elf/dl-runtime.c

   1 /* On-demand PLT fixup for shared objects.
   2 Copyright (C) 1995, 1996 Free Software Foundation, Inc.
   3 This file is part of the GNU C Library.
   4
   5 The GNU C Library is free software; you can redistribute it and/or
   6 modify it under the terms of the GNU Library General Public License as
   7 published by the Free Software Foundation; either version 2 of the
   8 License, or (at your option) any later version.
   9
  10 The GNU C Library is distributed in the hope that it will be useful,
  11 but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13 Library General Public License for more details.
  14
  15 You should have received a copy of the GNU Library General Public
  16 License along with the GNU C Library; see the file COPYING.LIB.  If
  17 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  18 Cambridge, MA 02139, USA.  */
  19
  20 #include <link.h>
  21 #include "dynamic-link.h"
  22
  23
  24 /* The global scope we will use for symbol lookups.
  25    This will be modified by _dl_open if RTLD_GLOBAL is used.  */
  26 struct link_map **_dl_global_scope = _dl_default_scope;
  27 struct link_map **_dl_global_scope_end = &_dl_default_scope[3];
  28
  29
  30 /* Hack _dl_global_scope[0] and [1] as necessary, and return a pointer into
  31    _dl_global_scope that should be passed to _dl_lookup_symbol for symbol
  32    references made in the object L's relocations.  */
  33 inline struct link_map **
  34 _dl_object_relocation_scope (struct link_map *l)
  35 {
  36   if (l->l_info[DT_SYMBOLIC])
  37     {
  38       /* This object's global references are to be resolved first
  39          in the object itself, and only secondarily in more global
  40          scopes.  */
  41
  42       if (! l->l_searchlist)
  43         /* We must construct the searchlist for this object.  */
  44         _dl_map_object_deps (l);
  45
  46       /* The primary scope is this object itself and its
  47          dependencies.  */
  48       _dl_global_scope[0] = l;
  49
  50       /* Secondary is the dependency tree that reached L; the object
  51          requested directly by the user is at the root of that tree.  */
  52       while (l->l_loader)
  53         l = l->l_loader;
  54       _dl_global_scope[1] = l;
  55
  56       /* Finally, the global scope follows.  */
  57
  58       return _dl_global_scope;
  59     }
  60   else
  61     {
  62       /* Use first the global scope, and then the scope of the root of the
  63          dependency tree that first caused this object to be loaded.  */
  64       while (l->l_loader)
  65         l = l->l_loader;
  66       *_dl_global_scope_end = l;
  67       return &_dl_global_scope[2];
  68     }
  69 }
  70 \f
  71 /* Figure out the right type, Rel or Rela.  */
  72 #define elf_machine_rel 1
  73 #define elf_machine_rela 2
  74 #if elf_machine_relplt == elf_machine_rel
  75 #define PLTREL ElfW(Rel)
  76 #elif elf_machine_relplt == elf_machine_rela
  77 #define PLTREL ElfW(Rela)
  78 #else
  79 #error "dl-machine.h bug: elf_machine_relplt not rel or rela"
  80 #endif
  81 #undef elf_machine_rel
  82 #undef elf_machine_rela
  83
  84 /* We need to define the function as a local symbol so that the reference
  85    in the trampoline code will be a local PC-relative call.  Tell the
  86    compiler not to worry that the function appears not to be called.  */
  87
  88 static ElfW(Addr) fixup (
  89 #ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
  90                          ELF_MACHINE_RUNTIME_FIXUP_ARGS,
  91 #endif
  92                          struct link_map *l, ElfW(Word) reloc_offset)
  93      __attribute__ ((unused));
  94
  95 /* This function is called through a special trampoline from the PLT the
  96    first time each PLT entry is called.  We must perform the relocation
  97    specified in the PLT of the given shared object, and return the resolved
  98    function address to the trampoline, which will restart the original call
  99    to that address.  Future calls will bounce directly from the PLT to the
 100    function.  */
 101
 102 static ElfW(Addr)
 103 fixup (
 104 #ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
 105        ELF_MACHINE_RUNTIME_FIXUP_ARGS,
 106 #endif
 107        struct link_map *l, ElfW(Word) reloc_offset)
 108 {
 109   const ElfW(Sym) *const symtab
 110     = (const ElfW(Sym) *) (l->l_addr + l->l_info[DT_SYMTAB]->d_un.d_ptr);
 111   const char *strtab =
 112     (const char *) (l->l_addr + l->l_info[DT_STRTAB]->d_un.d_ptr);
 113
 114   const PLTREL *const reloc
 115     = (const void *) (l->l_addr + l->l_info[DT_JMPREL]->d_un.d_ptr +
 116                       reloc_offset);
 117
 118   /* Set up the scope to find symbols referenced by this object.  */
 119   struct link_map **scope = _dl_object_relocation_scope (l);
 120
 121   /* Perform the specified relocation.  */
 122   ElfW(Addr) resolve (const ElfW(Sym) **ref,
 123                       ElfW(Addr) reloc_addr, int noplt)
 124     {
 125       return _dl_lookup_symbol (strtab + (*ref)->st_name, ref,
 126                                 scope, l->l_name, reloc_addr, noplt);
 127     }
 128   elf_machine_relplt (l, reloc, &symtab[ELFW(R_SYM) (reloc->r_info)], resolve);
 129
 130   *_dl_global_scope_end = NULL;
 131
 132   /* Return the address that was written by the relocation.  */
 133   return *(ElfW(Addr) *) (l->l_addr + reloc->r_offset);
 134 }
 135
 136
 137 /* This macro is defined in dl-machine.h to define the entry point called
 138    by the PLT.  The `fixup' function above does the real work, but a little
 139    more twiddling is needed to get the stack right and jump to the address
 140    finally resolved.  */
 141
 142 ELF_MACHINE_RUNTIME_TRAMPOLINE