dwarf-20161124/dwarfgen/dwarfgen.cc

/*
  Copyright (C) 2010-2016 David Anderson.  All rights reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:
  * Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.
  * Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the following disclaimer in the
    documentation and/or other materials provided with the distribution.
  * Neither the name of the example nor the
    names of its contributors may be used to endorse or promote products
    derived from this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY David Anderson ''AS IS'' AND ANY
  EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  DISCLAIMED. IN NO EVENT SHALL David Anderson BE LIABLE FOR ANY
  DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

*/
// dwarfgen.cc
//
// Using some information source, create a tree of dwarf
// information (speaking of a DIE tree).
// Turn the die tree into dwarfdata using libdwarf producer
// and write the resulting data in an object file.
// It is a bit inconsistent in error handling just to
// demonstrate the various possibilities using the producer
// library.
//
//  dwarfgen [-t def|obj|txt] [-o outpath] [-c cunum]  path

//  where -t means what sort of input to read
//         def means predefined (no input is read, the output
//         is based on some canned setups built into dwarfgen).
//         'path' is ignored in this case. This is the default.
//
//         obj means 'path' is required, it is the object file to
//             read (the dwarf sections are duplicated in the output file)
//
//         txt means 'path' is required, path must contain plain text
//             (in a form rather like output by dwarfdump)
//             that defines the dwarf that is to be output.
//
//  where  -o means specify the pathname of the output object. If not
//         supplied testout.o is used as the default output path.
//  where -c supplies a CU number of the obj input to output
//         because the dwarf producer wants just one CU.
//         Default is -1 which won't match anything.

#include "config.h"

/* Windows specific header files */
#ifdef HAVE_STDAFX_H
#include "stdafx.h"
#endif /* HAVE_STDAFX_H */

#include <unistd.h>
#include <stdlib.h> // for exit
#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>
#include <list>
#include <map>
#include <vector>
#include <string.h> // For memset etc
#include <sys/stat.h> //open
#include <fcntl.h> //open
#include "general.h"
#include "dwgetopt.h"
#include "gelf.h"
#include "strtabdata.h"
#include "dwarf.h"
#include "libdwarf.h"
#include "irepresentation.h"
#include "ireptodbg.h"
#include "createirepfrombinary.h"

using std::string;
using std::cout;
using std::cerr;
using std::endl;
using std::vector;

static void write_object_file(Dwarf_P_Debug dbg, IRepresentation &irep);
static void write_text_section(Elf * elf);
static void write_generated_dbg(Dwarf_P_Debug dbg,Elf * elf,
    IRepresentation &irep);

static string outfile("testout.o");
static string infile;
static enum  WhichInputSource { OptNone, OptReadText,OptReadBin,OptPredefined}
    whichinput(OptPredefined);

/*  Use a generic call to open the file, due to issues with Windows */
int open_a_file(const char * name);
int create_a_file(const char * name);
void close_a_file(int f);

// This is a global so thet CallbackFunc can get to it
// If we used the dwarf_producer_init_c() user_data pointer
// creatively we would not need a global.
static IRepresentation Irep;

static Elf * elf = 0;
static Elf32_Ehdr * ehp = 0;
static strtabdata secstrtab;

bool transformHighpcToConst = false;
int  defaultInfoStringForm = DW_FORM_string;
bool showrelocdetails = false;

// loff_t is signed for some reason (strange) but we make offsets unsigned.
#define LOFFTODWUNS(x)  ( (Dwarf_Unsigned)(x))

/*  See the Elf ABI for further definitions of these fields. */
class SectionFromDwarf {
public:
    std::string name_;
    Dwarf_Unsigned section_name_itself_;
    ElfSymIndex section_name_symidx_;
    int size_;
    Dwarf_Unsigned type_;
    Dwarf_Unsigned flags_;

    /*  type: SHT_REL, RELA: Section header index of the section
        relocation applies to.
        SHT_SYMTAB: Section header index of the associated string table. */
    Dwarf_Unsigned link_;

    /*  type: SHT_REL, RELA: Section header index of the section
        relocation applies to.
        SHT_SYMTAB: One greater than index of the last local symbol.. */
    Dwarf_Unsigned info_;
private:
    ElfSectIndex elf_sect_index_;
    Dwarf_Unsigned lengthWrittenToElf_;
public:
    Dwarf_Unsigned getNextOffset() { return lengthWrittenToElf_; }
    void setNextOffset(Dwarf_Unsigned v) { lengthWrittenToElf_ = v; }

    unsigned getSectionNameSymidx() {
        return section_name_symidx_.getSymIndex(); };
    SectionFromDwarf():section_name_itself_(0),
        section_name_symidx_(0),
        size_(0),type_(0),flags_(0),
        link_(0), info_(0), elf_sect_index_(0),
        lengthWrittenToElf_(0) {} ;
    ~SectionFromDwarf() {};
    void setSectIndex(ElfSectIndex v) { elf_sect_index_ = v;}
    ElfSectIndex getSectIndex() const { return elf_sect_index_;}
    SectionFromDwarf(const std::string&name,
        int size,Dwarf_Unsigned type,Dwarf_Unsigned flags,
        Dwarf_Unsigned link, Dwarf_Unsigned info):
        name_(name),
        size_(size),type_(type),flags_(flags),
        link_(link), info_(info), elf_sect_index_(0),
        lengthWrittenToElf_(0) {
            // Now create section name string section.
            section_name_itself_ = secstrtab.addString(name.c_str());
            ElfSymbols& es = Irep.getElfSymbols();
            // Now creat a symbol for the section name.
            // (which has its own string table)
            section_name_symidx_  = es.addElfSymbol(0,name);
    } ;
};

vector<SectionFromDwarf> dwsectab;

static ElfSectIndex create_dw_elf(SectionFromDwarf  &ds);

static SectionFromDwarf & FindMySection(const ElfSectIndex & elf_section_index)
{
    for(unsigned i =0; i < dwsectab.size(); ++i) {
        if(elf_section_index.getSectIndex() !=
            dwsectab[i].getSectIndex().getSectIndex()) {
            continue;
        }
        return dwsectab[i];
    }
    cerr << "dwarfgen: Unable to find my dw sec data for elf section " <<
        elf_section_index.getSectIndex() << endl;
    exit(1);
}

static int FindMySectionNum(const ElfSectIndex & elf_section_index)
{
    for(unsigned i =0; i < dwsectab.size(); ++i) {
        if(elf_section_index.getSectIndex() !=
            dwsectab[i].getSectIndex().getSectIndex()) {
            continue;
        }
        return i;
    }
    cerr << "dwarfgen: Unable to find my dw sec index for elf section " <<
        elf_section_index.getSectIndex() << endl;
    exit(1);
}

static unsigned
createnamestr(unsigned strtabstroff)
{
    Elf_Scn * strscn =elf_newscn(elf);
    if(!strscn) {
        cerr << "dwarfgen: Unable to elf_newscn() on " << outfile << endl;
        exit(1);
    }
    Elf_Data* shstr =elf_newdata(strscn);
    if(!shstr) {
        cerr << "dwarfgen: Unable to elf_newdata() on " << outfile << endl;
        exit(1);
    }
    shstr->d_buf = secstrtab.exposedata();
    shstr->d_type =  ELF_T_BYTE;
    shstr->d_size = secstrtab.exposelen();
    shstr->d_off = 0;
    shstr->d_align = 1;
    shstr->d_version = EV_CURRENT;

    Elf32_Shdr * strshdr = elf32_getshdr(strscn);
    if(!strshdr) {
        cerr << "dwarfgen: Unable to elf_getshdr() on " << outfile << endl;
        exit(1);
    }
    strshdr->sh_name =  strtabstroff;
    strshdr->sh_type= SHT_STRTAB;
    strshdr->sh_flags = SHF_STRINGS;
    strshdr->sh_addr = 0;
    strshdr->sh_offset = 0;
    strshdr->sh_size = 0;
    strshdr->sh_link  = 0;
    strshdr->sh_info = 0;
    strshdr->sh_addralign = 1;
    strshdr->sh_entsize = 0;
    return  elf_ndxscn(strscn);
}

// This functional interface is defined by libdwarf.
// Please see the comments in libdwarf2p.1.pdf
// (libdwarf2p.1.mm)  on this callback interface.
// Returns (to libdwarf) an Elf section number, so
// since 0 is always empty and dwarfgen sets 1 to be a fake
// text section on the first call this returns 2, second 3, etc.
int CallbackFunc(
    const char* name,
    int                 size,
    Dwarf_Unsigned      type,
    Dwarf_Unsigned      flags,
    Dwarf_Unsigned      link,
    Dwarf_Unsigned      info,
    Dwarf_Unsigned*     sect_name_symbol_index,
    void *              user_data,
    int*                error)
{
    // Create an elf section.
    // If the data is relocations, we suppress the generation
    // of a section when we intend to do the relocations
    // ourself (fine for dwarfgen but would
    // be really surprising for a normal compiler
    // back end using the producer code).

    // The section name appears both in the section strings
    // .shstrtab and
    // in the elf symtab .symtab and its strings .strtab.

    if (0 == strncmp(name,".rel",4))  {
        // It is relocation, create no section!
        return 0;
    }
    SectionFromDwarf ds(name,size,type,flags,link,info) ;

    // It is up to you to provide (to libdwarf,
    // to generate relocation records)
    // a symbol index for the section.
    // In Elf, each section gets an elf symbol table entry.
    // So that relocations have an address to refer to.
    // You will create the Elf symbol table, so you have to tell
    // libdwarf the index to put into relocation records for the
    // section newly defined here.
    *sect_name_symbol_index = ds.getSectionNameSymidx();
    ElfSectIndex createdsec = create_dw_elf(ds);

    // Do all the data creation before pushing
    // (copying) ds onto dwsectab!
    dwsectab.push_back(ds);
    // The number returned is elf section, not dwsectab[] index
    return createdsec.getSectIndex();
}

// Here we create a new Elf section
// This never happens for relocations in dwarfgen,
// only a few sections are created by dwarfgen.
static ElfSectIndex
create_dw_elf(SectionFromDwarf  &ds)
{
    Elf_Scn * scn =elf_newscn(elf);
    if(!scn) {
        cerr << "dwarfgen: Unable to elf_newscn() on " << ds.name_  << endl;
        exit(1);
    }
    Elf32_Shdr * shdr = elf32_getshdr(scn);
    if(!shdr) {
        cerr << "dwarfgen: Unable to elf_getshdr() on " << ds.name_ << endl;
        exit(1);
    }
    shdr->sh_name   = ds.section_name_itself_;
    shdr->sh_type   = ds.type_;
    shdr->sh_flags  = ds.flags_;
    shdr->sh_addr   = 0;
    shdr->sh_offset = 0;
    shdr->sh_size   = ds.size_;
    shdr->sh_link   = ds.link_;
    shdr->sh_info   = ds.info_;
    shdr->sh_addralign = 1;
    shdr->sh_entsize = 0;
    ElfSectIndex si(elf_ndxscn(scn));

    ds.setSectIndex(si);
    cout << "New Elf section: "<< ds.name_ <<
        " Type="<< ds.type_ <<
        " Flags="<< ds.flags_ <<
        " Elf secnum="<< si.getSectIndex() <<
        " link section=" << ds.link_<<
        " info=" << ds.info_ << endl ;
    return  si;
}

// Default error handler of libdwarf producer code.
void ErrorHandler(Dwarf_Error err,Dwarf_Ptr errarg)
{
    // FIXME do better error handling
    cerr <<"dwarfgen: Giving up, encountered an error" << endl;
    exit(1);
}


static void
setinput(enum  WhichInputSource *src,
    const string &type,
    bool *pathreq)
{
    if(type == "txt") {
        *src = OptReadText;
        *pathreq = true;
        return;
    } else if (type == "obj") {
        *src = OptReadBin;
        *pathreq = true;
        return;
    } else if (type == "def") {
        *src = OptPredefined;
        *pathreq = false;
        return;
    }
    cout << "dwarfgen: Giving up, only txt obj or def accepted after -t" << endl;
    exit(1);
}

int
main(int argc, char **argv)
{
    try {
        int opt;
        bool pathrequired(false);
        long cu_of_input_we_output = -1;
        while((opt=dwgetopt(argc,argv,"o:t:c:hsr")) != -1) {
            switch(opt) {
            case 'c':
                // At present we can only create a single
                // cu in the output of the libdwarf producer.
                cu_of_input_we_output = atoi(dwoptarg);
                break;
            case 'r':
                showrelocdetails=true;
                break;
            case 's':
                defaultInfoStringForm = DW_FORM_strp;
                break;
            case 't':
                setinput(&whichinput,dwoptarg,&pathrequired);
                break;
            case 'h':
                transformHighpcToConst = true;
                break;
            case 'o':
                outfile = dwoptarg;
                break;
            case '?':
                cerr << "dwarfgen: Invalid quest? option input " << endl;
                exit(1);
            default:
                cerr << "dwarfgen: Invalid option input " << endl;
                exit(1);
            }
        }
        if ( (dwoptind >= argc) && pathrequired) {
            cerr << "dwarfgen: Expected argument after options! Giving up."
                << endl;
            exit(EXIT_FAILURE);
        }
        if(pathrequired) {
            infile = argv[dwoptind];
        }

        if(whichinput == OptReadBin) {
            createIrepFromBinary(infile,Irep);
        } else if (whichinput == OptReadText) {
            cerr << "dwarfgen: dwarfgen: text read not supported yet" << endl;
            exit(EXIT_FAILURE);
        } else if (whichinput == OptPredefined) {
            cerr << "dwarfgen: predefined not supported yet" << endl;
            exit(EXIT_FAILURE);
        } else {
            cerr << "dwarfgen: Impossible: unknown input style." << endl;
            exit(EXIT_FAILURE);
        }
        // Example will return error value thru 'err' pointer
        // and return DW_DLV_BADADDR if there is an error.
        int ptrsizeflagbit = DW_DLC_POINTER32;
        int offsetsizeflagbit = DW_DLC_OFFSET32;
        const char * isa_name = "x86";
        const char *dwarf_version = "V2";
        int endian =  DW_DLC_TARGET_LITTLEENDIAN;
        Dwarf_Ptr errarg = 0;
        Dwarf_Error err = 0;
        void *user_data = 0;
        Dwarf_P_Debug dbg = 0;
        // We use DW_DLC_SYMBOLIC_RELOCATIONS so we can
        // read the relocations and do our own relocating.
        // See calls of dwarf_get_relocation_info().
        int res = dwarf_producer_init(
            DW_DLC_WRITE|ptrsizeflagbit|
            offsetsizeflagbit|DW_DLC_SYMBOLIC_RELOCATIONS|
            endian,
            CallbackFunc,
            0, // errhand
            errarg,
            user_data,
            isa_name,
            dwarf_version,
            0, // No extra identifying strings.
            &dbg,
            &err);
        if(res != DW_DLV_OK) {
            cerr << "dwarfgen: Failed init_b" << endl;
            exit(EXIT_FAILURE);
        }
        res = dwarf_pro_set_default_string_form(dbg,
            defaultInfoStringForm,&err);
        if(res != DW_DLV_OK) {
            cerr << "dwarfgen: Failed dwarf_pro_set_default_string_form"
                << endl;
            exit(EXIT_FAILURE);
        }
        transform_irep_to_dbg(dbg,Irep,cu_of_input_we_output);
        write_object_file(dbg,Irep);
        // Example calls ErrorHandler if there is an error
        // (which does not return, see above)
        // so no need to test for error.
        Dwarf_Unsigned str_count = 0;
        Dwarf_Unsigned str_len = 0;
        Dwarf_Unsigned debug_str_count = 0;
        Dwarf_Unsigned debug_str_len = 0;
        Dwarf_Unsigned reused_count = 0;
        Dwarf_Unsigned reused_len = 0;
        res = dwarf_pro_get_string_stats(dbg,
            &str_count,&str_len,
            &debug_str_count,
            &debug_str_len,
            &reused_count,
            &reused_len,&err);
        cout << "Debug_Str: debug_info str count " <<str_count <<
            ", byte total len " <<str_len << endl;
        cout << "Debug_Str: count " <<debug_str_count <<
            ", byte total len " <<debug_str_len << endl;
        cout << "Debug_Str: Reused count " <<reused_count <<
            ", byte total len not emitted " <<reused_len << endl;
        dwarf_producer_finish( dbg, 0);
        return 0;
    } // End try
    catch (std::bad_alloc &ba) {
        cout << "dwarfgen FAIL:bad alloc caught " << ba.what() << endl;
        exit(EXIT_FAILURE);
    }
    catch (std::exception &e) {
        cout << "dwarfgen FAIL:std lib exception " << e.what() << endl;
        exit(EXIT_FAILURE);
    }
    catch (...) {
        cout << "dwarfgen FAIL:other failure " << endl;
        exit(EXIT_FAILURE);
    }
    exit(1);
}

static void
write_object_file(Dwarf_P_Debug dbg, IRepresentation &irep)
{
    int fd = create_a_file(outfile.c_str());
    if(fd < 0 ) {
        cerr << "dwarfgen: Unable to open " << outfile <<
            " for writing." << endl;
        exit(1);
    }

    if(elf_version(EV_CURRENT) == EV_NONE) {
        cerr << "dwarfgen: Bad elf_version" << endl;
        exit(1);
    }

    Elf_Cmd cmd = ELF_C_WRITE;
    elf = elf_begin(fd,cmd,0);
    if(!elf) {
        cerr << "dwarfgen: Unable to elf_begin() on " << outfile << endl;
        exit(1);
    }
    ehp = elf32_newehdr(elf);
    if(!ehp) {
        cerr << "dwarfgen: Unable to elf_newehdr() on " << outfile << endl;
        exit(1);
    }
    ehp->e_ident[EI_MAG0] = ELFMAG0;
    ehp->e_ident[EI_MAG1] = ELFMAG1;
    ehp->e_ident[EI_MAG2] = ELFMAG2;
    ehp->e_ident[EI_MAG3] = ELFMAG3;
    ehp->e_ident[EI_CLASS] = ELFCLASS32;
    ehp->e_ident[EI_DATA] = ELFDATA2LSB;
    ehp->e_ident[EI_VERSION] = EV_CURRENT;
    ehp->e_machine = EM_386;
    //  We do not bother to create program headers, so
    //  mark this as ET_REL.
    ehp->e_type = ET_REL;
    ehp->e_version = EV_CURRENT;

    unsigned  strtabstroff = secstrtab.addString(".shstrtab");

    // an object section with fake .text data (just as an example).
    write_text_section(elf);

    write_generated_dbg(dbg,elf,Irep);

    // Now create section name string section.
    unsigned shstrindex = createnamestr(strtabstroff);
    ehp->e_shstrndx = shstrindex;

    off_t ures = elf_update(elf,cmd);
    if(ures == (off_t)(-1LL)) {
        cerr << "dwarfgen: Unable to elf_update() on " << outfile << endl;
        int eer = elf_errno();
        cerr << "Error is " << eer << " " << elf_errmsg(eer) << endl;
        exit(1);
    }
    cout << " output image size in bytes " << ures << endl;

    elf_end(elf);
    close_a_file(fd);
}


// an object section with fake .text data (just as an example).
static void
write_text_section(Elf * elf)
{
    unsigned  osecnameoff = secstrtab.addString(".text");
    Elf_Scn * scn1 =elf_newscn(elf);
    if(!scn1) {
        cerr << "dwarfgen: Unable to elf_newscn() on " << outfile << endl;
        exit(1);
    }

    Elf_Data* ed1 =elf_newdata(scn1);
    if(!ed1) {
        cerr << "dwarfgen: Unable to elf_newdata() on " << outfile << endl;
        exit(1);
    }
    const char *d = "data in section";
    ed1->d_buf = (void *)d;
    ed1->d_type =  ELF_T_BYTE;
    ed1->d_size = strlen(d) +1;
    ed1->d_off = 0;
    ed1->d_align = 4;
    ed1->d_version = EV_CURRENT;
    Elf32_Shdr * shdr1 = elf32_getshdr(scn1);
    if(!shdr1) {
        cerr << "dwarfgen: Unable to elf_getshdr() on " << outfile << endl;
        exit(1);
    }
    shdr1->sh_name =  osecnameoff;
    shdr1->sh_type= SHT_PROGBITS;
    shdr1->sh_flags = 0;
    shdr1->sh_addr = 0;
    shdr1->sh_offset = 0;
    shdr1->sh_size = 0;
    shdr1->sh_link  = 0;
    shdr1->sh_info = 0;
    shdr1->sh_addralign = 1;
    shdr1->sh_entsize = 0;
}
static void
InsertDataIntoElf(Dwarf_Signed d,Dwarf_P_Debug dbg,Elf *elf)
{
    Dwarf_Signed elf_section_index = 0;
    Dwarf_Unsigned length = 0;
    Dwarf_Ptr bytes = dwarf_get_section_bytes(dbg,d,
        &elf_section_index,&length,0);

    Elf_Scn *scn =  elf_getscn(elf,elf_section_index);
    if(!scn) {
        cerr << "dwarfgen: Unable to elf_getscn on disk transform # " << d << endl;
        exit(1);
    }

    ElfSectIndex si(elf_section_index);
    SectionFromDwarf & sfd  = FindMySection(si);

    Elf_Data* ed =elf_newdata(scn);
    if(!ed) {
        cerr << "dwarfgen: elf_newdata died on transformed index " << d << endl;
        exit(1);
    }
    ed->d_buf = bytes;
    ed->d_type =  ELF_T_BYTE;
    ed->d_size = length;
    ed->d_off = sfd.getNextOffset();
    sfd.setNextOffset(ed->d_off + length);
    ed->d_align = 1;
    ed->d_version = EV_CURRENT;
    cout << "Inserted " << length << " bytes into elf section index " <<
        elf_section_index << endl;
}

#if 0
static string
printable_rel_type(unsigned char reltype)
{
    enum Dwarf_Rel_Type t = (enum Dwarf_Rel_Type)reltype;
    switch(t) {
    case   dwarf_drt_none:
        return "dwarf_drt_none";
    case   dwarf_drt_data_reloc:
        return "dwarf_drt_data_reloc";
    case   dwarf_drt_segment_rel:
        return "dwarf_drt_segment_rel";
    case   dwarf_drt_first_of_length_pair:
        return "dwarf_drt_first_of_length_pair";
    case   dwarf_drt_second_of_length_pair:
        return "dwarf_drt_second_of_length_pair";
    default:
        break;
    }
    return "drt-unknown (impossible case)";
}
#endif

static Dwarf_Unsigned
FindSymbolValue(ElfSymIndex symi,IRepresentation &irep)
{
    ElfSymbols & syms = irep.getElfSymbols();
    ElfSymbol & es =  syms.getElfSymbol(symi);
    Dwarf_Unsigned symv = es.getSymbolValue();
    return symv;
}

static void
bitreplace(char *buf, Dwarf_Unsigned newval,
    size_t newvalsize,int length)
{
    if(length == 4) {
        uint32_t my4 = newval;
        uint32_t * p = reinterpret_cast<uint32_t *>(buf );
        uint32_t oldval = *p;
        *p = oldval + my4;
    } else if (length == 8) {
        uint64_t my8 = newval;
        uint64_t * p = reinterpret_cast<uint64_t *>(buf );
        uint64_t oldval = *p;
        *p = oldval + my8;
    } else {
        cerr << "dwarfgen:  Relocation is length " << length <<
            " which we do not yet handle." << endl;
        exit(1);
    }
}

// This remembers nothing, so is dreadfully slow.
static char *
findelfbuf(Elf *elf,Elf_Scn *scn,Dwarf_Unsigned offset, unsigned length)
{
    Elf_Data * edbase = 0;
    Elf_Data * ed = elf_getdata(scn,edbase);
    unsigned bct = 0;
    for (;ed; ed = elf_getdata(scn,ed)) {
        bct++;
        if(offset >= LOFFTODWUNS(ed->d_off + ed->d_size) ) {
            continue;
        }
        if(offset < LOFFTODWUNS(ed->d_off)) {
            cerr << "dwarfgen:  Relocation at offset  " <<
                offset << " cannot be accomplished, no buffer. "
                << endl;
            exit(1);
        }
        Dwarf_Unsigned localoff = offset - ed->d_off;
        if((localoff + length) > ed->d_size) {
            cerr << "dwarfgen:  Relocation at offset  " <<
                offset << " cannot be accomplished, size mismatch. "
                << endl;
            exit(1);
        }
        char *lclptr = reinterpret_cast<char *>(ed->d_buf) + localoff;
        return lclptr;
    }
    cerr << " Relocation at offset  " << offset  <<
        " cannot be accomplished,  past end of buffers" << endl;
    return 0;

}

static void
write_generated_dbg(Dwarf_P_Debug dbg,Elf * elf,IRepresentation &irep)
{
    Dwarf_Error err = 0;
    Dwarf_Signed sectioncount =
        dwarf_transform_to_disk_form(dbg,0);

    Dwarf_Signed d = 0;
    for(d = 0; d < sectioncount ; ++d) {
        InsertDataIntoElf(d,dbg,elf);
    }

    // Since we are emitting in final form sometimes, we may
    // do relocation processing here or we could (but do not)
    // emit relocation records into the object file.
    // The following is for DW_DLC_SYMBOLIC_RELOCATIONS.

    Dwarf_Unsigned reloc_sections_count = 0;
    int drd_version = 0;
    int res = dwarf_get_relocation_info_count(dbg,&reloc_sections_count,
        &drd_version,&err);
    if( res != DW_DLV_OK) {
        cerr << "dwarfgen: Error getting relocation info count." << endl;
        exit(1);

    }
    cout << "Relocations sections count= " << reloc_sections_count <<
        " relversion=" << drd_version << endl;
    for( Dwarf_Unsigned ct = 0; ct < reloc_sections_count ; ++ct) {
        // elf_section_index is the elf index of the
        // section to be relocated, and the section number
        // in the object file which we are creating.
        // In dwarfgen we do not use this as we do not create
        // relocation sections. Here it is always zero.
        Dwarf_Signed elf_section_index = 0;

        // elf_section_index_link is the elf index of the
        // section  the relocations apply to, such as .debug_info.
        // An elf index, not dwsectab[] index.
        Dwarf_Signed elf_section_index_link = 0;

        // relocation_buffer_count is the number of relocations
        // of this section.
        Dwarf_Unsigned relocation_buffer_count = 0;
        Dwarf_Relocation_Data reld;
        res = dwarf_get_relocation_info(dbg,&elf_section_index,
            &elf_section_index_link,
            &relocation_buffer_count,
            &reld,&err);
        // elf_section_index_link
        // refers to the output section numbers, not to dwsectab.
        if (res != DW_DLV_OK) {
            cerr << "dwarfgen: Error getting relocation record " <<
                ct << "."  << endl;
            exit(1);
        }

        int dwseclink =  FindMySectionNum(elf_section_index_link);
        ElfSectIndex sitarg = dwsectab[dwseclink].getSectIndex();
        string linktarg= dwsectab[dwseclink].name_;
        long int targsec = sitarg.getSectIndex();

        cout << "Relocs for sec=" << ct <<
            " ourlinkto="       << elf_section_index_link <<
            " linktoobjsecnum=" << targsec <<
            " name="            << linktarg <<
            " reloc-count="     << relocation_buffer_count << endl;
        Elf_Scn *scn =  elf_getscn(elf,elf_section_index_link);
        if(!scn) {
            cerr << "dwarfgen: Unable to elf_getscn  # " <<
                elf_section_index_link << endl;
            exit(1);
        }

        for (Dwarf_Unsigned r = 0; r < relocation_buffer_count; ++r) {
            Dwarf_Relocation_Data rec = reld+r;
            ElfSymIndex symi(rec->drd_symbol_index);
            Dwarf_Unsigned newval = FindSymbolValue(symi,irep);
            char *buf_to_update = findelfbuf(elf,scn,
                rec->drd_offset,rec->drd_length);

            if(buf_to_update) {
                if(showrelocdetails) {
                    cout << "Reloc "<< r <<
                        " symindex=" << rec->drd_symbol_index <<
                        " targoffset= " << IToHex(rec->drd_offset) <<
                        " newval = " << IToHex(newval) << endl;
                }
                bitreplace(buf_to_update, newval,sizeof(newval),
                    rec->drd_length);
            } else {
                if(showrelocdetails) {
                    cout << "Reloc "<< r << "does nothing"<<endl;
                }
            }
        }
    }
}

int
open_a_file(const char * name)
{
    /* Set to a file number that cannot be legal. */
    int f = -1;

#if defined(__CYGWIN__) || defined(_WIN32)
    /*  It is not possible to share file handles
        between applications or DLLs. Each application has its own
        file-handle table. For two applications to use the same file
        using a DLL, they must both open the file individually.
        Let the 'libelf' dll to open and close the file.  */

    /* For WIN32 open the file as binary */
    f = elf_open(name, O_RDONLY | O_BINARY);
#else
    f = open(name, O_RDONLY);
#endif
    return f;
}

int
create_a_file(const char * name)
{
    /* Set to a file number that cannot be legal. */
    int f = -1;

#if defined(__CYGWIN__) || defined(_WIN32)
    /*  It is not possible to share file handles
        between applications or DLLs. Each application has its own
        file-handle table. For two applications to use the same file
        using a DLL, they must both open the file individually.
        Let the 'libelf' dll to open and close the file.  */

    /* For WIN32 create the file as binary */
    f = elf_open(name, O_WRONLY | O_CREAT | O_BINARY);
#else
    int mode =  S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
    f = open(name,O_WRONLY | O_CREAT | O_TRUNC, mode);
#endif
    return f;
}

void
close_a_file(int f)
{
    close(f);
}