ladybird/Userland/Libraries/LibELF/DynamicObject.cpp

/*
 * Copyright (c) 2019-2020, Andrew Kaster <akaster@serenityos.org>
 * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Debug.h>
#include <AK/String.h>
#include <AK/StringBuilder.h>
#include <LibC/elf.h>
#include <LibELF/DynamicLoader.h>
#include <LibELF/DynamicObject.h>
#include <LibELF/Hashes.h>
#include <string.h>

namespace ELF {

static const char* name_for_dtag(ElfW(Sword) d_tag);

DynamicObject::DynamicObject(const String& filename, VirtualAddress base_address, VirtualAddress dynamic_section_address)
    : m_filename(filename)
    , m_base_address(base_address)
    , m_dynamic_address(dynamic_section_address)
{
    auto* header = (ElfW(Ehdr)*)base_address.as_ptr();
    auto* pheader = (ElfW(Phdr)*)(base_address.as_ptr() + header->e_phoff);
    m_elf_base_address = VirtualAddress(pheader->p_vaddr - pheader->p_offset);
    if (header->e_type == ET_DYN)
        m_is_elf_dynamic = true;
    else
        m_is_elf_dynamic = false;

    parse();
}

DynamicObject::~DynamicObject()
{
    // TODO: unmap the object
}

void DynamicObject::dump() const
{
    StringBuilder builder;
    builder.append("\nd_tag      tag_name         value\n");
    size_t num_dynamic_sections = 0;

    for_each_dynamic_entry([&](const DynamicObject::DynamicEntry& entry) {
        String name_field = String::formatted("({})", name_for_dtag(entry.tag()));
        builder.appendff("{:#08x} {:17} {:#08x}\n", entry.tag(), name_field, entry.val());
        num_dynamic_sections++;
    });

    if (m_has_soname)
        builder.appendff("DT_SONAME: {}\n", soname()); // FIXME: Validate that this string is null terminated?
    if (m_has_rpath)
        builder.appendff("DT_RPATH: {}\n", rpath());
    if (m_has_runpath)
        builder.appendff("DT_RUNPATH: {}\n", runpath());

    dbgln_if(DYNAMIC_LOAD_DEBUG, "Dynamic section at address {} contains {} entries:", m_dynamic_address.as_ptr(), num_dynamic_sections);
    dbgln_if(DYNAMIC_LOAD_DEBUG, "{}", builder.string_view());
}

void DynamicObject::parse()
{
    for_each_dynamic_entry([&](const DynamicEntry& entry) {
        switch (entry.tag()) {
        case DT_INIT:
            m_init_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_FINI:
            m_fini_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_INIT_ARRAY:
            m_init_array_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_INIT_ARRAYSZ:
            m_init_array_size = entry.val();
            break;
        case DT_FINI_ARRAY:
            m_fini_array_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_FINI_ARRAYSZ:
            m_fini_array_size = entry.val();
            break;
        case DT_HASH:
            // Use SYSV hash only if GNU hash is not available
            if (m_hash_type == HashType::SYSV) {
                m_hash_table_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            }
            break;
        case DT_GNU_HASH:
            m_hash_type = HashType::GNU;
            m_hash_table_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_SYMTAB:
            m_symbol_table_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_STRTAB:
            m_string_table_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_STRSZ:
            m_size_of_string_table = entry.val();
            break;
        case DT_SYMENT:
            m_size_of_symbol_table_entry = entry.val();
            break;
        case DT_PLTGOT:
            m_procedure_linkage_table_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_PLTRELSZ:
            m_size_of_plt_relocation_entry_list = entry.val();
            break;
        case DT_PLTREL:
            m_procedure_linkage_table_relocation_type = entry.val();
            VERIFY(m_procedure_linkage_table_relocation_type & (DT_REL | DT_RELA));
            break;
        case DT_JMPREL:
            m_plt_relocation_offset_location = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_RELA:
            m_addend_used = true;
            [[fallthrough]];
        case DT_REL:
            m_relocation_table_offset = entry.ptr() - (FlatPtr)m_elf_base_address.as_ptr();
            break;
        case DT_RELASZ:
        case DT_RELSZ:
            m_size_of_relocation_table = entry.val();
            break;
        case DT_RELAENT:
        case DT_RELENT:
            m_size_of_relocation_entry = entry.val();
            break;
        case DT_RELACOUNT:
        case DT_RELCOUNT:
            m_number_of_relocations = entry.val();
            break;
        case DT_FLAGS:
            m_dt_flags = entry.val();
            break;
        case DT_TEXTREL:
            m_dt_flags |= DF_TEXTREL; // This tag seems to exist for legacy reasons only?
            break;
        case DT_SONAME:
            m_soname_index = entry.val();
            m_has_soname = true;
            break;
        case DT_BIND_NOW:
            m_dt_flags |= DF_BIND_NOW;
            break;
        case DT_RPATH:
            m_rpath_index = entry.val();
            m_has_rpath = true;
            break;
        case DT_RUNPATH:
            m_runpath_index = entry.val();
            m_has_runpath = true;
            break;
        case DT_DEBUG:
            break;
        case DT_FLAGS_1:
            break;
        case DT_NEEDED:
            // We handle these in for_each_needed_library
            break;
        case DT_SYMBOLIC:
            break;
        default:
            dbgln("DynamicObject: DYNAMIC tag handling not implemented for DT_{}", name_for_dtag(entry.tag()));
            VERIFY_NOT_REACHED(); // FIXME: Maybe just break out here and return false?
            break;
        }
    });

    if (!m_size_of_relocation_entry) {
        // TODO: FIXME, this shouldn't be hardcoded
        // The reason we need this here is that for some reason, when there only PLT relocations, the compiler
        // doesn't insert a 'PLTRELSZ' entry to the dynamic section
        m_size_of_relocation_entry = sizeof(ElfW(Rel));
    }

    auto hash_section_address = hash_section().address().as_ptr();
    // TODO: consider base address - it might not be zero
    auto num_hash_chains = ((u32*)hash_section_address)[1];
    m_symbol_count = num_hash_chains;
}

DynamicObject::Relocation DynamicObject::RelocationSection::relocation(unsigned index) const
{
    VERIFY(index < entry_count());
    unsigned offset_in_section = index * entry_size();
    auto relocation_address = (ElfW(Rela)*)address().offset(offset_in_section).as_ptr();
    return Relocation(m_dynamic, *relocation_address, offset_in_section, m_addend_used);
}

DynamicObject::Relocation DynamicObject::RelocationSection::relocation_at_offset(unsigned offset) const
{
    VERIFY(offset <= (m_section_size_bytes - m_entry_size));
    auto relocation_address = (ElfW(Rela)*)address().offset(offset).as_ptr();
    return Relocation(m_dynamic, *relocation_address, offset, m_addend_used);
}

DynamicObject::Symbol DynamicObject::symbol(unsigned index) const
{
    auto symbol_section = Section(*this, m_symbol_table_offset, (m_symbol_count * m_size_of_symbol_table_entry), m_size_of_symbol_table_entry, "DT_SYMTAB");
    auto symbol_entry = (ElfW(Sym)*)symbol_section.address().offset(index * symbol_section.entry_size()).as_ptr();
    return Symbol(*this, index, *symbol_entry);
}

DynamicObject::Section DynamicObject::init_section() const
{
    return Section(*this, m_init_offset, sizeof(void (*)()), sizeof(void (*)()), "DT_INIT"sv);
}

DynamicObject::Section DynamicObject::fini_section() const
{
    return Section(*this, m_fini_offset, sizeof(void (*)()), sizeof(void (*)()), "DT_FINI"sv);
}

DynamicObject::Section DynamicObject::init_array_section() const
{
    return Section(*this, m_init_array_offset, m_init_array_size, sizeof(void (*)()), "DT_INIT_ARRAY"sv);
}

DynamicObject::Section DynamicObject::fini_array_section() const
{
    return Section(*this, m_fini_array_offset, m_fini_array_size, sizeof(void (*)()), "DT_FINI_ARRAY"sv);
}

DynamicObject::RelocationSection DynamicObject::relocation_section() const
{
    return RelocationSection(Section(*this, m_relocation_table_offset, m_size_of_relocation_table, m_size_of_relocation_entry, "DT_REL"sv), m_addend_used);
}

DynamicObject::RelocationSection DynamicObject::plt_relocation_section() const
{
    return RelocationSection(Section(*this, m_plt_relocation_offset_location, m_size_of_plt_relocation_entry_list, m_size_of_relocation_entry, "DT_JMPREL"sv), false);
}

ElfW(Half) DynamicObject::program_header_count() const
{
    auto* header = (const ElfW(Ehdr)*)m_base_address.as_ptr();
    return header->e_phnum;
}

const ElfW(Phdr) * DynamicObject::program_headers() const
{
    auto* header = (const ElfW(Ehdr)*)m_base_address.as_ptr();
    return (const ElfW(Phdr)*)(m_base_address.as_ptr() + header->e_phoff);
}

auto DynamicObject::HashSection::lookup_sysv_symbol(const StringView& name, u32 hash_value) const -> Optional<Symbol>
{
    u32* hash_table_begin = (u32*)address().as_ptr();
    size_t num_buckets = hash_table_begin[0];

    // This is here for completeness, but, since we're using the fact that every chain
    // will end at chain 0 (which means 'not found'), we don't need to check num_chains.
    // Interestingly, num_chains is required to be num_symbols

    // size_t num_chains = hash_table_begin[1];

    u32* buckets = &hash_table_begin[2];
    u32* chains = &buckets[num_buckets];

    for (u32 i = buckets[hash_value % num_buckets]; i; i = chains[i]) {
        auto symbol = m_dynamic.symbol(i);
        if (name == symbol.name()) {
            dbgln_if(DYNAMIC_LOAD_DEBUG, "Returning SYSV dynamic symbol with index {} for {}: {}", i, symbol.name(), symbol.address().as_ptr());
            return symbol;
        }
    }
    return {};
}

auto DynamicObject::HashSection::lookup_gnu_symbol(const StringView& name, u32 hash_value) const -> Optional<Symbol>
{
    // Algorithm reference: https://ent-voy.blogspot.com/2011/02/
    using BloomWord = FlatPtr;
    constexpr size_t bloom_word_size = sizeof(BloomWord) * 8;

    const u32* hash_table_begin = (u32*)address().as_ptr();

    const size_t num_buckets = hash_table_begin[0];
    const size_t num_omitted_symbols = hash_table_begin[1];
    const u32 num_maskwords = hash_table_begin[2];
    // This works because num_maskwords is required to be a power of 2
    const u32 num_maskwords_bitmask = num_maskwords - 1;
    const u32 shift2 = hash_table_begin[3];

    const BloomWord* bloom_words = (BloomWord const*)&hash_table_begin[4];
    const u32* const buckets = (u32 const*)&bloom_words[num_maskwords];
    const u32* const chains = &buckets[num_buckets];

    BloomWord hash1 = hash_value;
    BloomWord hash2 = hash1 >> shift2;
    const BloomWord bitmask = ((BloomWord)1 << (hash1 % bloom_word_size)) | ((BloomWord)1 << (hash2 % bloom_word_size));

    if ((bloom_words[(hash1 / bloom_word_size) & num_maskwords_bitmask] & bitmask) != bitmask)
        return {};

    size_t current_sym = buckets[hash1 % num_buckets];
    if (current_sym == 0)
        return {};
    const u32* current_chain = &chains[current_sym - num_omitted_symbols];

    for (hash1 &= ~1;; ++current_sym) {
        hash2 = *(current_chain++);
        auto symbol = m_dynamic.symbol(current_sym);
        if ((hash1 == (hash2 & ~1)) && name == symbol.raw_name())
            return symbol;
        if (hash2 & 1)
            break;
    }

    return {};
}

StringView DynamicObject::symbol_string_table_string(ElfW(Word) index) const
{
    return StringView { (const char*)base_address().offset(m_string_table_offset + index).as_ptr() };
}

const char* DynamicObject::raw_symbol_string_table_string(ElfW(Word) index) const
{
    return (const char*)base_address().offset(m_string_table_offset + index).as_ptr();
}

DynamicObject::InitializationFunction DynamicObject::init_section_function() const
{
    VERIFY(has_init_section());
    return (InitializationFunction)init_section().address().as_ptr();
}

static const char* name_for_dtag(ElfW(Sword) d_tag)
{
    switch (d_tag) {
    case DT_NULL:
        return "NULL"; /* marks end of _DYNAMIC array */
    case DT_NEEDED:
        return "NEEDED"; /* string table offset of needed lib */
    case DT_PLTRELSZ:
        return "PLTRELSZ"; /* size of relocation entries in PLT */
    case DT_PLTGOT:
        return "PLTGOT"; /* address PLT/GOT */
    case DT_HASH:
        return "HASH"; /* address of symbol hash table */
    case DT_STRTAB:
        return "STRTAB"; /* address of string table */
    case DT_SYMTAB:
        return "SYMTAB"; /* address of symbol table */
    case DT_RELA:
        return "RELA"; /* address of relocation table */
    case DT_RELASZ:
        return "RELASZ"; /* size of relocation table */
    case DT_RELAENT:
        return "RELAENT"; /* size of relocation entry */
    case DT_STRSZ:
        return "STRSZ"; /* size of string table */
    case DT_SYMENT:
        return "SYMENT"; /* size of symbol table entry */
    case DT_INIT:
        return "INIT"; /* address of initialization func. */
    case DT_FINI:
        return "FINI"; /* address of termination function */
    case DT_SONAME:
        return "SONAME"; /* string table offset of shared obj */
    case DT_RPATH:
        return "RPATH"; /* string table offset of library search path */
    case DT_SYMBOLIC:
        return "SYMBOLIC"; /* start sym search in shared obj. */
    case DT_REL:
        return "REL"; /* address of rel. tbl. w addends */
    case DT_RELSZ:
        return "RELSZ"; /* size of DT_REL relocation table */
    case DT_RELENT:
        return "RELENT"; /* size of DT_REL relocation entry */
    case DT_PLTREL:
        return "PLTREL"; /* PLT referenced relocation entry */
    case DT_DEBUG:
        return "DEBUG"; /* bugger */
    case DT_TEXTREL:
        return "TEXTREL"; /* Allow rel. mod. to unwritable seg */
    case DT_JMPREL:
        return "JMPREL"; /* add. of PLT's relocation entries */
    case DT_BIND_NOW:
        return "BIND_NOW"; /* Bind now regardless of env setting */
    case DT_INIT_ARRAY:
        return "INIT_ARRAY"; /* address of array of init func */
    case DT_FINI_ARRAY:
        return "FINI_ARRAY"; /* address of array of term func */
    case DT_INIT_ARRAYSZ:
        return "INIT_ARRAYSZ"; /* size of array of init func */
    case DT_FINI_ARRAYSZ:
        return "FINI_ARRAYSZ"; /* size of array of term func */
    case DT_RUNPATH:
        return "RUNPATH"; /* strtab offset of lib search path */
    case DT_FLAGS:
        return "FLAGS"; /* Set of DF_* flags */
    case DT_ENCODING:
        return "ENCODING"; /* further DT_* follow encoding rules */
    case DT_PREINIT_ARRAY:
        return "PREINIT_ARRAY"; /* address of array of preinit func */
    case DT_PREINIT_ARRAYSZ:
        return "PREINIT_ARRAYSZ"; /* size of array of preinit func */
    case DT_LOOS:
        return "LOOS"; /* reserved range for OS */
    case DT_HIOS:
        return "HIOS"; /*  specific dynamic array tags */
    case DT_LOPROC:
        return "LOPROC"; /* reserved range for processor */
    case DT_HIPROC:
        return "HIPROC"; /*  specific dynamic array tags */
    case DT_GNU_HASH:
        return "GNU_HASH"; /* address of GNU hash table */
    case DT_RELACOUNT:
        return "RELACOUNT"; /* if present, number of RELATIVE */
    case DT_RELCOUNT:
        return "RELCOUNT"; /* relocs, which must come first */
    case DT_FLAGS_1:
        return "FLAGS_1";
    default:
        return "??";
    }
}

auto DynamicObject::lookup_symbol(const StringView& name) const -> Optional<SymbolLookupResult>
{
    return lookup_symbol(HashSymbol { name });
}

auto DynamicObject::lookup_symbol(const HashSymbol& symbol) const -> Optional<SymbolLookupResult>
{
    auto result = hash_section().lookup_symbol(symbol);
    if (!result.has_value())
        return {};
    auto symbol_result = result.value();
    if (symbol_result.is_undefined())
        return {};
    return SymbolLookupResult { symbol_result.value(), symbol_result.size(), symbol_result.address(), symbol_result.bind(), this };
}

NonnullRefPtr<DynamicObject> DynamicObject::create(const String& filename, VirtualAddress base_address, VirtualAddress dynamic_section_address)
{
    return adopt_ref(*new DynamicObject(filename, base_address, dynamic_section_address));
}

// offset is in PLT relocation table
VirtualAddress DynamicObject::patch_plt_entry(u32 relocation_offset)
{
    auto relocation = plt_relocation_section().relocation_at_offset(relocation_offset);
#if ARCH(I386)
    VERIFY(relocation.type() == R_386_JMP_SLOT);
#else
    VERIFY(relocation.type() == R_X86_64_JUMP_SLOT);
#endif
    auto symbol = relocation.symbol();
    u8* relocation_address = relocation.address().as_ptr();

    VirtualAddress symbol_location;
    auto result = DynamicLoader::lookup_symbol(symbol);
    if (result.has_value()) {
        symbol_location = result.value().address;
    } else if (symbol.bind() != STB_WEAK) {
        dbgln("did not find symbol while doing relocations for library {}: {}", m_filename, symbol.name());
        VERIFY_NOT_REACHED();
    }

    dbgln_if(DYNAMIC_LOAD_DEBUG, "DynamicLoader: Jump slot relocation: putting {} ({}) into PLT at {}", symbol.name(), symbol_location, (void*)relocation_address);

    *(FlatPtr*)relocation_address = symbol_location.get();

    return symbol_location;
}

u32 DynamicObject::HashSymbol::gnu_hash() const
{
    if (!m_gnu_hash.has_value())
        m_gnu_hash = compute_gnu_hash(m_name);
    return m_gnu_hash.value();
}

u32 DynamicObject::HashSymbol::sysv_hash() const
{
    if (!m_sysv_hash.has_value())
        m_sysv_hash = compute_sysv_hash(m_name);
    return m_sysv_hash.value();
}

void* DynamicObject::symbol_for_name(const StringView& name)
{
    auto result = hash_section().lookup_symbol(name);
    if (!result.has_value())
        return nullptr;
    auto symbol = result.value();
    if (symbol.is_undefined())
        return nullptr;
    return base_address().offset(symbol.value()).as_ptr();
}
} // end namespace ELF