gold/ehframe.cc

   1 // ehframe.cc -- handle exception frame sections for gold
   2
   3 // Copyright 2006, 2007 Free Software Foundation, Inc.
   4 // Written by Ian Lance Taylor <iant@google.com>.
   5
   6 // This file is part of gold.
   7
   8 // This program is free software; you can redistribute it and/or modify
   9 // it under the terms of the GNU General Public License as published by
  10 // the Free Software Foundation; either version 3 of the License, or
  11 // (at your option) any later version.
  12
  13 // This program is distributed in the hope that it will be useful,
  14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16 // GNU General Public License for more details.
  17
  18 // You should have received a copy of the GNU General Public License
  19 // along with this program; if not, write to the Free Software
  20 // Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
  21 // MA 02110-1301, USA.
  22
  23 #include "gold.h"
  24
  25 #include "elfcpp.h"
  26 #include "dwarf.h"
  27 #include "ehframe.h"
  28
  29 namespace gold
  30 {
  31
  32 // This file handles generation of the exception frame header that
  33 // gcc's runtime support libraries use to find unwind information at
  34 // runtime.
  35
  36 // The exception frame header starts with four bytes:
  37
  38 // 0: The version number, currently 1.
  39
  40 // 1: The encoding of the pointer to the exception frames.  This can
  41 //    be any DWARF unwind encoding (DW_EH_PE_*).  It is normally a 4
  42 //    byte PC relative offset (DW_EH_PE_pcrel | DW_EH_PE_sdata4).
  43
  44 // 2: The encoding of the count of the number of FDE pointers in the
  45 //    lookup table.  This can be any DWARF unwind encoding, and in
  46 //    particular can be DW_EH_PE_omit if the count is omitted.  It is
  47 //    normally a 4 byte unsigned count (DW_EH_PE_udata4).
  48
  49 // 3: The encoding of the lookup table entries.  Currently gcc's
  50 //    libraries will only support DW_EH_PE_datarel | DW_EH_PE_sdata4,
  51 //    which means that the values are 4 byte offsets from the start of
  52 //    the table.
  53
  54 // The exception frame header is followed by a pointer to the contents
  55 // of the exception frame section (.eh_frame).  This pointer is
  56 // encoded as specified in the byte at offset 1 of the header (i.e.,
  57 // it is normally a 4 byte PC relative offset).
  58
  59 // If there is a lookup table, this is followed by the count of the
  60 // number of FDE pointers, encoded as specified in the byte at offset
  61 // 2 of the header (i.e., normally a 4 byte unsigned integer).
  62
  63 // This is followed by the table, which should start at an 4-byte
  64 // aligned address in memory.  Each entry in the table is 8 bytes.
  65 // Each entry represents an FDE.  The first four bytes of each entry
  66 // are an offset to the starting PC for the FDE.  The last four bytes
  67 // of each entry are an offset to the FDE data.  The offsets are from
  68 // the start of the exception frame header information.  The entries
  69 // are in sorted order by starting PC.
  70
  71 // FIXME: We currently always generate an empty exception frame
  72 // header.
  73
  74 const int eh_frame_hdr_size = 4;
  75
  76 // Construct the exception frame header.
  77
  78 Eh_frame_hdr::Eh_frame_hdr(Output_section* eh_frame_section)
  79   : Output_section_data(4),
  80     eh_frame_section_(eh_frame_section)
  81 {
  82 }
  83
  84 // Set the final address and size of the exception frame header.
  85
  86 void
  87 Eh_frame_hdr::do_set_address(uint64_t, off_t)
  88 {
  89   this->set_data_size(eh_frame_hdr_size + 4);
  90 }
  91
  92 // Write the data to the flie.
  93
  94 void
  95 Eh_frame_hdr::do_write(Output_file* of)
  96 {
  97   const off_t off = this->offset();
  98   const off_t oview_size = this->data_size();
  99   unsigned char* const oview = of->get_output_view(off, oview_size);
 100
 101   // Version number.
 102   oview[0] = 1;
 103
 104   // Write out a 4 byte PC relative offset to the address of the
 105   // .eh_frame section.
 106   oview[1] = elfcpp::DW_EH_PE_pcrel | elfcpp::DW_EH_PE_sdata4;
 107   uint64_t eh_frame_address = this->eh_frame_section_->address();
 108   uint64_t eh_frame_hdr_address = this->address();
 109   uint64_t eh_frame_offset = (eh_frame_address -
 110                               (eh_frame_hdr_address + 4));
 111   if (parameters->is_big_endian())
 112     elfcpp::Swap<32, true>::writeval(oview + 4, eh_frame_offset);
 113   else
 114     elfcpp::Swap<32, false>::writeval(oview + 4, eh_frame_offset);
 115
 116   // We don't currently write out the sorted table.
 117   oview[2] = elfcpp::DW_EH_PE_omit;
 118   oview[3] = elfcpp::DW_EH_PE_omit;
 119
 120   gold_assert(oview_size == 8);
 121
 122   of->write_output_view(off, oview_size, oview);
 123 }
 124
 125 } // End namespace gold.