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|
//! Simple `mmap`ed runtime.
//!
//! This runtime supports adding code to executable pages and turn the added code into user
//! specified function pointer.
use nix::sys::mman::{mmap, mprotect, munmap, MapFlags, ProtFlags};
#[cfg(not(target_os = "linux"))]
compile_error!("This runtime is only supported on linux");
/// A simple `mmap`ed runtime with executable pages.
pub struct Runtime {
buf: *mut u8,
len: usize,
idx: usize,
}
impl Runtime {
/// Create a new [Runtime].
///
/// # Panics
///
/// Panics if the `mmap` call fails.
pub fn new() -> Runtime {
// Allocate a single page.
let len = core::num::NonZeroUsize::new(4096).expect("Value is non zero");
let buf = unsafe {
mmap(
None,
len,
ProtFlags::PROT_NONE,
MapFlags::MAP_PRIVATE | MapFlags::MAP_ANONYMOUS,
0, /* fd */
0, /* off */
)
.expect("Failed to mmap runtime code page") as *mut u8
};
Runtime {
buf,
len: len.get(),
idx: 0,
}
}
/// Add the block of `code` to the runtime and a get function pointer of type `F`.
///
/// # Panics
///
/// Panics if the `code` does not fit on the `mmap`ed pages or is empty.
///
/// # Safety
///
/// The code added must fulfill the ABI of the specified function `F` and the returned function
/// pointer is only valid until the [`Runtime`] is dropped.
///
/// # Examples
///
/// ```
/// let mut rt = juicebox_asm::Runtime::new();
///
/// let code = [ 0x90 /* nop */, 0xc3 /* ret */ ];
/// let nop = unsafe { rt.add_code::<extern "C" fn()>(&code) };
///
/// nop();
/// ```
pub unsafe fn add_code<F>(&mut self, code: impl AsRef<[u8]>) -> F {
// Get pointer to start of next free byte.
assert!(self.idx < self.len, "Runtime code page full");
let fn_start = self.buf.add(self.idx);
// Copy over code.
let code = code.as_ref();
assert!(!code.is_empty(), "Adding empty code not supported");
assert!(
code.len() <= (self.len - self.idx),
"Code does not fit on the runtime code page"
);
self.unprotect();
unsafe { std::ptr::copy_nonoverlapping(code.as_ptr(), fn_start, code.len()) };
self.protect();
// Increment index to next free byte.
self.idx += code.len();
// Return function to newly added code.
unsafe { Self::as_fn::<F>(fn_start) }
}
/// Dump the code added so far to the runtime into a file called `jit.asm` in the processes
/// current working directory.
///
/// The code can be inspected with a disassembler as for example `ndiasm` from
/// [nasm.us](https://nasm.us/index.php).
/// ```sh
/// ndisasm -b 64 jit.asm
/// ```
///
/// # Panics
///
/// Panics if writing the file failed.
pub fn dump(&self) {
assert!(self.idx <= self.len);
let code = unsafe { core::slice::from_raw_parts(self.buf, self.idx) };
std::fs::write("jit.asm", code).expect("Failed to write file");
}
/// Reinterpret the block of code pointed to by `fn_start` as `F`.
#[inline]
unsafe fn as_fn<F>(fn_start: *mut u8) -> F {
unsafe { std::mem::transmute_copy(&fn_start) }
}
/// Add write protection the underlying code page(s).
///
/// # Panics
///
/// Panics if the `mprotect` call fails.
fn protect(&mut self) {
unsafe {
// Remove write permissions from code page and allow to read-execute from it.
mprotect(
self.buf.cast(),
self.len,
ProtFlags::PROT_READ | ProtFlags::PROT_EXEC,
)
.expect("Failed to RX mprotect runtime code page");
}
}
/// Remove write protection the underlying code page(s).
///
/// # Panics
///
/// Panics if the `mprotect` call fails.
fn unprotect(&mut self) {
unsafe {
// Add write permissions to code page.
mprotect(self.buf.cast(), self.len, ProtFlags::PROT_WRITE)
.expect("Failed to W mprotect runtime code page");
}
}
}
impl Drop for Runtime {
/// Unmaps the code page. This invalidates all the function pointer returned by
/// [`Runtime::add_code`].
fn drop(&mut self) {
unsafe {
munmap(self.buf.cast(), self.len).expect("Failed to munmap runtime");
}
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_code_max_size() {
let mut rt = Runtime::new();
let code = [0u8; 4096];
unsafe {
rt.add_code::<extern "C" fn()>(code);
}
}
#[test]
#[should_panic]
fn test_code_max_size_plus_1() {
let mut rt = Runtime::new();
let code = [0u8; 4097];
unsafe {
rt.add_code::<extern "C" fn()>(code);
}
}
#[test]
#[should_panic]
fn test_code_max_size_plus_1_2() {
let mut rt = Runtime::new();
let code = [0u8; 4096];
unsafe {
rt.add_code::<extern "C" fn()>(code);
}
let code = [0u8; 1];
unsafe {
rt.add_code::<extern "C" fn()>(code);
}
}
#[test]
#[should_panic]
fn test_empty_code() {
let mut rt = Runtime::new();
let code = [0u8; 0];
unsafe {
rt.add_code::<extern "C" fn()>(code);
}
}
}
|
