gcc/config/pa/fptr.c

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