elf/dl-runtime.c

   1 /* On-demand PLT fixup for shared objects.
   2    Copyright (C) 1995, 1996, 1997, 1998, 1999 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 not,
  17    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18    Boston, MA 02111-1307, USA.  */
  19
  20 #include <unistd.h>
  21 #include <elf/ldsodefs.h>
  22 #include "dynamic-link.h"
  23
  24 #if !defined ELF_MACHINE_NO_RELA || ELF_MACHINE_NO_REL
  25 # define PLTREL  ElfW(Rela)
  26 #else
  27 # define PLTREL  ElfW(Rel)
  28 #endif
  29
  30 #ifndef VERSYMIDX
  31 # define VERSYMIDX(sym) (DT_NUM + DT_PROCNUM + DT_VERSIONTAGIDX (sym))
  32 #endif
  33
  34
  35 /* This function is called through a special trampoline from the PLT the
  36    first time each PLT entry is called.  We must perform the relocation
  37    specified in the PLT of the given shared object, and return the resolved
  38    function address to the trampoline, which will restart the original call
  39    to that address.  Future calls will bounce directly from the PLT to the
  40    function.  */
  41
  42 #ifndef ELF_MACHINE_NO_PLT
  43 static ElfW(Addr) __attribute__ ((unused))
  44 fixup (
  45 # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
  46         ELF_MACHINE_RUNTIME_FIXUP_ARGS,
  47 # endif
  48        struct link_map *l, ElfW(Word) reloc_offset)
  49 {
  50   const ElfW(Sym) *const symtab
  51     = (const void *) l->l_info[DT_SYMTAB]->d_un.d_ptr;
  52   const char *strtab = (const void *) l->l_info[DT_STRTAB]->d_un.d_ptr;
  53
  54   const PLTREL *const reloc
  55     = (const void *) (l->l_info[DT_JMPREL]->d_un.d_ptr + reloc_offset);
  56   const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
  57   void *const rel_addr = (void *)(l->l_addr + reloc->r_offset);
  58   ElfW(Addr) value;
  59
  60   /* The use of `alloca' here looks ridiculous but it helps.  The goal is
  61      to prevent the function from being inlined and thus optimized out.
  62      There is no official way to do this so we use this trick.  gcc never
  63      inlines functions which use `alloca'.  */
  64   alloca (sizeof (int));
  65
  66   /* Sanity check that we're really looking at a PLT relocation.  */
  67   assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
  68
  69    /* Look up the target symbol.  */
  70   switch (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
  71     {
  72     default:
  73       {
  74         const ElfW(Half) *vernum =
  75           (const void *) l->l_info[VERSYMIDX (DT_VERSYM)]->d_un.d_ptr;
  76         ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)];
  77         const struct r_found_version *version = &l->l_versions[ndx];
  78
  79         if (version->hash != 0)
  80           {
  81             value = _dl_lookup_versioned_symbol(strtab + sym->st_name,
  82                                                 &sym, l->l_scope, l->l_name,
  83                                                 version, ELF_MACHINE_JMP_SLOT);
  84             break;
  85           }
  86       }
  87     case 0:
  88       value = _dl_lookup_symbol (strtab + sym->st_name, &sym, l->l_scope,
  89                                  l->l_name, ELF_MACHINE_JMP_SLOT);
  90     }
  91
  92   /* Currently value contains the base load address of the object
  93      that defines sym.  Now add in the symbol offset.  */
  94   value = (sym ? value + sym->st_value : 0);
  95
  96   /* And now perhaps the relocation addend.  */
  97   value = elf_machine_plt_value (l, reloc, value);
  98
  99   /* Finally, fix up the plt itself.  */
 100   elf_machine_fixup_plt (l, reloc, rel_addr, value);
 101
 102   return value;
 103 }
 104 #endif
 105
 106 #if !defined PROF && !defined ELF_MACHINE_NO_PLT
 107
 108 static ElfW(Addr) __attribute__ ((unused))
 109 profile_fixup (
 110 #ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
 111        ELF_MACHINE_RUNTIME_FIXUP_ARGS,
 112 #endif
 113        struct link_map *l, ElfW(Word) reloc_offset, ElfW(Addr) retaddr)
 114 {
 115   void (*mcount_fct) (ElfW(Addr), ElfW(Addr)) = _dl_mcount;
 116   ElfW(Addr) *resultp;
 117   ElfW(Addr) value;
 118
 119   /* The use of `alloca' here looks ridiculous but it helps.  The goal is
 120      to prevent the function from being inlined, and thus optimized out.
 121      There is no official way to do this so we use this trick.  gcc never
 122      inlines functions which use `alloca'.  */
 123   alloca (sizeof (int));
 124
 125   /* This is the address in the array where we store the result of previous
 126      relocations.  */
 127   resultp = &l->l_reloc_result[reloc_offset / sizeof (PLTREL)];
 128
 129   value = *resultp;
 130   if (value == 0)
 131     {
 132       /* This is the first time we have to relocate this object.  */
 133       const ElfW(Sym) *const symtab
 134         = (const void *) l->l_info[DT_SYMTAB]->d_un.d_ptr;
 135       const char *strtab = (const void *) l->l_info[DT_STRTAB]->d_un.d_ptr;
 136
 137       const PLTREL *const reloc
 138         = (const void *) (l->l_info[DT_JMPREL]->d_un.d_ptr + reloc_offset);
 139       const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
 140
 141       /* Sanity check that we're really looking at a PLT relocation.  */
 142       assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
 143
 144       /* Look up the target symbol.  */
 145       switch (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
 146         {
 147         default:
 148           {
 149             const ElfW(Half) *vernum =
 150               (const void *) l->l_info[VERSYMIDX (DT_VERSYM)]->d_un.d_ptr;
 151             ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)];
 152             const struct r_found_version *version = &l->l_versions[ndx];
 153
 154             if (version->hash != 0)
 155               {
 156                 value = _dl_lookup_versioned_symbol(strtab + sym->st_name,
 157                                                     &sym, l->l_scope,
 158                                                     l->l_name, version,
 159                                                     ELF_MACHINE_JMP_SLOT);
 160                 break;
 161               }
 162           }
 163         case 0:
 164           value = _dl_lookup_symbol (strtab + sym->st_name, &sym, l->l_scope,
 165                                      l->l_name, ELF_MACHINE_JMP_SLOT);
 166         }
 167
 168       /* Currently value contains the base load address of the object
 169          that defines sym.  Now add in the symbol offset.  */
 170       value = (sym ? value + sym->st_value : 0);
 171
 172       /* And now perhaps the relocation addend.  */
 173       value = elf_machine_plt_value (l, reloc, value);
 174
 175       /* Store the result for later runs.  */
 176       *resultp = value;
 177     }
 178
 179   (*mcount_fct) (retaddr, value);
 180
 181   return value;
 182 }
 183
 184 #endif /* PROF && ELF_MACHINE_NO_PLT */
 185
 186
 187 /* This macro is defined in dl-machine.h to define the entry point called
 188    by the PLT.  The `fixup' function above does the real work, but a little
 189    more twiddling is needed to get the stack right and jump to the address
 190    finally resolved.  */
 191
 192 ELF_MACHINE_RUNTIME_TRAMPOLINE