elfreader.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
 
#include "elfreader.h"
 
#include <QDir>
 
QT_BEGIN_NAMESPACE
 
using namespace Qt::StringLiterals;
 
/* This is a copy of the ELF reader contained in Qt Creator (src/libs/utils),
 * extended by the dependencies() function to read out the dependencies of a dynamic executable. */
 
quint16 getHalfWord(const unsigned char *&s, const ElfData &context)
{
    quint16 res;
    if (context.endian == Elf_ELFDATA2MSB)
        res = qFromBigEndian<quint16>(s);
    else
        res = qFromLittleEndian<quint16>(s);
    s += 2;
    return res;
}
 
quint32 getWord(const unsigned char *&s, const ElfData &context)
{
    quint32 res;
    if (context.endian == Elf_ELFDATA2MSB)
        res = qFromBigEndian<quint32>(s);
    else
        res = qFromLittleEndian<quint32>(s);
    s += 4;
    return res;
}
 
quint64 getAddress(const unsigned char *&s, const ElfData &context)
{
    quint64 res;
    if (context.elfclass == Elf_ELFCLASS32) {
        if (context.endian == Elf_ELFDATA2MSB)
            res = qFromBigEndian<quint32>(s);
        else
            res = qFromLittleEndian<quint32>(s);
        s += 4;
    } else {
        if (context.endian == Elf_ELFDATA2MSB)
            res = qFromBigEndian<quint64>(s);
        else
            res = qFromLittleEndian<quint64>(s);
        s += 8;
    }
    return res;
}
 
quint64 getOffset(const unsigned char *&s, const ElfData &context)
{
    return getAddress(s, context);
}
 
static void parseSectionHeader(const uchar *s, ElfSectionHeader *sh, const ElfData &context)
{
    sh->index = getWord(s, context);
    sh->type = getWord(s, context);
    sh->flags = quint32(getOffset(s, context));
    sh->addr = getAddress(s, context);
    sh->offset = getOffset(s, context);
    sh->size = getOffset(s, context);
}
 
static void parseProgramHeader(const uchar *s, ElfProgramHeader *sh, const ElfData &context)
{
    sh->type = getWord(s, context);
    sh->offset = getOffset(s, context);
    /* p_vaddr = */ getAddress(s, context);
    /* p_paddr = */ getAddress(s, context);
    sh->filesz = getWord(s, context);
    sh->memsz = getWord(s, context);
}
 
class ElfMapper
{
public:
    ElfMapper(const ElfReader *reader) : file(reader->m_binary) {}
 
    bool map()
    {
        if (!file.open(QIODevice::ReadOnly))
            return false;
 
        fdlen = quint64(file.size());
        ustart = file.map(0, qint64(fdlen));
        if (ustart == 0) {
            // Try reading the data into memory instead.
            raw = file.readAll();
            start = raw.constData();
            fdlen = quint64(raw.size());
        }
        return true;
    }
 
public:
    QFile file;
    QByteArray raw;
    union { const char *start; const uchar *ustart; };
    quint64 fdlen;
};
 
ElfReader::ElfReader(const QString &binary)
    : m_binary(binary)
{
}
 
ElfData ElfReader::readHeaders()
{
    readIt();
    return m_elfData;
}
 
static inline QString msgInvalidElfObject(const QString &binary, const QString &why)
{
    return QStringLiteral("'%1' is an invalid ELF object (%2)")
           .arg(QDir::toNativeSeparators(binary), why);
}
 
ElfReader::Result ElfReader::readIt()
{
    if (!m_elfData.sectionHeaders.isEmpty())
        return Ok;
    if (!m_elfData.programHeaders.isEmpty())
        return Ok;
 
    ElfMapper mapper(this);
    if (!mapper.map())
        return Corrupt;
 
    const quint64 fdlen = mapper.fdlen;
 
    if (fdlen < 64) {
        m_errorString = QStringLiteral("'%1' is not an ELF object (file too small)").arg(QDir::toNativeSeparators(m_binary));
        return NotElf;
    }
 
    if (strncmp(mapper.start, "\177ELF", 4) != 0) {
        m_errorString = QStringLiteral("'%1' is not an ELF object").arg(QDir::toNativeSeparators(m_binary));
        return NotElf;
    }
 
    // 32 or 64 bit
    m_elfData.elfclass = ElfClass(mapper.start[4]);
    const bool is64Bit = m_elfData.elfclass == Elf_ELFCLASS64;
    if (m_elfData.elfclass != Elf_ELFCLASS32 && m_elfData.elfclass != Elf_ELFCLASS64) {
        m_errorString = msgInvalidElfObject(m_binary, QStringLiteral("odd cpu architecture"));
        return Corrupt;
    }
 
    // int bits = (data[4] << 5);
    // If you remove this check to read ELF objects of a different arch,
    // please make sure you modify the typedefs
    // to match the _plugin_ architecture.
    // if ((sizeof(void*) == 4 && bits != 32)
    //     || (sizeof(void*) == 8 && bits != 64)) {
    //     if (errorString)
    //         *errorString = QLibrary::QStringLiteral("'%1' is an invalid ELF object (%2)")
    //         .arg(m_binary).arg("wrong cpu architecture"_L1);
    //     return Corrupt;
    // }
 
    // Read Endianhness.
    m_elfData.endian = ElfEndian(mapper.ustart[5]);
    if (m_elfData.endian != Elf_ELFDATA2LSB && m_elfData.endian != Elf_ELFDATA2MSB) {
        m_errorString = msgInvalidElfObject(m_binary, QStringLiteral("odd endianness"));
        return Corrupt;
    }
 
    const uchar *data = mapper.ustart + 16; // e_ident
    m_elfData.elftype    = ElfType(getHalfWord(data, m_elfData));
    m_elfData.elfmachine = ElfMachine(getHalfWord(data, m_elfData));
    /* e_version = */   getWord(data, m_elfData);
    m_elfData.entryPoint = getAddress(data, m_elfData);
 
    quint64 e_phoff   = getOffset(data, m_elfData);
    quint64 e_shoff   = getOffset(data, m_elfData);
    /* e_flags = */     getWord(data, m_elfData);
 
    quint32 e_shsize  = getHalfWord(data, m_elfData);
 
    if (e_shsize > fdlen) {
        m_errorString = msgInvalidElfObject(m_binary, QStringLiteral("unexpected e_shsize"));
        return Corrupt;
    }
 
    quint32 e_phentsize = getHalfWord(data, m_elfData);
    if (e_phentsize != (is64Bit ? 56 : 32)) {
        m_errorString = msgInvalidElfObject(m_binary, QStringLiteral("invalid structure"));
        return ElfReader::Corrupt;
    }
    quint32 e_phnum     = getHalfWord(data, m_elfData);
 
    quint32 e_shentsize = getHalfWord(data, m_elfData);
 
    if (e_shentsize % 4) {
        m_errorString = msgInvalidElfObject(m_binary, QStringLiteral("unexpected e_shentsize"));
        return Corrupt;
    }
 
    quint32 e_shnum     = getHalfWord(data, m_elfData);
    quint32 e_shtrndx   = getHalfWord(data, m_elfData);
    if (data != mapper.ustart + (is64Bit ? 64 : 52)) {
        m_errorString = msgInvalidElfObject(m_binary, QStringLiteral("unexpected e_phentsize"));
        return ElfReader::Corrupt;
    }
 
    if (quint64(e_shnum) * e_shentsize > fdlen) {
        const QString reason = QStringLiteral("announced %1 sections, each %2 bytes, exceed file size").arg(e_shnum).arg(e_shentsize);
        m_errorString = msgInvalidElfObject(m_binary, reason);
        return Corrupt;
    }
 
    quint64 soff = e_shoff + e_shentsize * e_shtrndx;
 
//    if ((soff + e_shentsize) > fdlen || soff % 4 || soff == 0) {
//        m_errorString = QLibrary::QStringLiteral("'%1' is an invalid ELF object (%2)")
//           .arg(m_binary)
//           .arg("shstrtab section header seems to be at %1"_L1)
//           .arg(QString::number(soff, 16));
//        return Corrupt;
//    }
 
    if (e_shoff) {
        ElfSectionHeader strtab;
        parseSectionHeader(mapper.ustart + soff, &strtab, m_elfData);
        const quint64 stringTableFileOffset = strtab.offset;
        if (quint32(stringTableFileOffset + e_shentsize) >= fdlen
                || stringTableFileOffset == 0) {
            const QString reason = QStringLiteral("string table seems to be at 0x%1").arg(soff, 0, 16);
            m_errorString = msgInvalidElfObject(m_binary, reason);
            return Corrupt;
        }
 
        for (quint32 i = 0; i < e_shnum; ++i) {
            const uchar *s = mapper.ustart + e_shoff + i * e_shentsize;
            ElfSectionHeader sh;
            parseSectionHeader(s, &sh, m_elfData);
 
            if (stringTableFileOffset + sh.index > fdlen) {
                const QString reason = QStringLiteral("section name %1 of %2 behind end of file")
                                       .arg(i).arg(e_shnum);
                m_errorString = msgInvalidElfObject(m_binary, reason);
                return Corrupt;
            }
 
            sh.name = mapper.start + stringTableFileOffset + sh.index;
            if (sh.name == ".gdb_index") {
                m_elfData.symbolsType = FastSymbols;
            } else if (sh.name == ".debug_info") {
                m_elfData.symbolsType = PlainSymbols;
            } else if (sh.name == ".gnu_debuglink") {
                m_elfData.debugLink = QByteArray(mapper.start + sh.offset);
                m_elfData.symbolsType = LinkedSymbols;
            } else if (sh.name == ".note.gnu.build-id") {
                m_elfData.symbolsType = BuildIdSymbols;
                if (sh.size > 16)
                    m_elfData.buildId = QByteArray(mapper.start + sh.offset + 16,
                                                   int(sh.size) - 16).toHex();
            }
            m_elfData.sectionHeaders.append(sh);
        }
    }
 
    if (e_phoff) {
        for (quint32 i = 0; i < e_phnum; ++i) {
            const uchar *s = mapper.ustart + e_phoff + i * e_phentsize;
            ElfProgramHeader ph;
            parseProgramHeader(s, &ph, m_elfData);
            m_elfData.programHeaders.append(ph);
        }
    }
    return Ok;
}
 
QByteArray ElfReader::readSection(const QByteArray &name)
{
    readIt();
    int i = m_elfData.indexOf(name);
    if (i == -1)
        return QByteArray();
 
    ElfMapper mapper(this);
    if (!mapper.map())
        return QByteArray();
 
    const ElfSectionHeader &section = m_elfData.sectionHeaders.at(i);
    return QByteArray(mapper.start + section.offset, int(section.size));
}
 
static QByteArray cutout(const char *s)
{
    QByteArray res(s, 80);
    const int pos = res.indexOf('\0');
    if (pos != -1)
        res.resize(pos - 1);
    return res;
}
 
QByteArray ElfReader::readCoreName(bool *isCore)
{
    *isCore = false;
 
    readIt();
 
    ElfMapper mapper(this);
    if (!mapper.map())
        return QByteArray();
 
    if (m_elfData.elftype != Elf_ET_CORE)
        return QByteArray();
 
    *isCore = true;
 
    for (int i = 0, n = m_elfData.sectionHeaders.size(); i != n; ++i)
        if (m_elfData.sectionHeaders.at(i).type == Elf_SHT_NOTE) {
            const ElfSectionHeader &header = m_elfData.sectionHeaders.at(i);
            return cutout(mapper.start + header.offset + 0x40);
        }
 
    for (int i = 0, n = m_elfData.programHeaders.size(); i != n; ++i)
        if (m_elfData.programHeaders.at(i).type == Elf_PT_NOTE) {
            const ElfProgramHeader &header = m_elfData.programHeaders.at(i);
            return cutout(mapper.start + header.offset + 0xec);
        }
 
    return QByteArray();
}
 
int ElfData::indexOf(const QByteArray &name) const
{
    for (int i = 0, n = sectionHeaders.size(); i != n; ++i)
        if (sectionHeaders.at(i).name == name)
            return i;
    return -1;
}
 
/* Helpers for reading out the .dynamic section containing the dependencies.
 * The ".dynamic" section is an array of
 * typedef struct {
 *      Elf32_Sword d_tag;
 *      union {
 *          Elf32_Word  d_val;
 *          dElf32_Addr d_ptr;
 *     } d_un;
 *  } Elf32_Dyn
 * with entries where a tag DT_NEEDED indicates that m_val is an offset into
 * the string table ".dynstr". The documentation states that entries with the
 * tag DT_STRTAB contain an offset for the string table to be used, but that
 * has been found not to contain valid entries. */
 
enum DynamicSectionTags {
    DT_NULL = 0,
    DT_NEEDED = 1,
    DT_STRTAB = 5,
    DT_SONAME = 14,
    DT_RPATH = 15
};
 
QList<QByteArray> ElfReader::dependencies()
{
    QList<QByteArray> result;
 
    ElfMapper mapper(this);
    if (!mapper.map()) {
        m_errorString = QStringLiteral("Mapper failure");
        return result;
    }
    quint64 dynStrOffset = 0;
    quint64 dynamicOffset = 0;
    quint64 dynamicSize = 0;
 
    const QList<ElfSectionHeader> &headers = readHeaders().sectionHeaders;
    for (const ElfSectionHeader &eh : headers) {
        if (eh.name  == QByteArrayLiteral(".dynstr")) {
            dynStrOffset = eh.offset;
        } else if (eh.name  == QByteArrayLiteral(".dynamic")) {
            dynamicOffset = eh.offset;
            dynamicSize = eh.size;
        }
        if (dynStrOffset && dynamicOffset)
            break;
    }
 
    if (!dynStrOffset || !dynamicOffset) {
        m_errorString = QStringLiteral("Not a dynamically linked executable.");
        return result;
    }
 
    const unsigned char *dynamicData = mapper.ustart + dynamicOffset;
    const unsigned char *dynamicDataEnd = dynamicData + dynamicSize;
    while (dynamicData < dynamicDataEnd) {
        const quint32 tag = getWord(dynamicData, m_elfData);
        if (tag == DT_NULL)
            break;
        if (m_elfData.elfclass == Elf_ELFCLASS64)
            dynamicData += sizeof(quint32); // padding to d_val/d_ptr.
        if (tag == DT_NEEDED) {
            const quint32 offset = getWord(dynamicData, m_elfData);
            if (m_elfData.elfclass == Elf_ELFCLASS64)
                dynamicData += sizeof(quint32); // past d_ptr.
            const char *name = mapper.start + dynStrOffset + offset;
            result.push_back(name);
        } else {
             dynamicData += m_elfData.elfclass == Elf_ELFCLASS64 ? 8 : 4;
        }
    }
    return result;
}
 
QT_END_NAMESPACE