path: root/src/patchelf.cc

/*
 *  PatchELF is a utility to modify properties of ELF executables and libraries
 *  Copyright (C) 2004-2016  Eelco Dolstra <edolstra@gmail.com>
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or (at
 *  your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful, but
 *  WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 *  General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include <string>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <memory>
#include <sstream>
#include <limits>
#include <stdexcept>

#include <cstdlib>
#include <cstdio>
#include <cstdarg>
#include <cassert>
#include <cstring>
#include <cerrno>

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>

#include "elf.h"


static bool debugMode = false;

static bool forceRPath = false;

static std::vector<std::string> fileNames;
static int pageSize = PAGESIZE;

typedef std::shared_ptr<std::vector<unsigned char>> FileContents;


#define ElfFileParams class Elf_Ehdr, class Elf_Phdr, class Elf_Shdr, class Elf_Addr, class Elf_Off, class Elf_Dyn, class Elf_Sym, class Elf_Verneed
#define ElfFileParamNames Elf_Ehdr, Elf_Phdr, Elf_Shdr, Elf_Addr, Elf_Off, Elf_Dyn, Elf_Sym, Elf_Verneed


static std::vector<std::string> splitColonDelimitedString(const char * s)
{
    std::vector<std::string> parts;
    const char * pos = s;
    while (*pos) {
        const char * end = strchr(pos, ':');
        if (!end) end = strchr(pos, 0);

        parts.push_back(std::string(pos, end - pos));
        if (*end == ':') ++end;
        pos = end;
    }

    return parts;
}

static bool hasAllowedPrefix(const std::string & s, const std::vector<std::string> & allowedPrefixes)
{
    for (auto & i : allowedPrefixes)
        if (!s.compare(0, i.size(), i)) return true;
    return false;
}


static unsigned int getPageSize()
{
    return pageSize;
}


template<ElfFileParams>
class ElfFile
{
public:

    const FileContents fileContents;

private:

    unsigned char * contents;

    Elf_Ehdr * hdr;
    std::vector<Elf_Phdr> phdrs;
    std::vector<Elf_Shdr> shdrs;

    bool littleEndian;

    bool changed = false;

    bool isExecutable = false;

    typedef std::string SectionName;
    typedef std::map<SectionName, std::string> ReplacedSections;

    ReplacedSections replacedSections;

    std::string sectionNames; /* content of the .shstrtab section */

    /* Align on 4 or 8 bytes boundaries on 32- or 64-bit platforms
       respectively. */
    size_t sectionAlignment = sizeof(Elf_Off);

    std::vector<SectionName> sectionsByOldIndex;

public:

    ElfFile(FileContents fileContents);

    bool isChanged()
    {
        return changed;
    }

private:

    struct CompPhdr
    {
        ElfFile * elfFile;
        bool operator ()(const Elf_Phdr & x, const Elf_Phdr & y)
        {
            // A PHDR comes before everything else.
            if (y.p_type == PT_PHDR) return false;
            if (x.p_type == PT_PHDR) return true;

            // Sort non-PHDRs by address.
            return elfFile->rdi(x.p_paddr) < elfFile->rdi(y.p_paddr);
        }
    };

    friend struct CompPhdr;

    void sortPhdrs();

    struct CompShdr
    {
        ElfFile * elfFile;
        bool operator ()(const Elf_Shdr & x, const Elf_Shdr & y)
        {
            return elfFile->rdi(x.sh_offset) < elfFile->rdi(y.sh_offset);
        }
    };

    friend struct CompShdr;

    void sortShdrs();

    void shiftFile(unsigned int extraPages, Elf_Addr startPage);

    std::string getSectionName(const Elf_Shdr & shdr);

    Elf_Shdr & findSection(const SectionName & sectionName);

    Elf_Shdr * findSection2(const SectionName & sectionName);

    unsigned int findSection3(const SectionName & sectionName);

    std::string & replaceSection(const SectionName & sectionName,
        unsigned int size);

    bool haveReplacedSection(const SectionName & sectionName);

    void writeReplacedSections(Elf_Off & curOff,
        Elf_Addr startAddr, Elf_Off startOffset);

    void rewriteHeaders(Elf_Addr phdrAddress);

    void rewriteSectionsLibrary();

    void rewriteSectionsExecutable();

public:

    void rewriteSections();

    std::string getInterpreter();

    typedef enum { printSoname, replaceSoname } sonameMode;

    void modifySoname(sonameMode op, const std::string & newSoname);

    void setInterpreter(const std::string & newInterpreter);

    typedef enum { rpPrint, rpShrink, rpSet, rpRemove } RPathOp;

    void modifyRPath(RPathOp op, const std::vector<std::string> & allowedRpathPrefixes, std::string newRPath);

    void addNeeded(const std::set<std::string> & libs);

    void removeNeeded(const std::set<std::string> & libs);

    void replaceNeeded(const std::map<std::string, std::string> & libs);

    void printNeededLibs();

    void noDefaultLib();

private:

    /* Convert an integer in big or little endian representation (as
       specified by the ELF header) to this platform's integer
       representation. */
    template<class I>
    I rdi(I i);

    /* Convert back to the ELF representation. */
    template<class I>
    I wri(I & t, unsigned long long i)
    {
        t = rdi((I) i);
        return i;
    }
};


/* !!! G++ creates broken code if this function is inlined, don't know
   why... */
template<ElfFileParams>
template<class I>
I ElfFile<ElfFileParamNames>::rdi(I i)
{
    I r = 0;
    if (littleEndian) {
        for (unsigned int n = 0; n < sizeof(I); ++n) {
            r |= ((I) *(((unsigned char *) &i) + n)) << (n * 8);
        }
    } else {
        for (unsigned int n = 0; n < sizeof(I); ++n) {
            r |= ((I) *(((unsigned char *) &i) + n)) << ((sizeof(I) - n - 1) * 8);
        }
    }
    return r;
}


/* Ugly: used to erase DT_RUNPATH when using --force-rpath. */
#define DT_IGNORE       0x00726e67


static void debug(const char * format, ...)
{
    if (debugMode) {
        va_list ap;
        va_start(ap, format);
        vfprintf(stderr, format, ap);
        va_end(ap);
    }
}


void fmt2(std::ostringstream & out)
{
}


template<typename T, typename... Args>
void fmt2(std::ostringstream & out, T x, Args... args)
{
    out << x;
    fmt2(out, args...);
}


template<typename... Args>
std::string fmt(Args... args)
{
    std::ostringstream out;
    fmt2(out, args...);
    return out.str();
}


struct SysError : std::runtime_error
{
    int errNo;
    SysError(const std::string & msg)
        : std::runtime_error(fmt(msg + ": " + strerror(errno)))
        , errNo(errno)
    { }
};


__attribute__((noreturn)) static void error(std::string msg)
{
    if (errno)
        throw SysError(msg);
    else
        throw std::runtime_error(msg);
}


static void growFile(FileContents contents, size_t newSize)
{
    if (newSize > contents->capacity()) error("maximum file size exceeded");
    if (newSize <= contents->size()) return;
    contents->resize(newSize, 0);
}


static FileContents readFile(std::string fileName,
    size_t cutOff = std::numeric_limits<size_t>::max())
{
    struct stat st;
    if (stat(fileName.c_str(), &st) != 0)
        throw SysError(fmt("getting info about '", fileName, "'"));

    if ((uint64_t) st.st_size > (uint64_t) std::numeric_limits<size_t>::max())
        throw SysError(fmt("cannot read file of size ", st.st_size, " into memory"));

    size_t size = std::min(cutOff, (size_t) st.st_size);

    FileContents contents = std::make_shared<std::vector<unsigned char>>();
    contents->reserve(size + 32 * 1024 * 1024);
    contents->resize(size, 0);

    int fd = open(fileName.c_str(), O_RDONLY);
    if (fd == -1) throw SysError(fmt("opening '", fileName, "'"));

    size_t bytesRead = 0;
    ssize_t portion;
    while ((portion = read(fd, contents->data() + bytesRead, size - bytesRead)) > 0)
        bytesRead += portion;

    if (bytesRead != size)
        throw SysError(fmt("reading '", fileName, "'"));

    close(fd);

    return contents;
}


struct ElfType
{
    bool is32Bit;
    int machine; // one of EM_*
};


ElfType getElfType(const FileContents & fileContents)
{
    /* Check the ELF header for basic validity. */
    if (fileContents->size() < (off_t) sizeof(Elf32_Ehdr)) error("missing ELF header");

    auto contents = fileContents->data();

    if (memcmp(contents, ELFMAG, SELFMAG) != 0)
        error("not an ELF executable");

    if (contents[EI_VERSION] != EV_CURRENT)
        error("unsupported ELF version");

    if (contents[EI_CLASS] != ELFCLASS32 && contents[EI_CLASS] != ELFCLASS64)
        error("ELF executable is not 32 or 64 bit");

    bool is32Bit = contents[EI_CLASS] == ELFCLASS32;

    // FIXME: endianness
    return ElfType{is32Bit, is32Bit ? ((Elf32_Ehdr *) contents)->e_machine : ((Elf64_Ehdr *) contents)->e_machine};
}


static void checkPointer(const FileContents & contents, void * p, unsigned int size)
{
    unsigned char * q = (unsigned char *) p;
    assert(q >= contents->data() && q + size <= contents->data() + contents->size());
}


template<ElfFileParams>
ElfFile<ElfFileParamNames>::ElfFile(FileContents fileContents)
    : fileContents(fileContents)
    , contents(fileContents->data())
{
    /* Check the ELF header for basic validity. */
    if (fileContents->size() < (off_t) sizeof(Elf_Ehdr)) error("missing ELF header");

    hdr = (Elf_Ehdr *) fileContents->data();

    if (memcmp(hdr->e_ident, ELFMAG, SELFMAG) != 0)
        error("not an ELF executable");

    littleEndian = hdr->e_ident[EI_DATA] == ELFDATA2LSB;

    if (rdi(hdr->e_type) != ET_EXEC && rdi(hdr->e_type) != ET_DYN)
        error("wrong ELF type");

    if ((size_t) (rdi(hdr->e_phoff) + rdi(hdr->e_phnum) * rdi(hdr->e_phentsize)) > fileContents->size())
        error("program header table out of bounds");

    if (rdi(hdr->e_shnum) == 0)
        error("no section headers. The input file is probably a statically linked, self-decompressing binary");

    if ((size_t) (rdi(hdr->e_shoff) + rdi(hdr->e_shnum) * rdi(hdr->e_shentsize)) > fileContents->size())
        error("section header table out of bounds");

    if (rdi(hdr->e_phentsize) != sizeof(Elf_Phdr))
        error("program headers have wrong size");

    /* Copy the program and section headers. */
    for (int i = 0; i < rdi(hdr->e_phnum); ++i) {
        phdrs.push_back(* ((Elf_Phdr *) (contents + rdi(hdr->e_phoff)) + i));
        if (rdi(phdrs[i].p_type) == PT_INTERP) isExecutable = true;
    }

    for (int i = 0; i < rdi(hdr->e_shnum); ++i)
        shdrs.push_back(* ((Elf_Shdr *) (contents + rdi(hdr->e_shoff)) + i));

    /* Get the section header string table section (".shstrtab").  Its
       index in the section header table is given by e_shstrndx field
       of the ELF header. */
    unsigned int shstrtabIndex = rdi(hdr->e_shstrndx);
    assert(shstrtabIndex < shdrs.size());
    unsigned int shstrtabSize = rdi(shdrs[shstrtabIndex].sh_size);
    char * shstrtab = (char * ) contents + rdi(shdrs[shstrtabIndex].sh_offset);
    checkPointer(fileContents, shstrtab, shstrtabSize);

    assert(shstrtabSize > 0);
    assert(shstrtab[shstrtabSize - 1] == 0);

    sectionNames = std::string(shstrtab, shstrtabSize);

    sectionsByOldIndex.resize(hdr->e_shnum);
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        sectionsByOldIndex[i] = getSectionName(shdrs[i]);
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::sortPhdrs()
{
    /* Sort the segments by offset. */
    CompPhdr comp;
    comp.elfFile = this;
    sort(phdrs.begin(), phdrs.end(), comp);
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::sortShdrs()
{
    /* Translate sh_link mappings to section names, since sorting the
       sections will invalidate the sh_link fields. */
    std::map<SectionName, SectionName> linkage;
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        if (rdi(shdrs[i].sh_link) != 0)
            linkage[getSectionName(shdrs[i])] = getSectionName(shdrs[rdi(shdrs[i].sh_link)]);

    /* Idem for sh_info on certain sections. */
    std::map<SectionName, SectionName> info;
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        if (rdi(shdrs[i].sh_info) != 0 &&
            (rdi(shdrs[i].sh_type) == SHT_REL || rdi(shdrs[i].sh_type) == SHT_RELA))
            info[getSectionName(shdrs[i])] = getSectionName(shdrs[rdi(shdrs[i].sh_info)]);

    /* Idem for the index of the .shstrtab section in the ELF header. */
    SectionName shstrtabName = getSectionName(shdrs[rdi(hdr->e_shstrndx)]);

    /* Sort the sections by offset. */
    CompShdr comp;
    comp.elfFile = this;
    sort(shdrs.begin() + 1, shdrs.end(), comp);

    /* Restore the sh_link mappings. */
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        if (rdi(shdrs[i].sh_link) != 0)
            wri(shdrs[i].sh_link,
                findSection3(linkage[getSectionName(shdrs[i])]));

    /* And the st_info mappings. */
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        if (rdi(shdrs[i].sh_info) != 0 &&
            (rdi(shdrs[i].sh_type) == SHT_REL || rdi(shdrs[i].sh_type) == SHT_RELA))
            wri(shdrs[i].sh_info,
                findSection3(info[getSectionName(shdrs[i])]));

    /* And the .shstrtab index. */
    wri(hdr->e_shstrndx, findSection3(shstrtabName));
}


static void writeFile(std::string fileName, FileContents contents)
{
    int fd = open(fileName.c_str(), O_TRUNC | O_WRONLY);
    if (fd == -1)
        error("open");

    size_t bytesWritten = 0;
    ssize_t portion;
    while ((portion = write(fd, contents->data() + bytesWritten, contents->size() - bytesWritten)) > 0)
        bytesWritten += portion;

    if (bytesWritten != contents->size())
        error("write");

    if (close(fd) != 0)
        error("close");
}


static unsigned int roundUp(unsigned int n, unsigned int m)
{
    return ((n - 1) / m + 1) * m;
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::shiftFile(unsigned int extraPages, Elf_Addr startPage)
{
    /* Move the entire contents of the file 'extraPages' pages
       further. */
    unsigned int oldSize = fileContents->size();
    unsigned int shift = extraPages * getPageSize();
    growFile(fileContents, fileContents->size() + extraPages * getPageSize());
    memmove(contents + extraPages * getPageSize(), contents, oldSize);
    memset(contents + sizeof(Elf_Ehdr), 0, shift - sizeof(Elf_Ehdr));

    /* Adjust the ELF header. */
    wri(hdr->e_phoff, sizeof(Elf_Ehdr));
    wri(hdr->e_shoff, rdi(hdr->e_shoff) + shift);

    /* Update the offsets in the section headers. */
    for (int i = 1; i < rdi(hdr->e_shnum); ++i)
        wri(shdrs[i].sh_offset, rdi(shdrs[i].sh_offset) + shift);

    /* Update the offsets in the program headers. */
    for (int i = 0; i < rdi(hdr->e_phnum); ++i) {
        wri(phdrs[i].p_offset, rdi(phdrs[i].p_offset) + shift);
        if (rdi(phdrs[i].p_align) != 0 &&
            (rdi(phdrs[i].p_vaddr) - rdi(phdrs[i].p_offset)) % rdi(phdrs[i].p_align) != 0) {
            debug("changing alignment of program header %d from %d to %d\n", i,
                rdi(phdrs[i].p_align), getPageSize());
            wri(phdrs[i].p_align, getPageSize());
        }
    }

    /* Add a segment that maps the new program/section headers and
       PT_INTERP segment into memory.  Otherwise glibc will choke. */
    phdrs.resize(rdi(hdr->e_phnum) + 1);
    wri(hdr->e_phnum, rdi(hdr->e_phnum) + 1);
    Elf_Phdr & phdr = phdrs[rdi(hdr->e_phnum) - 1];
    wri(phdr.p_type, PT_LOAD);
    wri(phdr.p_offset, 0);
    wri(phdr.p_vaddr, wri(phdr.p_paddr, startPage));
    wri(phdr.p_filesz, wri(phdr.p_memsz, shift));
    wri(phdr.p_flags, PF_R | PF_W);
    wri(phdr.p_align, getPageSize());
}


template<ElfFileParams>
std::string ElfFile<ElfFileParamNames>::getSectionName(const Elf_Shdr & shdr)
{
    return std::string(sectionNames.c_str() + rdi(shdr.sh_name));
}


template<ElfFileParams>
Elf_Shdr & ElfFile<ElfFileParamNames>::findSection(const SectionName & sectionName)
{
    Elf_Shdr * shdr = findSection2(sectionName);
    if (!shdr) {
        std::string extraMsg = "";
        if (sectionName == ".interp" || sectionName == ".dynamic" || sectionName == ".dynstr")
            extraMsg = ". The input file is most likely statically linked";
        error("cannot find section '" + sectionName + "'" + extraMsg);
    }
    return *shdr;
}


template<ElfFileParams>
Elf_Shdr * ElfFile<ElfFileParamNames>::findSection2(const SectionName & sectionName)
{
    unsigned int i = findSection3(sectionName);
    return i ? &shdrs[i] : 0;
}


template<ElfFileParams>
unsigned int ElfFile<ElfFileParamNames>::findSection3(const SectionName & sectionName)
{
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        if (getSectionName(shdrs[i]) == sectionName) return i;
    return 0;
}

template<ElfFileParams>
bool ElfFile<ElfFileParamNames>::haveReplacedSection(const SectionName & sectionName)
{
    ReplacedSections::iterator i = replacedSections.find(sectionName);

    if (i != replacedSections.end())
        return true;
    return false;
}

template<ElfFileParams>
std::string & ElfFile<ElfFileParamNames>::replaceSection(const SectionName & sectionName,
    unsigned int size)
{
    ReplacedSections::iterator i = replacedSections.find(sectionName);
    std::string s;

    if (i != replacedSections.end()) {
        s = std::string(i->second);
    } else {
        Elf_Shdr & shdr = findSection(sectionName);
        s = std::string((char *) contents + rdi(shdr.sh_offset), rdi(shdr.sh_size));
    }

    s.resize(size);
    replacedSections[sectionName] = s;

    return replacedSections[sectionName];
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::writeReplacedSections(Elf_Off & curOff,
    Elf_Addr startAddr, Elf_Off startOffset)
{
    /* Overwrite the old section contents with 'X's.  Do this
       *before* writing the new section contents (below) to prevent
       clobbering previously written new section contents. */
    for (auto & i : replacedSections) {
        std::string sectionName = i.first;
        Elf_Shdr & shdr = findSection(sectionName);
        if (shdr.sh_type != SHT_NOBITS)
            memset(contents + rdi(shdr.sh_offset), 'X', rdi(shdr.sh_size));
    }

    for (auto & i : replacedSections) {
        std::string sectionName = i.first;
        Elf_Shdr & shdr = findSection(sectionName);
        debug("rewriting section '%s' from offset 0x%x (size %d) to offset 0x%x (size %d)\n",
            sectionName.c_str(), rdi(shdr.sh_offset), rdi(shdr.sh_size), curOff, i.second.size());

        memcpy(contents + curOff, (unsigned char *) i.second.c_str(),
            i.second.size());

        /* Update the section header for this section. */
        wri(shdr.sh_offset, curOff);
        wri(shdr.sh_addr, startAddr + (curOff - startOffset));
        wri(shdr.sh_size, i.second.size());
        wri(shdr.sh_addralign, sectionAlignment);

        /* If this is the .interp section, then the PT_INTERP segment
           must be sync'ed with it. */
        if (sectionName == ".interp") {
            for (unsigned int j = 0; j < phdrs.size(); ++j)
                if (rdi(phdrs[j].p_type) == PT_INTERP) {
                    phdrs[j].p_offset = shdr.sh_offset;
                    phdrs[j].p_vaddr = phdrs[j].p_paddr = shdr.sh_addr;
                    phdrs[j].p_filesz = phdrs[j].p_memsz = shdr.sh_size;
                }
        }

        /* If this is the .dynamic section, then the PT_DYNAMIC segment
           must be sync'ed with it. */
        if (sectionName == ".dynamic") {
            for (unsigned int j = 0; j < phdrs.size(); ++j)
                if (rdi(phdrs[j].p_type) == PT_DYNAMIC) {
                    phdrs[j].p_offset = shdr.sh_offset;
                    phdrs[j].p_vaddr = phdrs[j].p_paddr = shdr.sh_addr;
                    phdrs[j].p_filesz = phdrs[j].p_memsz = shdr.sh_size;
                }
        }

        curOff += roundUp(i.second.size(), sectionAlignment);
    }

    replacedSections.clear();
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::rewriteSectionsLibrary()
{
    /* For dynamic libraries, we just place the replacement sections
       at the end of the file.  They're mapped into memory by a
       PT_LOAD segment located directly after the last virtual address
       page of other segments. */
    Elf_Addr startPage = 0;
    for (unsigned int i = 0; i < phdrs.size(); ++i) {
        Elf_Addr thisPage = roundUp(rdi(phdrs[i].p_vaddr) + rdi(phdrs[i].p_memsz), getPageSize());
        if (thisPage > startPage) startPage = thisPage;
    }

    debug("last page is 0x%llx\n", (unsigned long long) startPage);

    /* Because we're adding a new section header, we're necessarily increasing
       the size of the program header table.  This can cause the first section
       to overlap the program header table in memory; we need to shift the first
       few segments to someplace else. */
    /* Some sections may already be replaced so account for that */
    unsigned int i = 1;
    Elf_Addr pht_size = sizeof(Elf_Ehdr) + (phdrs.size() + 1)*sizeof(Elf_Phdr);
    while( shdrs[i].sh_addr <= pht_size && i < rdi(hdr->e_shnum) ) {
        if (not haveReplacedSection(getSectionName(shdrs[i])))
            replaceSection(getSectionName(shdrs[i]), shdrs[i].sh_size);
        i++;
    }

    /* Compute the total space needed for the replaced sections */
    off_t neededSpace = 0;
    for (auto & i : replacedSections)
        neededSpace += roundUp(i.second.size(), sectionAlignment);
    debug("needed space is %d\n", neededSpace);

    size_t startOffset = roundUp(fileContents->size(), getPageSize());

    growFile(fileContents, startOffset + neededSpace);

    /* Even though this file is of type ET_DYN, it could actually be
       an executable.  For instance, Gold produces executables marked
       ET_DYN as does LD when linking with pie. If we move PT_PHDR, it
       has to stay in the first PT_LOAD segment or any subsequent ones
       if they're continuous in memory due to linux kernel constraints
       (see BUGS). Since the end of the file would be after bss, we can't 
       move PHDR there, we therefore choose to leave PT_PHDR where it is but
       move enough following sections such that we can add the extra PT_LOAD
       section to it. This PT_LOAD segment ensures the sections at the end of
       the file are mapped into memory for ld.so to process.
       We can't use the approach in rewriteSectionsExecutable()
       since DYN executables tend to start at virtual address 0, so
       rewriteSectionsExecutable() won't work because it doesn't have
       any virtual address space to grow downwards into. */
    if (isExecutable) {
        if (startOffset >= startPage) {
            debug("shifting new PT_LOAD segment by %d bytes to work around a Linux kernel bug\n", startOffset - startPage);
        }
        startPage = startOffset;
    }

    /* Add a segment that maps the replaced sections into memory. */
    phdrs.resize(rdi(hdr->e_phnum) + 1);
    wri(hdr->e_phnum, rdi(hdr->e_phnum) + 1);
    Elf_Phdr & phdr = phdrs[rdi(hdr->e_phnum) - 1];
    wri(phdr.p_type, PT_LOAD);
    wri(phdr.p_offset, startOffset);
    wri(phdr.p_vaddr, wri(phdr.p_paddr, startPage));
    wri(phdr.p_filesz, wri(phdr.p_memsz, neededSpace));
    wri(phdr.p_flags, PF_R | PF_W);
    wri(phdr.p_align, getPageSize());


    /* Write out the replaced sections. */
    Elf_Off curOff = startOffset;
    writeReplacedSections(curOff, startPage, startOffset);
    assert(curOff == startOffset + neededSpace);

    /* Write out the updated program and section headers */
    rewriteHeaders(hdr->e_phoff);
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::rewriteSectionsExecutable()
{
    /* Sort the sections by offset, otherwise we won't correctly find
       all the sections before the last replaced section. */
    sortShdrs();


    /* What is the index of the last replaced section? */
    unsigned int lastReplaced = 0;
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i) {
        std::string sectionName = getSectionName(shdrs[i]);
        if (replacedSections.find(sectionName) != replacedSections.end()) {
            debug("using replaced section '%s'\n", sectionName.c_str());
            lastReplaced = i;
        }
    }

    assert(lastReplaced != 0);

    debug("last replaced is %d\n", lastReplaced);

    /* Try to replace all sections before that, as far as possible.
       Stop when we reach an irreplacable section (such as one of type
       SHT_PROGBITS).  These cannot be moved in virtual address space
       since that would invalidate absolute references to them. */
    assert(lastReplaced + 1 < shdrs.size()); /* !!! I'm lazy. */
    size_t startOffset = rdi(shdrs[lastReplaced + 1].sh_offset);
    Elf_Addr startAddr = rdi(shdrs[lastReplaced + 1].sh_addr);
    std::string prevSection;
    for (unsigned int i = 1; i <= lastReplaced; ++i) {
        Elf_Shdr & shdr(shdrs[i]);
        std::string sectionName = getSectionName(shdr);
        debug("looking at section '%s'\n", sectionName.c_str());
        /* !!! Why do we stop after a .dynstr section? I can't
           remember! */
        if ((rdi(shdr.sh_type) == SHT_PROGBITS && sectionName != ".interp")
            || prevSection == ".dynstr")
        {
            startOffset = rdi(shdr.sh_offset);
            startAddr = rdi(shdr.sh_addr);
            lastReplaced = i - 1;
            break;
        } else {
            if (replacedSections.find(sectionName) == replacedSections.end()) {
                debug("replacing section '%s' which is in the way\n", sectionName.c_str());
                replaceSection(sectionName, rdi(shdr.sh_size));
            }
        }
        prevSection = sectionName;
    }

    debug("first reserved offset/addr is 0x%x/0x%llx\n",
        startOffset, (unsigned long long) startAddr);

    assert(startAddr % getPageSize() == startOffset % getPageSize());
    Elf_Addr firstPage = startAddr - startOffset;
    debug("first page is 0x%llx\n", (unsigned long long) firstPage);

    if (rdi(hdr->e_shoff) < startOffset) {
        /* The section headers occur too early in the file and would be
           overwritten by the replaced sections. Move them to the end of the file
           before proceeding. */
        off_t shoffNew = fileContents->size();
        off_t shSize = rdi(hdr->e_shoff) + rdi(hdr->e_shnum) * rdi(hdr->e_shentsize);
        growFile(fileContents, fileContents->size() + shSize);
        wri(hdr->e_shoff, shoffNew);

        /* Rewrite the section header table.  For neatness, keep the
           sections sorted. */
        assert(rdi(hdr->e_shnum) == shdrs.size());
        sortShdrs();
        for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
            * ((Elf_Shdr *) (contents + rdi(hdr->e_shoff)) + i) = shdrs[i];
    }


    /* Compute the total space needed for the replaced sections, the
       ELF header, and the program headers. */
    size_t neededSpace = sizeof(Elf_Ehdr) + phdrs.size() * sizeof(Elf_Phdr);
    for (auto & i : replacedSections)
        neededSpace += roundUp(i.second.size(), sectionAlignment);

    debug("needed space is %d\n", neededSpace);

    /* If we need more space at the start of the file, then grow the
       file by the minimum number of pages and adjust internal
       offsets. */
    if (neededSpace > startOffset) {

        /* We also need an additional program header, so adjust for that. */
        neededSpace += sizeof(Elf_Phdr);
        debug("needed space is %d\n", neededSpace);

        unsigned int neededPages = roundUp(neededSpace - startOffset, getPageSize()) / getPageSize();
        debug("needed pages is %d\n", neededPages);
        if (neededPages * getPageSize() > firstPage)
            error("virtual address space underrun!");

        firstPage -= neededPages * getPageSize();
        startOffset += neededPages * getPageSize();

        shiftFile(neededPages, firstPage);
    }


    /* Clear out the free space. */
    Elf_Off curOff = sizeof(Elf_Ehdr) + phdrs.size() * sizeof(Elf_Phdr);
    debug("clearing first %d bytes\n", startOffset - curOff);
    memset(contents + curOff, 0, startOffset - curOff);


    /* Write out the replaced sections. */
    writeReplacedSections(curOff, firstPage, 0);
    assert(curOff == neededSpace);


    rewriteHeaders(firstPage + rdi(hdr->e_phoff));
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::rewriteSections()
{
    if (replacedSections.empty()) return;

    for (auto & i : replacedSections)
        debug("replacing section '%s' with size %d\n",
            i.first.c_str(), i.second.size());

    if (rdi(hdr->e_type) == ET_DYN) {
        debug("this is a dynamic library\n");
        rewriteSectionsLibrary();
    } else if (rdi(hdr->e_type) == ET_EXEC) {
        debug("this is an executable\n");
        rewriteSectionsExecutable();
    } else error("unknown ELF type");
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::rewriteHeaders(Elf_Addr phdrAddress)
{
    /* Rewrite the program header table. */

    /* If there is a segment for the program header table, update it.
       (According to the ELF spec, there can only be one.) */
    for (unsigned int i = 0; i < phdrs.size(); ++i) {
        if (rdi(phdrs[i].p_type) == PT_PHDR) {
            phdrs[i].p_offset = hdr->e_phoff;
            wri(phdrs[i].p_vaddr, wri(phdrs[i].p_paddr, phdrAddress));
            wri(phdrs[i].p_filesz, wri(phdrs[i].p_memsz, phdrs.size() * sizeof(Elf_Phdr)));
            break;
        }
    }

    sortPhdrs();

    for (unsigned int i = 0; i < phdrs.size(); ++i)
        * ((Elf_Phdr *) (contents + rdi(hdr->e_phoff)) + i) = phdrs[i];


    /* Rewrite the section header table.  For neatness, keep the
       sections sorted. */
    assert(rdi(hdr->e_shnum) == shdrs.size());
    sortShdrs();
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
        * ((Elf_Shdr *) (contents + rdi(hdr->e_shoff)) + i) = shdrs[i];


    /* Update all those nasty virtual addresses in the .dynamic
       section.  Note that not all executables have .dynamic sections
       (e.g., those produced by klibc's klcc). */
    Elf_Shdr * shdrDynamic = findSection2(".dynamic");
    if (shdrDynamic) {
        Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic->sh_offset));
        unsigned int d_tag;
        for ( ; (d_tag = rdi(dyn->d_tag)) != DT_NULL; dyn++)
            if (d_tag == DT_STRTAB)
                dyn->d_un.d_ptr = findSection(".dynstr").sh_addr;
            else if (d_tag == DT_STRSZ)
                dyn->d_un.d_val = findSection(".dynstr").sh_size;
            else if (d_tag == DT_SYMTAB)
                dyn->d_un.d_ptr = findSection(".dynsym").sh_addr;
            else if (d_tag == DT_HASH)
                dyn->d_un.d_ptr = findSection(".hash").sh_addr;
            else if (d_tag == DT_GNU_HASH)
                dyn->d_un.d_ptr = findSection(".gnu.hash").sh_addr;
            else if (d_tag == DT_JMPREL) {
                Elf_Shdr * shdr = findSection2(".rel.plt");
                if (!shdr) shdr = findSection2(".rela.plt"); /* 64-bit Linux, x86-64 */
                if (!shdr) shdr = findSection2(".rela.IA_64.pltoff"); /* 64-bit Linux, IA-64 */
                if (!shdr) error("cannot find section corresponding to DT_JMPREL");
                dyn->d_un.d_ptr = shdr->sh_addr;
            }
            else if (d_tag == DT_REL) { /* !!! hack! */
                Elf_Shdr * shdr = findSection2(".rel.dyn");
                /* no idea if this makes sense, but it was needed for some
                   program */
                if (!shdr) shdr = findSection2(".rel.got");
                /* some programs have neither section, but this doesn't seem
                   to be a problem */
                if (!shdr) continue;
                dyn->d_un.d_ptr = shdr->sh_addr;
            }
            else if (d_tag == DT_RELA) {
                Elf_Shdr * shdr = findSection2(".rela.dyn");
                /* some programs lack this section, but it doesn't seem to
                   be a problem */
                if (!shdr) continue;
                dyn->d_un.d_ptr = shdr->sh_addr;
            }
            else if (d_tag == DT_VERNEED)
                dyn->d_un.d_ptr = findSection(".gnu.version_r").sh_addr;
            else if (d_tag == DT_VERSYM)
                dyn->d_un.d_ptr = findSection(".gnu.version").sh_addr;
    }


    /* Rewrite the .dynsym section.  It contains the indices of the
       sections in which symbols appear, so these need to be
       remapped. */
    for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i) {
        if (rdi(shdrs[i].sh_type) != SHT_SYMTAB && rdi(shdrs[i].sh_type) != SHT_DYNSYM) continue;
        debug("rewriting symbol table section %d\n", i);
        for (size_t entry = 0; (entry + 1) * sizeof(Elf_Sym) <= rdi(shdrs[i].sh_size); entry++) {
            Elf_Sym * sym = (Elf_Sym *) (contents + rdi(shdrs[i].sh_offset) + entry * sizeof(Elf_Sym));
            unsigned int shndx = rdi(sym->st_shndx);
            if (shndx != SHN_UNDEF && shndx < SHN_LORESERVE) {
                if (shndx >= sectionsByOldIndex.size()) {
                    fprintf(stderr, "warning: entry %d in symbol table refers to a non-existent section, skipping\n", shndx);
                    continue;
                }
                std::string section = sectionsByOldIndex.at(shndx);
                assert(!section.empty());
                unsigned int newIndex = findSection3(section); // inefficient
                //debug("rewriting symbol %d: index = %d (%s) -> %d\n", entry, shndx, section.c_str(), newIndex);
                wri(sym->st_shndx, newIndex);
                /* Rewrite st_value.  FIXME: we should do this for all
                   types, but most don't actually change. */
                if (ELF32_ST_TYPE(rdi(sym->st_info)) == STT_SECTION)
                    wri(sym->st_value, rdi(shdrs[newIndex].sh_addr));
            }
        }
    }
}



static void setSubstr(std::string & s, unsigned int pos, const std::string & t)
{
    assert(pos + t.size() <= s.size());
    copy(t.begin(), t.end(), s.begin() + pos);
}


template<ElfFileParams>
std::string ElfFile<ElfFileParamNames>::getInterpreter()
{
    Elf_Shdr & shdr = findSection(".interp");
    return std::string((char *) contents + rdi(shdr.sh_offset), rdi(shdr.sh_size));
}

template<ElfFileParams>
void ElfFile<ElfFileParamNames>::modifySoname(sonameMode op, const std::string & newSoname)
{
    if (rdi(hdr->e_type) != ET_DYN) {
        debug("this is not a dynamic library\n");
        return;
    }

    Elf_Shdr & shdrDynamic = findSection(".dynamic");
    Elf_Shdr & shdrDynStr = findSection(".dynstr");
    char * strTab = (char *) contents + rdi(shdrDynStr.sh_offset);

    /* Walk through the dynamic section, look for the DT_SONAME entry. */
    Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));
    Elf_Dyn * dynSoname = 0;
    char * soname = 0;
    for ( ; rdi(dyn->d_tag) != DT_NULL; dyn++) {
        if (rdi(dyn->d_tag) == DT_SONAME) {
            dynSoname = dyn;
            soname = strTab + rdi(dyn->d_un.d_val);
        }
    }

    if (op == printSoname) {
        if (soname) {
            if (std::string(soname ? soname : "") == "")
                debug("DT_SONAME is empty\n");
            else
                printf("%s\n", soname);
        } else {
            debug("no DT_SONAME found\n");
        }
        return;
    }

    if (std::string(soname ? soname : "") == newSoname) {
        debug("current and proposed new SONAMEs are equal keeping DT_SONAME entry\n");
        return;
    }

    /* Zero out the previous SONAME */
    unsigned int sonameSize = 0;
    if (soname) {
        sonameSize = strlen(soname);
        memset(soname, 'X', sonameSize);
    }

    debug("new SONAME is '%s'\n", newSoname.c_str());

    /* Grow the .dynstr section to make room for the new SONAME. */
    debug("SONAME is too long, resizing...\n");

    std::string & newDynStr = replaceSection(".dynstr", rdi(shdrDynStr.sh_size) + newSoname.size() + 1);
    setSubstr(newDynStr, rdi(shdrDynStr.sh_size), newSoname + '\0');

    /* Update the DT_SONAME entry. */
    if (dynSoname) {
        dynSoname->d_un.d_val = shdrDynStr.sh_size;
    } else {
        /* There is no DT_SONAME entry in the .dynamic section, so we
           have to grow the .dynamic section. */
        std::string & newDynamic = replaceSection(".dynamic", rdi(shdrDynamic.sh_size) + sizeof(Elf_Dyn));

        unsigned int idx = 0;
        for (; rdi(((Elf_Dyn *) newDynamic.c_str())[idx].d_tag) != DT_NULL; idx++);
        debug("DT_NULL index is %d\n", idx);

        /* Shift all entries down by one. */
        setSubstr(newDynamic, sizeof(Elf_Dyn), std::string(newDynamic, 0, sizeof(Elf_Dyn) * (idx + 1)));

        /* Add the DT_SONAME entry at the top. */
        Elf_Dyn newDyn;
        wri(newDyn.d_tag, DT_SONAME);
        newDyn.d_un.d_val = shdrDynStr.sh_size;
        setSubstr(newDynamic, 0, std::string((char *)&newDyn, sizeof(Elf_Dyn)));
    }

    changed = true;
}

template<ElfFileParams>
void ElfFile<ElfFileParamNames>::setInterpreter(const std::string & newInterpreter)
{
    std::string & section = replaceSection(".interp", newInterpreter.size() + 1);
    setSubstr(section, 0, newInterpreter + '\0');
    changed = true;
}


static void concatToRPath(std::string & rpath, const std::string & path)
{
    if (!rpath.empty()) rpath += ":";
    rpath += path;
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::modifyRPath(RPathOp op,
    const std::vector<std::string> & allowedRpathPrefixes, std::string newRPath)
{
    Elf_Shdr & shdrDynamic = findSection(".dynamic");

    /* !!! We assume that the virtual address in the DT_STRTAB entry
       of the dynamic section corresponds to the .dynstr section. */
    Elf_Shdr & shdrDynStr = findSection(".dynstr");
    char * strTab = (char *) contents + rdi(shdrDynStr.sh_offset);


    /* Walk through the dynamic section, look for the RPATH/RUNPATH
       entry.

       According to the ld.so docs, DT_RPATH is obsolete, we should
       use DT_RUNPATH.  DT_RUNPATH has two advantages: it can be
       overriden by LD_LIBRARY_PATH, and it's scoped (the DT_RUNPATH
       for an executable or library doesn't affect the search path for
       libraries used by it).  DT_RPATH is ignored if DT_RUNPATH is
       present.  The binutils 'ld' still generates only DT_RPATH,
       unless you use its '--enable-new-dtag' option, in which case it
       generates a DT_RPATH and DT_RUNPATH pointing at the same
       string. */
    std::vector<std::string> neededLibs;
    Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));
    Elf_Dyn * dynRPath = 0, * dynRunPath = 0;
    char * rpath = 0;
    for ( ; rdi(dyn->d_tag) != DT_NULL; dyn++) {
        if (rdi(dyn->d_tag) == DT_RPATH) {
            dynRPath = dyn;
            /* Only use DT_RPATH if there is no DT_RUNPATH. */
            if (!dynRunPath)
                rpath = strTab + rdi(dyn->d_un.d_val);
        }
        else if (rdi(dyn->d_tag) == DT_RUNPATH) {
            dynRunPath = dyn;
            rpath = strTab + rdi(dyn->d_un.d_val);
        }
        else if (rdi(dyn->d_tag) == DT_NEEDED)
            neededLibs.push_back(std::string(strTab + rdi(dyn->d_un.d_val)));
    }

    if (op == rpPrint) {
        printf("%s\n", rpath ? rpath : "");
        return;
    }

    if (op == rpShrink && !rpath) {
        debug("no RPATH to shrink\n");
        return;
    }


    /* For each directory in the RPATH, check if it contains any
       needed library. */
    if (op == rpShrink) {
        std::vector<bool> neededLibFound(neededLibs.size(), false);

        newRPath = "";

        for (auto & dirName : splitColonDelimitedString(rpath)) {

            /* Non-absolute entries are allowed (e.g., the special
               "$ORIGIN" hack). */
            if (dirName[0] != '/') {
                concatToRPath(newRPath, dirName);
                continue;
            }

            /* If --allowed-rpath-prefixes was given, reject directories
               not starting with any of the (colon-delimited) prefixes. */
            if (!allowedRpathPrefixes.empty() && !hasAllowedPrefix(dirName, allowedRpathPrefixes)) {
                debug("removing directory '%s' from RPATH because of non-allowed prefix\n", dirName.c_str());
                continue;
            }

            /* For each library that we haven't found yet, see if it
               exists in this directory. */
            bool libFound = false;
            for (unsigned int j = 0; j < neededLibs.size(); ++j)
                if (!neededLibFound[j]) {
                    std::string libName = dirName + "/" + neededLibs[j];
                    try {
                        if (getElfType(readFile(libName, sizeof(Elf32_Ehdr))).machine == rdi(hdr->e_machine)) {
                            neededLibFound[j] = true;
                            libFound = true;
                        } else
                            debug("ignoring library '%s' because its machine type differs\n", libName.c_str());
                    } catch (SysError & e) {
                        if (e.errNo != ENOENT) throw;
                    }
                }

            if (!libFound)
                debug("removing directory '%s' from RPATH\n", dirName.c_str());
            else
                concatToRPath(newRPath, dirName);
        }
    }

    if (op == rpRemove) {
        if (!rpath) {
            debug("no RPATH to delete\n");
            return;
        }

        Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));
        Elf_Dyn * last = dyn;
        for ( ; rdi(dyn->d_tag) != DT_NULL; dyn++) {
            if (rdi(dyn->d_tag) == DT_RPATH) {
                debug("removing DT_RPATH entry\n");
                changed = true;
            } else if (rdi(dyn->d_tag) == DT_RUNPATH) {
                debug("removing DT_RUNPATH entry\n");
                changed = true;
            } else {
                *last++ = *dyn;
            }
        }
        memset(last, 0, sizeof(Elf_Dyn) * (dyn - last));
        return;
    }


    if (std::string(rpath ? rpath : "") == newRPath) return;

    changed = true;

    /* Zero out the previous rpath to prevent retained dependencies in
       Nix. */
    unsigned int rpathSize = 0;
    if (rpath) {
        rpathSize = strlen(rpath);
        memset(rpath, 'X', rpathSize);
    }

    debug("new rpath is '%s'\n", newRPath.c_str());

    if (!forceRPath && dynRPath && !dynRunPath) { /* convert DT_RPATH to DT_RUNPATH */
        dynRPath->d_tag = DT_RUNPATH;
        dynRunPath = dynRPath;
        dynRPath = 0;
    }

    if (forceRPath && dynRPath && dynRunPath) { /* convert DT_RUNPATH to DT_RPATH */
        dynRunPath->d_tag = DT_IGNORE;
    }

    if (newRPath.size() <= rpathSize) {
        strcpy(rpath, newRPath.c_str());
        return;
    }

    /* Grow the .dynstr section to make room for the new RPATH. */
    debug("rpath is too long, resizing...\n");

    std::string & newDynStr = replaceSection(".dynstr",
        rdi(shdrDynStr.sh_size) + newRPath.size() + 1);
    setSubstr(newDynStr, rdi(shdrDynStr.sh_size), newRPath + '\0');

    /* Update the DT_RUNPATH and DT_RPATH entries. */
    if (dynRunPath || dynRPath) {
        if (dynRunPath) dynRunPath->d_un.d_val = shdrDynStr.sh_size;
        if (dynRPath) dynRPath->d_un.d_val = shdrDynStr.sh_size;
    }

    else {
        /* There is no DT_RUNPATH entry in the .dynamic section, so we
           have to grow the .dynamic section. */
        std::string & newDynamic = replaceSection(".dynamic",
            rdi(shdrDynamic.sh_size) + sizeof(Elf_Dyn));

        unsigned int idx = 0;
        for ( ; rdi(((Elf_Dyn *) newDynamic.c_str())[idx].d_tag) != DT_NULL; idx++) ;
        debug("DT_NULL index is %d\n", idx);

        /* Shift all entries down by one. */
        setSubstr(newDynamic, sizeof(Elf_Dyn),
            std::string(newDynamic, 0, sizeof(Elf_Dyn) * (idx + 1)));

        /* Add the DT_RUNPATH entry at the top. */
        Elf_Dyn newDyn;
        wri(newDyn.d_tag, forceRPath ? DT_RPATH : DT_RUNPATH);
        newDyn.d_un.d_val = shdrDynStr.sh_size;
        setSubstr(newDynamic, 0, std::string((char *) &newDyn, sizeof(Elf_Dyn)));
    }
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::removeNeeded(const std::set<std::string> & libs)
{
    if (libs.empty()) return;

    Elf_Shdr & shdrDynamic = findSection(".dynamic");
    Elf_Shdr & shdrDynStr = findSection(".dynstr");
    char * strTab = (char *) contents + rdi(shdrDynStr.sh_offset);

    Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));
    Elf_Dyn * last = dyn;
    for ( ; rdi(dyn->d_tag) != DT_NULL; dyn++) {
        if (rdi(dyn->d_tag) == DT_NEEDED) {
            char * name = strTab + rdi(dyn->d_un.d_val);
            if (libs.find(name) != libs.end()) {
                debug("removing DT_NEEDED entry '%s'\n", name);
                changed = true;
            } else {
                debug("keeping DT_NEEDED entry '%s'\n", name);
                *last++ = *dyn;
            }
        } else
            *last++ = *dyn;
    }

    memset(last, 0, sizeof(Elf_Dyn) * (dyn - last));
}

template<ElfFileParams>
void ElfFile<ElfFileParamNames>::replaceNeeded(const std::map<std::string, std::string> & libs)
{
    if (libs.empty()) return;

    Elf_Shdr & shdrDynamic = findSection(".dynamic");
    Elf_Shdr & shdrDynStr = findSection(".dynstr");
    char * strTab = (char *) contents + rdi(shdrDynStr.sh_offset);

    Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));

    unsigned int verNeedNum = 0;

    unsigned int dynStrAddedBytes = 0;

    for ( ; rdi(dyn->d_tag) != DT_NULL; dyn++) {
        if (rdi(dyn->d_tag) == DT_NEEDED) {
            char * name = strTab + rdi(dyn->d_un.d_val);
            auto i = libs.find(name);
            if (i != libs.end()) {
                auto replacement = i->second;

                debug("replacing DT_NEEDED entry '%s' with '%s'\n", name, replacement.c_str());

                // technically, the string referred by d_val could be used otherwise, too (although unlikely)
                // we'll therefore add a new string
                debug("resizing .dynstr ...\n");

                std::string & newDynStr = replaceSection(".dynstr",
                    rdi(shdrDynStr.sh_size) + replacement.size() + 1 + dynStrAddedBytes);
                setSubstr(newDynStr, rdi(shdrDynStr.sh_size) + dynStrAddedBytes, replacement + '\0');

                wri(dyn->d_un.d_val, rdi(shdrDynStr.sh_size) + dynStrAddedBytes);

                dynStrAddedBytes += replacement.size() + 1;

                changed = true;
            } else {
                debug("keeping DT_NEEDED entry '%s'\n", name);
            }
        }
        if (rdi(dyn->d_tag) == DT_VERNEEDNUM) {
            verNeedNum = rdi(dyn->d_un.d_val);
        }
    }

    // If a replaced library uses symbol versions, then there will also be
    // references to it in the "version needed" table, and these also need to
    // be replaced.

    if (verNeedNum) {
        Elf_Shdr & shdrVersionR = findSection(".gnu.version_r");
        // The filename strings in the .gnu.version_r are different from the
        // ones in .dynamic: instead of being in .dynstr, they're in some
        // arbitrary section and we have to look in ->sh_link to figure out
        // which one.
        Elf_Shdr & shdrVersionRStrings = shdrs[rdi(shdrVersionR.sh_link)];
        // this is where we find the actual filename strings
        char * verStrTab = (char *) contents + rdi(shdrVersionRStrings.sh_offset);
        // and we also need the name of the section containing the strings, so
        // that we can pass it to replaceSection
        std::string versionRStringsSName = getSectionName(shdrVersionRStrings);

        debug("found .gnu.version_r with %i entries, strings in %s\n", verNeedNum, versionRStringsSName.c_str());

        unsigned int verStrAddedBytes = 0;

        Elf_Verneed * need = (Elf_Verneed *) (contents + rdi(shdrVersionR.sh_offset));
        while (verNeedNum > 0) {
            char * file = verStrTab + rdi(need->vn_file);
            auto i = libs.find(file);
            if (i != libs.end()) {
                auto replacement = i->second;

                debug("replacing .gnu.version_r entry '%s' with '%s'\n", file, replacement.c_str());
                debug("resizing string section %s ...\n", versionRStringsSName.c_str());

                std::string & newVerDynStr = replaceSection(versionRStringsSName,
                    rdi(shdrVersionRStrings.sh_size) + replacement.size() + 1 + verStrAddedBytes);
                setSubstr(newVerDynStr, rdi(shdrVersionRStrings.sh_size) + verStrAddedBytes, replacement + '\0');

                wri(need->vn_file, rdi(shdrVersionRStrings.sh_size) + verStrAddedBytes);

                verStrAddedBytes += replacement.size() + 1;

                changed = true;
            } else {
                debug("keeping .gnu.version_r entry '%s'\n", file);
            }
            // the Elf_Verneed structures form a linked list, so jump to next entry
            need = (Elf_Verneed *) (((char *) need) + rdi(need->vn_next));
            --verNeedNum;
        }
    }
}

template<ElfFileParams>
void ElfFile<ElfFileParamNames>::addNeeded(const std::set<std::string> & libs)
{
    if (libs.empty()) return;

    Elf_Shdr & shdrDynamic = findSection(".dynamic");
    Elf_Shdr & shdrDynStr = findSection(".dynstr");

    /* add all new libs to the dynstr string table */
    unsigned int length = 0;
    for (auto & i : libs) length += i.size() + 1;

    std::string & newDynStr = replaceSection(".dynstr",
        rdi(shdrDynStr.sh_size) + length + 1);
    std::set<Elf64_Xword> libStrings;
    unsigned int pos = 0;
    for (auto & i : libs) {
        setSubstr(newDynStr, rdi(shdrDynStr.sh_size) + pos, i + '\0');
        libStrings.insert(rdi(shdrDynStr.sh_size) + pos);
        pos += i.size() + 1;
    }

    /* add all new needed entries to the dynamic section */
    std::string & newDynamic = replaceSection(".dynamic",
        rdi(shdrDynamic.sh_size) + sizeof(Elf_Dyn) * libs.size());

    unsigned int idx = 0;
    for ( ; rdi(((Elf_Dyn *) newDynamic.c_str())[idx].d_tag) != DT_NULL; idx++) ;
    debug("DT_NULL index is %d\n", idx);

    /* Shift all entries down by the number of new entries. */
    setSubstr(newDynamic, sizeof(Elf_Dyn) * libs.size(),
        std::string(newDynamic, 0, sizeof(Elf_Dyn) * (idx + 1)));

    /* Add the DT_NEEDED entries at the top. */
    unsigned int i = 0;
    for (auto & j : libStrings) {
        Elf_Dyn newDyn;
        wri(newDyn.d_tag, DT_NEEDED);
        wri(newDyn.d_un.d_val, j);
        setSubstr(newDynamic, i * sizeof(Elf_Dyn), std::string((char *) &newDyn, sizeof(Elf_Dyn)));
        i++;
    }

    changed = true;
}

template<ElfFileParams>
void ElfFile<ElfFileParamNames>::printNeededLibs()
{
    Elf_Shdr & shdrDynamic = findSection(".dynamic");
    Elf_Shdr & shdrDynStr = findSection(".dynstr");
    char *strTab = (char *)contents + rdi(shdrDynStr.sh_offset);

    Elf_Dyn *dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));

    for (; rdi(dyn->d_tag) != DT_NULL; dyn++) {
        if (rdi(dyn->d_tag) == DT_NEEDED) {
            char *name = strTab + rdi(dyn->d_un.d_val);
            printf("%s\n", name);
        }
    }
}


template<ElfFileParams>
void ElfFile<ElfFileParamNames>::noDefaultLib()
{
    Elf_Shdr & shdrDynamic = findSection(".dynamic");

    Elf_Dyn * dyn = (Elf_Dyn *) (contents + rdi(shdrDynamic.sh_offset));
    Elf_Dyn * dynFlags1 = 0;
    for ( ; rdi(dyn->d_tag) != DT_NULL; dyn++) {
        if (rdi(dyn->d_tag) == DT_FLAGS_1) {
            dynFlags1 = dyn;
            break;
        }
    }
    if (dynFlags1) {
        if (dynFlags1->d_un.d_val & DF_1_NODEFLIB)
            return;
        dynFlags1->d_un.d_val |= DF_1_NODEFLIB;
    } else {
        std::string & newDynamic = replaceSection(".dynamic",
                rdi(shdrDynamic.sh_size) + sizeof(Elf_Dyn));

        unsigned int idx = 0;
        for ( ; rdi(((Elf_Dyn *) newDynamic.c_str())[idx].d_tag) != DT_NULL; idx++) ;
        debug("DT_NULL index is %d\n", idx);

        /* Shift all entries down by one. */
        setSubstr(newDynamic, sizeof(Elf_Dyn),
                std::string(newDynamic, 0, sizeof(Elf_Dyn) * (idx + 1)));

        /* Add the DT_FLAGS_1 entry at the top. */
        Elf_Dyn newDyn;
        wri(newDyn.d_tag, DT_FLAGS_1);
        newDyn.d_un.d_val = DF_1_NODEFLIB;
        setSubstr(newDynamic, 0, std::string((char *) &newDyn, sizeof(Elf_Dyn)));
    }

    changed = true;
}


static bool printInterpreter = false;
static bool printSoname = false;
static bool setSoname = false;
static std::string newSoname;
static std::string newInterpreter;
static bool shrinkRPath = false;
static std::vector<std::string> allowedRpathPrefixes;
static bool removeRPath = false;
static bool setRPath = false;
static bool printRPath = false;
static std::string newRPath;
static std::set<std::string> neededLibsToRemove;
static std::map<std::string, std::string> neededLibsToReplace;
static std::set<std::string> neededLibsToAdd;
static bool printNeeded = false;
static bool noDefaultLib = false;

template<class ElfFile>
static void patchElf2(ElfFile && elfFile, std::string fileName)
{
    if (printInterpreter)
        printf("%s\n", elfFile.getInterpreter().c_str());

    if (printSoname)
        elfFile.modifySoname(elfFile.printSoname, "");

    if (setSoname)
        elfFile.modifySoname(elfFile.replaceSoname, newSoname);

    if (newInterpreter != "")
        elfFile.setInterpreter(newInterpreter);

    if (printRPath)
        elfFile.modifyRPath(elfFile.rpPrint, {}, "");

    if (shrinkRPath)
        elfFile.modifyRPath(elfFile.rpShrink, allowedRpathPrefixes, "");
    else if (removeRPath)
        elfFile.modifyRPath(elfFile.rpRemove, {}, "");
    else if (setRPath)
        elfFile.modifyRPath(elfFile.rpSet, {}, newRPath);

    if (printNeeded) elfFile.printNeededLibs();

    elfFile.removeNeeded(neededLibsToRemove);
    elfFile.replaceNeeded(neededLibsToReplace);
    elfFile.addNeeded(neededLibsToAdd);

    if (noDefaultLib)
        elfFile.noDefaultLib();

    if (elfFile.isChanged()){
        elfFile.rewriteSections();
        writeFile(fileName, elfFile.fileContents);
    }
}


static void patchElf()
{
    for (auto fileName : fileNames) {
        if (!printInterpreter && !printRPath && !printSoname && !printNeeded)
            debug("patching ELF file '%s'\n", fileName.c_str());

        debug("Kernel page size is %u bytes\n", getPageSize());

        auto fileContents = readFile(fileName);

        if (getElfType(fileContents).is32Bit)
            patchElf2(ElfFile<Elf32_Ehdr, Elf32_Phdr, Elf32_Shdr, Elf32_Addr, Elf32_Off, Elf32_Dyn, Elf32_Sym, Elf32_Verneed>(fileContents), fileName);
        else
            patchElf2(ElfFile<Elf64_Ehdr, Elf64_Phdr, Elf64_Shdr, Elf64_Addr, Elf64_Off, Elf64_Dyn, Elf64_Sym, Elf64_Verneed>(fileContents), fileName);
    }
}


void showHelp(const std::string & progName)
{
        fprintf(stderr, "syntax: %s\n\
  [--set-interpreter FILENAME]\n\
  [--page-size SIZE]\n\
  [--print-interpreter]\n\
  [--print-soname]\t\tPrints 'DT_SONAME' entry of .dynamic section. Raises an error if DT_SONAME doesn't exist\n\
  [--set-soname SONAME]\t\tSets 'DT_SONAME' entry to SONAME.\n\
  [--set-rpath RPATH]\n\
  [--remove-rpath]\n\
  [--shrink-rpath]\n\
  [--allowed-rpath-prefixes PREFIXES]\t\tWith '--shrink-rpath', reject rpath entries not starting with the allowed prefix\n\
  [--print-rpath]\n\
  [--force-rpath]\n\
  [--add-needed LIBRARY]\n\
  [--remove-needed LIBRARY]\n\
  [--replace-needed LIBRARY NEW_LIBRARY]\n\
  [--print-needed]\n\
  [--no-default-lib]\n\
  [--debug]\n\
  [--version]\n\
  FILENAME...\n", progName.c_str());
}


int mainWrapped(int argc, char * * argv)
{
    if (argc <= 1) {
        showHelp(argv[0]);
        return 1;
    }

    if (getenv("PATCHELF_DEBUG") != 0) debugMode = true;

    int i;
    for (i = 1; i < argc; ++i) {
        std::string arg(argv[i]);
        if (arg == "--set-interpreter" || arg == "--interpreter") {
            if (++i == argc) error("missing argument");
            newInterpreter = argv[i];
        }
        else if (arg == "--page-size") {
            if (++i == argc) error("missing argument");
            pageSize = atoi(argv[i]);
            if (pageSize <= 0) error("invalid argument to --page-size");
        }
        else if (arg == "--print-interpreter") {
            printInterpreter = true;
        }
        else if (arg == "--print-soname") {
            printSoname = true;
        }
        else if (arg == "--set-soname") {
            if (++i == argc) error("missing argument");
            setSoname = true;
            newSoname = argv[i];
        }
        else if (arg == "--remove-rpath") {
            removeRPath = true;
        }
        else if (arg == "--shrink-rpath") {
            shrinkRPath = true;
        }
        else if (arg == "--allowed-rpath-prefixes") {
            if (++i == argc) error("missing argument");
            allowedRpathPrefixes = splitColonDelimitedString(argv[i]);
        }
        else if (arg == "--set-rpath") {
            if (++i == argc) error("missing argument");
            setRPath = true;
            newRPath = argv[i];
        }
        else if (arg == "--print-rpath") {
            printRPath = true;
        }
        else if (arg == "--force-rpath") {
            /* Generally we prefer to emit DT_RUNPATH instead of
               DT_RPATH, as the latter is obsolete.  However, there is
               a slight semantic difference: DT_RUNPATH is "scoped",
               it only affects the executable or library in question,
               not its recursive imports.  So maybe you really want to
               force the use of DT_RPATH.  That's what this option
               does.  Without it, DT_RPATH (if encountered) is
               converted to DT_RUNPATH, and if neither is present, a
               DT_RUNPATH is added.  With it, DT_RPATH isn't converted
               to DT_RUNPATH, and if neither is present, a DT_RPATH is
               added. */
            forceRPath = true;
        }
        else if (arg == "--print-needed") {
            printNeeded = true;
        }
        else if (arg == "--add-needed") {
            if (++i == argc) error("missing argument");
            neededLibsToAdd.insert(argv[i]);
        }
        else if (arg == "--remove-needed") {
            if (++i == argc) error("missing argument");
            neededLibsToRemove.insert(argv[i]);
        }
        else if (arg == "--replace-needed") {
            if (i+2 >= argc) error("missing argument(s)");
            neededLibsToReplace[ argv[i+1] ] = argv[i+2];
            i += 2;
        }
        else if (arg == "--debug") {
            debugMode = true;
        }
        else if (arg == "--no-default-lib") {
            noDefaultLib = true;
        }
        else if (arg == "--help" || arg == "-h" ) {
            showHelp(argv[0]);
            return 0;
        }
        else if (arg == "--version") {
            printf(PACKAGE_STRING "\n");
            return 0;
        }
        else {
            fileNames.push_back(arg);
        }
    }

    if (fileNames.empty()) error("missing filename");

    patchElf();

    return 0;
}

int main(int argc, char * * argv)
{
    try {
        return mainWrapped(argc, argv);
    } catch (std::exception & e) {
        fprintf(stderr, "patchelf: %s\n", e.what());
        return 1;
    }
}