gcc/config/pa/fptr.c

   1 /* Subroutine for function pointer canonicalization on PA-RISC with ELF32.
   2    Copyright 2002, 2003, 2004, 2007, 2009 Free Software Foundation, Inc.
   3    Contributed by John David Anglin (dave.anglin@nrc.ca).
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it under
   8 the terms of the GNU General Public License as published by the Free
   9 Software Foundation; either version 2, or (at your option) any later
  10 version.
  11
  12 In addition to the permissions in the GNU General Public License, the
  13 Free Software Foundation gives you unlimited permission to link the
  14 compiled version of this file into combinations with other programs,
  15 and to distribute those combinations without any restriction coming
  16 from the use of this file.  (The General Public License restrictions
  17 do apply in other respects; for example, they cover modification of
  18 the file, and distribution when not linked into a combine
  19 executable.)
  20
  21 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  22 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  23 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  24 for more details.
  25
  26 You should have received a copy of the GNU General Public License
  27 along with GCC; see the file COPYING.  If not, write to the Free
  28 Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
  29 02110-1301, USA.  */
  30
  31 /* WARNING: The code is this function depends on internal and undocumented
  32    details of the GNU linker and dynamic loader as implemented for parisc
  33    linux.  */
  34
  35 /* This MUST match the defines sysdeps/hppa/dl-machine.h and
  36    bfd/elf32-hppa.c.  */
  37 #define GOT_FROM_PLT_STUB (4*4)
  38
  39 /* List of byte offsets in _dl_runtime_resolve to search for "bl" branches.
  40    The first "bl" branch instruction found MUST be a call to fixup.  See
  41    the define for TRAMPOLINE_TEMPLATE in sysdeps/hppa/dl-machine.h.  If
  42    the trampoline template is changed, the list must be appropriately
  43    updated.  The offset of -4 allows for a magic branch at the start of
  44    the template should it be necessary to change the current branch
  45    position.  */
  46 #define NOFFSETS 2
  47 static int fixup_branch_offset[NOFFSETS] = { 32, -4 };
  48
  49 #define GET_FIELD(X, FROM, TO) \
  50   ((X) >> (31 - (TO)) & ((1 << ((TO) - (FROM) + 1)) - 1))
  51 #define SIGN_EXTEND(VAL,BITS) \
  52   ((int) ((VAL) >> ((BITS) - 1) ? (-1 << (BITS)) | (VAL) : (VAL)))
  53
  54 struct link_map;
  55 typedef int (*fptr_t) (void);
  56 typedef int (*fixup_t) (struct link_map *, unsigned int);
  57 extern unsigned int _GLOBAL_OFFSET_TABLE_;
  58
  59 /* __canonicalize_funcptr_for_compare must be hidden so that it is not
  60    placed in the dynamic symbol table.  Like millicode functions, it
  61    must be linked into all binaries in order access the got table of
  62    that binary.  However, we don't use the millicode calling convention
  63    and the routine must be a normal function so that it can be compiled
  64    as pic code.  */
  65 unsigned int __canonicalize_funcptr_for_compare (fptr_t)
  66       __attribute__ ((visibility ("hidden")));
  67
  68 unsigned int
  69 __canonicalize_funcptr_for_compare (fptr_t fptr)
  70 {
  71   static unsigned int fixup_plabel[2];
  72   static fixup_t fixup;
  73   unsigned int *plabel, *got;
  74
  75   /* -1 and page 0 are special.  -1 is used in crtend to mark the end of
  76      a list of function pointers.  Also return immediately if the plabel
  77      bit is not set in the function pointer.  In this case, the function
  78      pointer points directly to the function.  */
  79   if ((int) fptr == -1 || (unsigned int) fptr < 4096 || !((int) fptr & 2))
  80     return (unsigned int) fptr;
  81
  82   /* The function pointer points to a function descriptor (plabel).  If
  83      the plabel hasn't been resolved, the first word of the plabel points
  84      to the entry of the PLT stub just before the global offset table.
  85      The second word in the plabel contains the relocation offset for the
  86      function.  */
  87   plabel = (unsigned int *) ((unsigned int) fptr & ~3);
  88   got = (unsigned int *) (plabel[0] + GOT_FROM_PLT_STUB);
  89
  90   /* Return the address of the function if the plabel has been resolved.  */
  91   if (got !=  &_GLOBAL_OFFSET_TABLE_)
  92     return plabel[0];
  93
  94   /* Initialize our plabel for calling fixup if we haven't done so already.
  95      This code needs to be thread safe but we don't have to be too careful
  96      as the result is invariant.  */
  97   if (!fixup)
  98     {
  99       int i;
 100       unsigned int *iptr;
 101
 102       /* Find the first "bl" branch in the offset search list.  This is a
 103          call to fixup or a magic branch to fixup at the beginning of the
 104          trampoline template.  The fixup function does the actual runtime
 105          resolution of function descriptors.  We only look for "bl" branches
 106          with a 17-bit pc-relative displacement.  */
 107       for (i = 0; i < NOFFSETS; i++)
 108         {
 109           iptr = (unsigned int *) (got[-2] + fixup_branch_offset[i]);
 110           if ((*iptr & 0xfc00e000) == 0xe8000000)
 111             break;
 112         }
 113
 114       /* This should not happen... */
 115       if (i == NOFFSETS)
 116         return ~0;
 117
 118       /* Extract the 17-bit displacement from the instruction.  */
 119       iptr += SIGN_EXTEND (GET_FIELD (*iptr, 19, 28) |
 120                            GET_FIELD (*iptr, 29, 29) << 10 |
 121                            GET_FIELD (*iptr, 11, 15) << 11 |
 122                            GET_FIELD (*iptr, 31, 31) << 16, 17);
 123
 124       /* Build a plabel for an indirect call to fixup.  */
 125       fixup_plabel[0] = (unsigned int) iptr + 8;  /* address of fixup */
 126       fixup_plabel[1] = got[-1];                  /* ltp for fixup */
 127       fixup = (fixup_t) ((int) fixup_plabel | 3);
 128     }
 129
 130   /* Call fixup to resolve the function address.  got[1] contains the
 131      link_map pointer and plabel[1] the relocation offset.  */
 132   fixup ((struct link_map *) got[1], plabel[1]);
 133
 134   return plabel[0];
 135 }