#![no_std]
#[derive(Debug)]
pub enum Error {
/// Wrong or missing ELF file magic.
WrongElfMagic,
/// No more bytes left while parsing the ELF file.
OutOfBytes,
/// Const generic on `Elf` is too small to hold all `LOAD` from the ELF file.
OutOfLoadSegments,
/// Unknown value in `e_ident:EI_CLASS` byte.
UnknownBitness(u8),
/// Unknown value in `e_ident:EI_DATA` byte.
UnknownEndianess(u8),
/// Unknown value in `e_machine` bytes.
UnknownMachine(u16),
}
type Result<T> = core::result::Result<T, Error>;
trait FromEndian: Sized {
const N: usize = core::mem::size_of::<Self>();
fn from_le_bytes<B: AsRef<[u8]>>(bytes: B) -> Option<Self>;
fn from_be_bytes<B: AsRef<[u8]>>(bytes: B) -> Option<Self>;
}
macro_rules! impl_from_endian {
($ty: ty) => {
impl FromEndian for $ty {
fn from_le_bytes<B: AsRef<[u8]>>(bytes: B) -> Option<Self> {
bytes
.as_ref()
.get(..Self::N)?
.try_into()
.map(<$ty>::from_le_bytes)
.ok()
}
fn from_be_bytes<B: AsRef<[u8]>>(bytes: B) -> Option<Self> {
bytes
.as_ref()
.get(..Self::N)?
.try_into()
.map(<$ty>::from_be_bytes)
.ok()
}
}
};
}
impl_from_endian!(u16);
impl_from_endian!(u32);
impl_from_endian!(u64);
#[derive(Debug, Clone, Copy)]
enum Endian {
Little,
Big,
}
impl TryFrom<u8> for Endian {
type Error = Error;
fn try_from(v: u8) -> Result<Self> {
match v {
1 => Ok(Endian::Little),
2 => Ok(Endian::Big),
_ => Err(Error::UnknownEndianess(v)),
}
}
}
#[derive(Debug, Clone, Copy)]
enum Bit {
Bit32,
Bit64,
}
impl TryFrom<u8> for Bit {
type Error = Error;
fn try_from(v: u8) -> Result<Self> {
match v {
1 => Ok(Bit::Bit32),
2 => Ok(Bit::Bit64),
_ => Err(Error::UnknownBitness(v)),
}
}
}
impl Into<usize> for Bit {
fn into(self) -> usize {
match self {
Bit::Bit32 => 4,
Bit::Bit64 => 8,
}
}
}
#[derive(Debug, Clone, Copy)]
pub enum Machine {
X86_64,
RiscV,
}
impl TryFrom<u16> for Machine {
type Error = Error;
fn try_from(v: u16) -> Result<Self> {
match v {
62 => Ok(Machine::X86_64),
243 => Ok(Machine::RiscV),
_ => Err(Error::UnknownMachine(v)),
}
}
}
struct ElfReader<'bytes> {
bytes: &'bytes [u8],
pos: usize,
}
impl<'bytes> ElfReader<'bytes> {
const fn new(bytes: &'bytes [u8]) -> ElfReader<'_> {
ElfReader { bytes, pos: 0 }
}
fn read_slice(&mut self, len: usize) -> Result<&'bytes [u8]> {
if let Some(bytes) = self.bytes.get(self.pos..self.pos + len) {
self.bump(len);
Ok(bytes)
} else {
Err(Error::OutOfBytes)
}
}
fn read_u8(&mut self) -> Result<u8> {
if let Some(byte) = self.bytes.get(self.pos) {
self.bump(1);
Ok(*byte)
} else {
Err(Error::OutOfBytes)
}
}
fn read<E: FromEndian>(&mut self, en: Endian) -> Result<E> {
let bytes = self.bytes.get(self.pos..).ok_or(Error::OutOfBytes)?;
let val = match en {
Endian::Little => E::from_le_bytes(&bytes),
Endian::Big => E::from_be_bytes(&bytes),
};
if val.is_some() {
self.bump(E::N);
}
val.ok_or(Error::OutOfBytes)
}
fn read_native(&mut self, en: Endian, bt: Bit) -> Result<u64> {
match bt {
Bit::Bit32 => self.read::<u32>(en).map(u64::from),
Bit::Bit64 => self.read::<u64>(en),
}
}
#[inline]
fn bump(&mut self, inc: usize) {
self.pos += inc;
}
#[inline]
fn set_pos(&mut self, pos: usize) {
self.pos = pos;
}
#[inline]
const fn pos(&self) -> usize {
self.pos
}
}
#[derive(Debug, Clone, Copy)]
pub struct LoadSegment<'bytes> {
pub vaddr: u64,
pub bytes: &'bytes [u8],
pub zero_pad: usize,
pub x: bool,
pub w: bool,
pub r: bool,
}
#[derive(Debug)]
pub struct Elf<'bytes, const N: usize> {
machine: Machine,
entry: u64,
load_segments: [Option<LoadSegment<'bytes>>; N],
}
impl<const N: usize> Elf<'_, N> {
pub fn parse(b: &[u8]) -> Result<Elf<'_, N>> {
let mut r = ElfReader::new(b);
//
// Parse ELF header.
//
if !matches!(r.read_slice(4), Ok(b"\x7fELF")) {
return Err(Error::WrongElfMagic);
}
let bit = r.read_u8().map(Bit::try_from)??;
let en = r.read_u8().map(Endian::try_from)??;
// Consume rest of e_ident.
r.bump(10);
let _type = r.read::<u16>(en)?;
let machine = r.read::<u16>(en).map(Machine::try_from)??;
let _version = r.read::<u32>(en)?;
let entry = r.read_native(en, bit)?;
let phoff = r.read_native(en, bit)?;
let _shoff = r.read_native(en, bit)?;
let _flags = r.read::<u32>(en)?;
let ehsize = r.read::<u16>(en)?;
let phentsize = r.read::<u16>(en)?;
let phnum = r.read::<u16>(en)?;
let _shentsize = r.read::<u16>(en)?;
let _shnum = r.read::<u16>(en)?;
let _shstrndf = r.read::<u16>(en)?;
assert_eq!(r.pos(), usize::from(ehsize));
//
// Parse load program header.
//
let mut load_segments = [None; N];
let mut load_segments_slice = &mut load_segments[..];
const PT_LOAD: u32 = 1;
const PF_X: u32 = 1 << 0;
const PF_W: u32 = 1 << 1;
const PF_R: u32 = 1 << 2;
let phoff = usize::try_from(phoff).expect("phoff too large!");
for ph in 0..phnum {
let off = ph
.checked_mul(phentsize)
.map(usize::from)
.expect("phdr offset overflowed");
let pos = phoff.checked_add(off).expect("phdr position overflowed");
r.set_pos(pos);
// We only care about load segments.
if r.read::<u32>(en)? != PT_LOAD {
continue;
}
let mut flags = 0;
// Elf64 program header has flags field here.
if matches!(bit, Bit::Bit64) {
flags = r.read::<u32>(en)?
}
let offset = r.read_native(en, bit)?;
let vaddr = r.read_native(en, bit)?;
let _paddr = r.read_native(en, bit)?;
let filesz = r.read_native(en, bit)?;
let memsz = r.read_native(en, bit)?;
// Elf32 program header has flags field here.
if matches!(bit, Bit::Bit32) {
flags = r.read::<u32>(en)?
}
let _align = r.read_native(en, bit)?;
let offset = usize::try_from(offset).expect("file offset too large");
let filesz = usize::try_from(filesz).expect("file size too large");
let memsz = usize::try_from(memsz).expect("mem size too large");
// Seek to start of PT_LOAD segment bytes.
r.set_pos(offset);
// Get slice of PT_LOAD segment bytes.
let bytes = r.read_slice(filesz)?;
let x = (flags & PF_X) != 0;
let w = (flags & PF_W) != 0;
let r = (flags & PF_R) != 0;
load_segments_slice = load_segments_slice
.split_first_mut()
.map(|(slot, rest)| {
*slot = Some(LoadSegment {
vaddr,
bytes,
zero_pad: memsz - filesz,
x,
w,
r,
});
rest
})
.ok_or(Error::OutOfLoadSegments)?;
}
Ok(Elf {
machine,
entry,
load_segments,
})
}
#[inline]
pub fn machine(&self) -> Machine {
self.machine
}
#[inline]
pub fn entry(&self) -> u64 {
self.entry
}
#[inline]
pub fn load_segments(&self) -> impl Iterator<Item = &LoadSegment<'_>> {
self.load_segments.iter().flatten()
}
}