libgcc/config/pa/fptr.c

   1 /* Subroutine for function pointer canonicalization on PA-RISC with ELF32.
   2    Copyright (C) 2002-2019 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] = { -4, 32 };
  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) ? ((unsigned)(-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 static inline int
  56 _dl_read_access_allowed (unsigned int *addr)
  57 {
  58   int result;
  59
  60   asm ("proberi (%1),3,%0" : "=r" (result) : "r" (addr) : );
  61
  62   return result;
  63 }
  64
  65 /* __canonicalize_funcptr_for_compare must be hidden so that it is not
  66    placed in the dynamic symbol table.  Like millicode functions, it
  67    must be linked into all binaries in order access the got table of
  68    that binary.  However, we don't use the millicode calling convention
  69    and the routine must be a normal function so that it can be compiled
  70    as pic code.  */
  71 unsigned int __canonicalize_funcptr_for_compare (fptr_t)
  72       __attribute__ ((visibility ("hidden")));
  73
  74 unsigned int
  75 __canonicalize_funcptr_for_compare (fptr_t fptr)
  76 {
  77   static unsigned int fixup_plabel[2] __attribute__((used));
  78   fixup_t fixup;
  79   unsigned int *got, *iptr, *plabel;
  80   int i;
  81
  82   /* -1 and page 0 are special.  -1 is used in crtend to mark the end of
  83      a list of function pointers.  Also return immediately if the plabel
  84      bit is not set in the function pointer.  In this case, the function
  85      pointer points directly to the function.  */
  86   if ((int) fptr == -1 || (unsigned int) fptr < 4096 || !((int) fptr & 2))
  87     return (unsigned int) fptr;
  88
  89   /* The function pointer points to a function descriptor (plabel).  If
  90      the plabel hasn't been resolved, the first word of the plabel points
  91      to the entry of the PLT stub just before the global offset table.
  92      The second word in the plabel contains the relocation offset for the
  93      function.  */
  94   plabel = (unsigned int *) ((unsigned int) fptr & ~3);
  95   if (!_dl_read_access_allowed (plabel))
  96     return (unsigned int) fptr;
  97
  98   /* Load first word of candidate descriptor.  It should be a pointer
  99      with word alignment and point to memory that can be read.  */
 100   got = (unsigned int *) plabel[0];
 101   if (((unsigned int) got & 3) != 0
 102       || !_dl_read_access_allowed (got))
 103     return (unsigned int) fptr;
 104
 105   got = (unsigned int *) (plabel[0] + GOT_FROM_PLT_STUB);
 106
 107   /* Return the address of the function if the plabel has been resolved.  */
 108   if (got !=  &_GLOBAL_OFFSET_TABLE_)
 109     return plabel[0];
 110
 111   /* Find the first "bl" branch in the offset search list.  This is a
 112      call to _dl_fixup or a magic branch to fixup at the beginning of the
 113      trampoline template.  The fixup function does the actual runtime
 114      resolution of function descriptors.  We only look for "bl" branches
 115      with a 17-bit pc-relative displacement.  */
 116   for (i = 0; i < NOFFSETS; i++)
 117     {
 118       iptr = (unsigned int *) (got[-2] + fixup_branch_offset[i]);
 119       if ((*iptr & 0xfc00e000) == 0xe8000000)
 120         break;
 121     }
 122
 123   /* This should not happen... */
 124   if (i == NOFFSETS)
 125     return ~0;
 126
 127   /* Extract the 17-bit displacement from the instruction.  */
 128   iptr += SIGN_EXTEND (GET_FIELD (*iptr, 19, 28) |
 129                        GET_FIELD (*iptr, 29, 29) << 10 |
 130                        GET_FIELD (*iptr, 11, 15) << 11 |
 131                        GET_FIELD (*iptr, 31, 31) << 16, 17);
 132
 133   /* Build a plabel for an indirect call to _dl_fixup.  */
 134   fixup_plabel[0] = (unsigned int) iptr + 8;    /* address of fixup */
 135   fixup_plabel[1] = got[-1];                    /* ltp for fixup */
 136   fixup = (fixup_t) ((int) fixup_plabel | 2);
 137
 138   /* Call fixup to resolve the function address.  got[1] contains the
 139      link_map pointer and plabel[1] the relocation offset.  */
 140   fixup ((struct link_map *) got[1], plabel[1]);
 141
 142   return plabel[0];
 143 }