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elf.rs
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elf.rs
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//! Parsing and loading kernel objects from ELF files.
use core::mem::{self, MaybeUninit};
use core::{fmt, str};
use align_address::Align;
use goblin::elf::note::Nhdr32;
use goblin::elf64::dynamic::{self, Dyn, DynamicInfo};
use goblin::elf64::header::{self, Header};
use goblin::elf64::program_header::{self, ProgramHeader};
use goblin::elf64::reloc::{self, Rela};
use log::{info, warn};
use plain::Plain;
use crate::boot_info::{LoadInfo, TlsInfo};
#[cfg(target_arch = "x86_64")]
const ELF_ARCH: u16 = goblin::elf::header::EM_X86_64;
#[cfg(target_arch = "x86_64")]
const R_RELATIVE: u32 = goblin::elf::reloc::R_X86_64_RELATIVE;
#[cfg(target_arch = "aarch64")]
const ELF_ARCH: u16 = goblin::elf::header::EM_AARCH64;
#[cfg(target_arch = "aarch64")]
const R_RELATIVE: u32 = goblin::elf::reloc::R_AARCH64_RELATIVE;
#[cfg(target_arch = "riscv64")]
const ELF_ARCH: u16 = goblin::elf::header::EM_RISCV;
#[cfg(target_arch = "riscv64")]
const R_RELATIVE: u32 = goblin::elf::reloc::R_RISCV_RELATIVE;
/// A parsed kernel object ready for loading.
pub struct KernelObject<'a> {
/// The raw bytes of the parsed ELF file.
elf: &'a [u8],
/// The ELF file header at the beginning of [`Self::elf`].
header: &'a Header,
/// The kernel's program headers.
///
/// Loadable program segments will be copied for execution.
///
/// The thread-local storage segment will be used for creating [`TlsInfo`] for the kernel.
phs: &'a [ProgramHeader],
/// Relocations with an explicit addend.
relas: &'a [Rela],
}
struct NoteIterator<'a> {
bytes: &'a [u8],
align: usize,
}
#[derive(Debug)]
struct Note<'a> {
ty: u32,
name: &'a str,
desc: &'a [u8],
}
impl<'a> Iterator for NoteIterator<'a> {
type Item = Note<'a>;
fn next(&mut self) -> Option<Self::Item> {
let header = Nhdr32::from_bytes(self.bytes).ok()?;
let mut offset = mem::size_of_val(header);
let name = str::from_utf8(&self.bytes[offset..][..header.n_namesz as usize - 1]).unwrap();
offset = (offset + header.n_namesz as usize).align_up(self.align);
let desc = &self.bytes[offset..][..header.n_descsz as usize];
offset = (offset + header.n_descsz as usize).align_up(self.align);
self.bytes = &self.bytes[offset..];
Some(Note {
ty: header.n_type,
name,
desc,
})
}
}
fn iter_notes(bytes: &[u8], align: usize) -> NoteIterator<'_> {
NoteIterator { bytes, align }
}
/// An error returned when parsing a kernel ELF fails.
#[derive(Debug)]
pub struct ParseKernelError(&'static str);
impl fmt::Display for ParseKernelError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let info = self.0;
write!(f, "invalid ELF: {info}")
}
}
impl<'a> KernelObject<'a> {
/// Parses raw bytes of an ELF file into a loadable kernel object.
pub fn parse(elf: &[u8]) -> Result<KernelObject<'_>, ParseKernelError> {
{
let range = elf.as_ptr_range();
let len = elf.len();
info!("Parsing kernel from ELF at {range:?} (len = {len:#x} B / {len} B)");
}
let header = plain::from_bytes::<Header>(elf).unwrap();
let phs = {
let start = header.e_phoff as usize;
let len = header.e_phnum as usize;
ProgramHeader::slice_from_bytes_len(&elf[start..], len).unwrap()
};
// General compatibility checks
{
let class = header.e_ident[header::EI_CLASS];
if class != header::ELFCLASS64 {
return Err(ParseKernelError("kernel ist not a 64-bit object"));
}
let data_encoding = header.e_ident[header::EI_DATA];
if data_encoding != header::ELFDATA2LSB {
return Err(ParseKernelError("kernel object is not little endian"));
}
let os_abi = header.e_ident[header::EI_OSABI];
if os_abi != header::ELFOSABI_STANDALONE {
warn!("Kernel is not a hermit application");
}
let note_section = phs
.iter()
.find(|ph| ph.p_type == program_header::PT_NOTE)
.ok_or(ParseKernelError("Kernel does not have note section"))?;
let mut note_iter = iter_notes(
&elf[note_section.p_offset as usize..][..note_section.p_filesz as usize],
note_section.p_align as usize,
);
let note = note_iter
.find(|note| note.name == "HERMIT" && note.ty == crate::NT_HERMIT_ENTRY_VERSION)
.ok_or(ParseKernelError(
"Kernel does not specify hermit entry version",
))?;
if note.desc[0] != crate::HERMIT_ENTRY_VERSION {
return Err(ParseKernelError("hermit entry version does not match"));
}
if !matches!(header.e_type, header::ET_DYN | header::ET_EXEC) {
return Err(ParseKernelError("kernel has unsupported ELF type"));
}
if header.e_machine != ELF_ARCH {
return Err(ParseKernelError(
"kernel is not compiled for the correct architecture",
));
}
}
let dyns = phs
.iter()
.find(|program_header| program_header.p_type == program_header::PT_DYNAMIC)
.map(|ph| {
let start = ph.p_offset as usize;
let len = (ph.p_filesz as usize) / dynamic::SIZEOF_DYN;
Dyn::slice_from_bytes_len(&elf[start..], len).unwrap()
})
.unwrap_or_default();
if dyns.iter().any(|d| d.d_tag == dynamic::DT_NEEDED) {
return Err(ParseKernelError(
"kernel was linked against dynamic libraries",
));
}
let dynamic_info = DynamicInfo::new(dyns, phs);
assert_eq!(0, dynamic_info.relcount);
let relas = {
let start = dynamic_info.rela;
let len = dynamic_info.relacount;
Rela::slice_from_bytes_len(&elf[start..], len).unwrap()
};
assert!(relas
.iter()
.all(|rela| reloc::r_type(rela.r_info) == R_RELATIVE));
Ok(KernelObject {
elf,
header,
phs,
relas,
})
}
/// Required memory size for loading.
pub fn mem_size(&self) -> usize {
let first_ph = self
.phs
.iter()
.find(|ph| ph.p_type == program_header::PT_LOAD)
.unwrap();
let start_addr = first_ph.p_vaddr;
let last_ph = self
.phs
.iter()
.rev()
.find(|ph| ph.p_type == program_header::PT_LOAD)
.unwrap();
let end_addr = last_ph.p_vaddr + last_ph.p_memsz;
let mem_size = end_addr - start_addr;
mem_size.try_into().unwrap()
}
fn is_relocatable(&self) -> bool {
match self.header.e_type {
header::ET_DYN => true,
header::ET_EXEC => false,
_ => unreachable!(),
}
}
/// Returns the required start address.
///
/// If this returns [`None`], the kernel is relocatable and does not require a certain start address.
pub fn start_addr(&self) -> Option<u64> {
(!self.is_relocatable()).then(|| {
self.phs
.iter()
.find(|ph| ph.p_type == program_header::PT_LOAD)
.unwrap()
.p_vaddr
})
}
fn tls_info(&self, start_addr: u64) -> Option<TlsInfo> {
self.phs
.iter()
.find(|ph| ph.p_type == program_header::PT_TLS)
.map(|ph| {
let mut tls_start = ph.p_vaddr;
if self.is_relocatable() {
tls_start += start_addr;
}
let tls_info = TlsInfo {
start: tls_start,
filesz: ph.p_filesz,
memsz: ph.p_memsz,
align: ph.p_align,
};
let range =
tls_info.start as *const ()..(tls_info.start + tls_info.memsz) as *const ();
let len = tls_info.memsz;
info!("TLS is at {range:?} (len = {len:#x} B / {len} B)",);
tls_info
})
}
fn entry_point(&self, start_addr: u64) -> u64 {
let mut entry_point = self.header.e_entry;
if self.is_relocatable() {
entry_point += start_addr;
}
entry_point
}
/// Loads the kernel into the provided memory.
pub fn load_kernel(&self, memory: &mut [MaybeUninit<u8>], start_addr: u64) -> LoadedKernel {
info!(
"Loading kernel to {:?} (len = {len:#x} B / {len} B)",
memory.as_ptr_range(),
len = memory.len()
);
if !self.is_relocatable() {
assert_eq!(self.start_addr().unwrap(), start_addr);
}
assert_eq!(self.mem_size(), memory.len());
// Load program segments
// Contains TLS initialization image
let load_start_addr = self.start_addr().unwrap_or_default();
self.phs
.iter()
.filter(|ph| ph.p_type == program_header::PT_LOAD)
.for_each(|ph| {
let ph_memory = {
let mem_start = (ph.p_vaddr - load_start_addr) as usize;
let mem_len = ph.p_memsz as usize;
&mut memory[mem_start..][..mem_len]
};
let file_len = ph.p_filesz as usize;
let ph_file = &self.elf[ph.p_offset as usize..][..file_len];
// FIXME: Replace with `maybe_uninit_write_slice` once stable
let ph_file = unsafe { mem::transmute(ph_file) };
ph_memory[..file_len].copy_from_slice(ph_file);
for byte in &mut ph_memory[file_len..] {
byte.write(0);
}
});
if self.is_relocatable() {
// Perform relocations
self.relas.iter().for_each(|rela| {
assert_eq!(R_RELATIVE, reloc::r_type(rela.r_info));
let relocated = (start_addr as i64 + rela.r_addend).to_ne_bytes();
let buf = &relocated[..];
// FIXME: Replace with `maybe_uninit_write_slice` once stable
let buf = unsafe { mem::transmute(buf) };
memory[rela.r_offset as usize..][..mem::size_of_val(&relocated)]
.copy_from_slice(buf);
});
}
LoadedKernel {
load_info: LoadInfo {
kernel_image_addr_range: start_addr..start_addr + self.mem_size() as u64,
tls_info: self.tls_info(start_addr),
},
entry_point: self.entry_point(start_addr),
}
}
}
/// Load information required by the loader.
#[derive(Debug)]
pub struct LoadedKernel {
/// Load information required by the kernel.
pub load_info: LoadInfo,
/// The kernel's entry point.
pub entry_point: u64,
}