libgcc/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 3, or (at your option) any later
  10 version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 Under Section 7 of GPL version 3, you are granted additional
  18 permissions described in the GCC Runtime Library Exception, version
  19 3.1, as published by the Free Software Foundation.
  20
  21 You should have received a copy of the GNU General Public License and
  22 a copy of the GCC Runtime Library Exception along with this program;
  23 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  24 <http://www.gnu.org/licenses/>.  */
  25
  26
  27 /* WARNING: The code is this function depends on internal and undocumented
  28    details of the GNU linker and dynamic loader as implemented for parisc
  29    linux.  */
  30
  31 /* This MUST match the defines sysdeps/hppa/dl-machine.h and
  32    bfd/elf32-hppa.c.  */
  33 #define GOT_FROM_PLT_STUB (4*4)
  34
  35 /* List of byte offsets in _dl_runtime_resolve to search for "bl" branches.
  36    The first "bl" branch instruction found MUST be a call to fixup.  See
  37    the define for TRAMPOLINE_TEMPLATE in sysdeps/hppa/dl-machine.h.  If
  38    the trampoline template is changed, the list must be appropriately
  39    updated.  The offset of -4 allows for a magic branch at the start of
  40    the template should it be necessary to change the current branch
  41    position.  */
  42 #define NOFFSETS 2
  43 static int fixup_branch_offset[NOFFSETS] = { 32, -4 };
  44
  45 #define GET_FIELD(X, FROM, TO) \
  46   ((X) >> (31 - (TO)) & ((1 << ((TO) - (FROM) + 1)) - 1))
  47 #define SIGN_EXTEND(VAL,BITS) \
  48   ((int) ((VAL) >> ((BITS) - 1) ? (-1 << (BITS)) | (VAL) : (VAL)))
  49
  50 struct link_map;
  51 typedef int (*fptr_t) (void);
  52 typedef int (*fixup_t) (struct link_map *, unsigned int);
  53 extern unsigned int _GLOBAL_OFFSET_TABLE_;
  54
  55 /* __canonicalize_funcptr_for_compare must be hidden so that it is not
  56    placed in the dynamic symbol table.  Like millicode functions, it
  57    must be linked into all binaries in order access the got table of
  58    that binary.  However, we don't use the millicode calling convention
  59    and the routine must be a normal function so that it can be compiled
  60    as pic code.  */
  61 unsigned int __canonicalize_funcptr_for_compare (fptr_t)
  62       __attribute__ ((visibility ("hidden")));
  63
  64 unsigned int
  65 __canonicalize_funcptr_for_compare (fptr_t fptr)
  66 {
  67   static unsigned int fixup_plabel[2];
  68   static fixup_t fixup;
  69   unsigned int *plabel, *got;
  70
  71   /* -1 and page 0 are special.  -1 is used in crtend to mark the end of
  72      a list of function pointers.  Also return immediately if the plabel
  73      bit is not set in the function pointer.  In this case, the function
  74      pointer points directly to the function.  */
  75   if ((int) fptr == -1 || (unsigned int) fptr < 4096 || !((int) fptr & 2))
  76     return (unsigned int) fptr;
  77
  78   /* The function pointer points to a function descriptor (plabel).  If
  79      the plabel hasn't been resolved, the first word of the plabel points
  80      to the entry of the PLT stub just before the global offset table.
  81      The second word in the plabel contains the relocation offset for the
  82      function.  */
  83   plabel = (unsigned int *) ((unsigned int) fptr & ~3);
  84   got = (unsigned int *) (plabel[0] + GOT_FROM_PLT_STUB);
  85
  86   /* Return the address of the function if the plabel has been resolved.  */
  87   if (got !=  &_GLOBAL_OFFSET_TABLE_)
  88     return plabel[0];
  89
  90   /* Initialize our plabel for calling fixup if we haven't done so already.
  91      This code needs to be thread safe but we don't have to be too careful
  92      as the result is invariant.  */
  93   if (!fixup)
  94     {
  95       int i;
  96       unsigned int *iptr;
  97
  98       /* Find the first "bl" branch in the offset search list.  This is a
  99          call to fixup or a magic branch to fixup at the beginning of the
 100          trampoline template.  The fixup function does the actual runtime
 101          resolution of function descriptors.  We only look for "bl" branches
 102          with a 17-bit pc-relative displacement.  */
 103       for (i = 0; i < NOFFSETS; i++)
 104         {
 105           iptr = (unsigned int *) (got[-2] + fixup_branch_offset[i]);
 106           if ((*iptr & 0xfc00e000) == 0xe8000000)
 107             break;
 108         }
 109
 110       /* This should not happen... */
 111       if (i == NOFFSETS)
 112         return ~0;
 113
 114       /* Extract the 17-bit displacement from the instruction.  */
 115       iptr += SIGN_EXTEND (GET_FIELD (*iptr, 19, 28) |
 116                            GET_FIELD (*iptr, 29, 29) << 10 |
 117                            GET_FIELD (*iptr, 11, 15) << 11 |
 118                            GET_FIELD (*iptr, 31, 31) << 16, 17);
 119
 120       /* Build a plabel for an indirect call to fixup.  */
 121       fixup_plabel[0] = (unsigned int) iptr + 8;  /* address of fixup */
 122       fixup_plabel[1] = got[-1];                  /* ltp for fixup */
 123       fixup = (fixup_t) ((int) fixup_plabel | 3);
 124     }
 125
 126   /* Call fixup to resolve the function address.  got[1] contains the
 127      link_map pointer and plabel[1] the relocation offset.  */
 128   fixup ((struct link_map *) got[1], plabel[1]);
 129
 130   return plabel[0];
 131 }