sled/3party/libelfin/elf/elf.cc

// Copyright (c) 2013 Austin T. Clements. All rights reserved.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.

#include "elf++.hh"

#include <cstring>

using namespace std;

ELFPP_BEGIN_NAMESPACE

template<template<typename E, byte_order Order> class Hdr>
void canon_hdr(Hdr<Elf64, byte_order::native> *out, const void *data,
               elfclass ei_class, elfdata ei_data)
{
        switch (ei_class) {
        case elfclass::_32:
                switch (ei_data) {
                case elfdata::lsb:
                        out->from(*(Hdr<Elf32, byte_order::lsb>*)data);
                        break;
                case elfdata::msb:
                        out->from(*(Hdr<Elf32, byte_order::msb>*)data);
                        break;
                }
                break;
        case elfclass::_64:
                switch (ei_data) {
                case elfdata::lsb:
                        out->from(*(Hdr<Elf64, byte_order::lsb>*)data);
                        break;
                case elfdata::msb:
                        out->from(*(Hdr<Elf64, byte_order::msb>*)data);
                        return;
                }
        }
}

//////////////////////////////////////////////////////////////////
// class elf
//

struct elf::impl
{
        impl(const shared_ptr<loader> &l)
                : l(l) { }

        const shared_ptr<loader> l;
        Ehdr<> hdr;
        vector<section> sections;
        vector<segment> segments;

        section invalid_section;
        segment invalid_segment;
};

elf::elf(const std::shared_ptr<loader> &l)
        : m(make_shared<impl>(l))
{
        // Read the first six bytes to check the magic number, ELF
        // class, and byte order.
        struct core_hdr
        {
                char ei_magic[4];
                elfclass ei_class;
                elfdata ei_data;
                unsigned char ei_version;
        } *core_hdr = (struct core_hdr*)l->load(0, sizeof *core_hdr);

        // Check basic header
        if (strncmp(core_hdr->ei_magic, "\x7f" "ELF", 4) != 0)
                throw format_error("bad ELF magic number");
        if (core_hdr->ei_version != 1)
                throw format_error("unknown ELF version");
        if (core_hdr->ei_class != elfclass::_32 &&
            core_hdr->ei_class != elfclass::_64)
                throw format_error("bad ELF class");
        if (core_hdr->ei_data != elfdata::lsb &&
            core_hdr->ei_data != elfdata::msb)
                throw format_error("bad ELF data order");

        // Read in the real header and canonicalize it
        size_t hdr_size = (core_hdr->ei_class == elfclass::_32 ?
                           sizeof(Ehdr<Elf32>) : sizeof(Ehdr<Elf64>));
        const void *hdr = l->load(0, hdr_size);
        canon_hdr(&m->hdr, hdr, core_hdr->ei_class, core_hdr->ei_data);

        // More checks
        if (m->hdr.version != 1)
                throw format_error("bad section ELF version");
        if (m->hdr.shnum && m->hdr.shstrndx >= m->hdr.shnum)
                throw format_error("bad section name string table index");

        // Load segments
        const void *seg_data = l->load(m->hdr.phoff,
                                       m->hdr.phentsize * m->hdr.phnum);
        for (unsigned i = 0; i < m->hdr.phnum; i++) {
                const void *seg = ((const char*)seg_data) + i * m->hdr.phentsize;
                m->segments.push_back(segment(*this, seg));
        }

        // Load sections
        const void *sec_data = l->load(m->hdr.shoff,
                                       m->hdr.shentsize * m->hdr.shnum);
        for (unsigned i = 0; i < m->hdr.shnum; i++) {
                const void *sec = ((const char*)sec_data) + i * m->hdr.shentsize;
                // XXX Circular reference.  Maybe this should be
                // constructed on the fly?  Canonicalizing the header
                // isn't super-cheap.
                m->sections.push_back(section(*this, sec));
        }
}

const Ehdr<> &
elf::get_hdr() const
{
        return m->hdr;
}

shared_ptr<loader>
elf::get_loader() const
{
        return m->l;
}

const std::vector<section> &
elf::sections() const
{
        return m->sections;
}

const std::vector<segment> &
elf::segments() const
{
        return m->segments;
}

const section &
elf::get_section(const std::string &name) const
{
        for (auto &sec : sections())
                if (name == sec.get_name(nullptr))
                        return sec;
        return m->invalid_section;
}

const section &
elf::get_section(unsigned index) const
{
        if (index >= sections().size())
                return m->invalid_section;
        return sections().at(index);
}

const segment&
elf::get_segment(unsigned index) const
{
        if (index >= segments().size())
                return m->invalid_segment;
        return segments().at(index);
}

//////////////////////////////////////////////////////////////////
// class segment
//

struct segment::impl {
        impl(const elf &f)
                : f(f), data(nullptr) { }

        const elf f;
        Phdr<> hdr;
        const void *data;
};

segment::segment(const elf &f, const void *hdr)
    : m(make_shared<impl>(f)) {
        canon_hdr(&m->hdr, hdr, f.get_hdr().ei_class, f.get_hdr().ei_data);
}

const Phdr<> &
segment::get_hdr() const {
        return m->hdr;
}

const void *
segment::data() const {
        if (!m->data)
                m->data = m->f.get_loader()->load(m->hdr.offset,
                                                  m->hdr.filesz);
        return m->data;
}

size_t
segment::file_size() const {
        return m->hdr.filesz;
}

size_t
segment::mem_size() const {
        return m->hdr.memsz;
}

//////////////////////////////////////////////////////////////////
// class section
//

std::string
enums::to_string(shn v)
{
        if (v == shn::undef)
                return "undef";
        if (v == shn::abs)
                return "abs";
        if (v == shn::common)
                return "common";
        return std::to_string(v);
}

struct section::impl
{
        impl(const elf &f)
                : f(f), name(nullptr), data(nullptr) { }

        const elf f;
        Shdr<> hdr;
        const char *name;
        size_t name_len;
        const void *data;
};

section::section(const elf &f, const void *hdr)
        : m(make_shared<impl>(f))
{
        canon_hdr(&m->hdr, hdr, f.get_hdr().ei_class, f.get_hdr().ei_data);
}

const Shdr<> &
section::get_hdr() const
{
        return m->hdr;
}

const char *
section::get_name(size_t *len_out) const
{
        // XXX Should the section name strtab be cached?
        if (!m->name)
                m->name = m->f.get_section(m->f.get_hdr().shstrndx)
                        .as_strtab().get(m->hdr.name, &m->name_len);
        if (len_out)
                *len_out = m->name_len;
        return m->name;
}

string
section::get_name() const
{
        return get_name(nullptr);
}

const void *
section::data() const
{
        if (m->hdr.type == sht::nobits)
                return nullptr;
        if (!m->data)
                m->data = m->f.get_loader()->load(m->hdr.offset, m->hdr.size);
        return m->data;
}

size_t
section::size() const
{
        return m->hdr.size;
}

strtab
section::as_strtab() const
{
        if (m->hdr.type != sht::strtab)
                throw section_type_mismatch("cannot use section as strtab");
        return strtab(m->f, data(), size());
}

symtab
section::as_symtab() const
{
        if (m->hdr.type != sht::symtab && m->hdr.type != sht::dynsym)
                throw section_type_mismatch("cannot use section as symtab");
        return symtab(m->f, data(), size(),
                      m->f.get_section(get_hdr().link).as_strtab());
}

//////////////////////////////////////////////////////////////////
// class strtab
//

struct strtab::impl
{
        impl(const elf &f, const char *data, const char *end)
                : f(f), data(data), end(end) { }

        const elf f;
        const char *data, *end;
};

strtab::strtab(elf f, const void *data, size_t size)
        : m(make_shared<impl>(f, (const char*)data, (const char *)data + size))
{
}

const char *
strtab::get(Elf64::Off offset, size_t *len_out) const
{
        const char *start = m->data + offset;

        if (start >= m->end)
                throw range_error("string offset " + std::to_string(offset) + " exceeds section size");

        // Find the null terminator
        const char *p = start;
        while (p < m->end && *p)
                p++;
        if (p == m->end)
                throw format_error("unterminated string");

        if (len_out)
                *len_out = p - start;
        return start;
}

std::string
strtab::get(Elf64::Off offset) const
{
        return get(offset, nullptr);
}

//////////////////////////////////////////////////////////////////
// class sym
//

sym::sym(elf f, const void *data, strtab strs)
        : strs(strs)
{
        canon_hdr(&this->data, data, f.get_hdr().ei_class, f.get_hdr().ei_data);
}

const char *
sym::get_name(size_t *len_out) const
{
        return strs.get(get_data().name, len_out);
}

std::string
sym::get_name() const
{
        return strs.get(get_data().name);
}

//////////////////////////////////////////////////////////////////
// class symtab
//

struct symtab::impl
{
        impl(const elf &f, const char *data, const char *end, strtab strs)
                : f(f), data(data), end(end), strs(strs) { }

        const elf f;
        const char *data, *end;
        const strtab strs;
};

symtab::symtab(elf f, const void *data, size_t size, strtab strs)
        : m(make_shared<impl>(f, (const char*)data, (const char *)data + size,
                              strs))
{
}

symtab::iterator::iterator(const symtab &tab, const char *pos)
        : f(tab.m->f), strs(tab.m->strs), pos(pos)
{
        if (f.get_hdr().ei_class == elfclass::_32)
                stride = sizeof(Sym<Elf32>);
        else
                stride = sizeof(Sym<Elf64>);
}

symtab::iterator
symtab::begin() const
{
        return iterator(*this, m->data);
}

symtab::iterator
symtab::end() const
{
        return iterator(*this, m->end);
}

ELFPP_END_NAMESPACE