA (a value of zero).
 179  *
 180  * Another type of generic relocation, GENERIC_RELOC_SECTDIFF, is to support
 181  * the difference of two symbols defined in different sections.  That is the
 182  * expression "symbol1 - symbol2 + constant" is a relocatable expression when
 183  * both symbols are defined in some section.  For this type of relocation the
 184  * both relocations entries are scattered relocation entries.  The value of
 185  * symbol1 is stored in the first relocation entry's r_value field and the
 186  * value of symbol2 is stored in the pair's r_value field.
 187  *
 188  * A special case for a prebound lazy pointer is needed to beable to set the
 189  * value of the lazy pointer back to its non-prebound state.  This is done
 190  * using the GENERIC_RELOC_PB_LA_PTR r_type.  This is a scattered relocation
 191  * entry where the r_value feild is the value of the lazy pointer not prebound.
 192  */
 193 enum reloc_type_generic
 194 {
 195     GENERIC_RELOC_VANILLA,      /* generic relocation as discribed above */
 196     GENERIC_RELOC_PAIR,         /* Only follows a GENERIC_RELOC_SECTDIFF */
 197     GENERIC_RELOC_SECTDIFF,
 198     GENERIC_RELOC_PB_LA_PTR,    /* prebound lazy pointer */
 199     GENERIC_RELOC_LOCAL_SECTDIFF,
 200     GENERIC_RELOC_TLV           /* thread local variables */
 201 };
 202
 203 #endif /* _MACHO_RELOC_H_ */