1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269
//! Simple `mmap`ed runtime.
//!
//! This runtime supports adding code to executable pages and turn the added code into user
//! specified function pointer.
#[cfg(not(target_os = "linux"))]
compile_error!("This runtime is only supported on linux");
use nix::sys::mman::{mmap, mprotect, munmap, MapFlags, ProtFlags};
mod perf {
use std::fs;
use std::io::Write;
/// Provide support for the simple [perf jit interface][perf-jit].
///
/// This allows a simple (static) jit runtime to generate meta data describing the generated
/// functions, which is used during post-processing by `perf report` to symbolize addresses
/// captured while executing jitted code.
///
/// By the nature of this format, this can not be used for dynamic jit runtimes, which reuses
/// memory which previously contained jitted code.
///
/// [perf-jit]: https://elixir.bootlin.com/linux/v6.6.6/source/tools/perf/Documentation/jit-interface.txt
pub(super) struct PerfMap {
file: std::fs::File,
}
impl PerfMap {
/// Create an empty perf map file.
pub(super) fn new() -> Self {
let name = format!("/tmp/perf-{}.map", nix::unistd::getpid());
let file = fs::OpenOptions::new()
.truncate(true)
.create(true)
.write(true)
.open(&name)
.unwrap_or_else(|_| panic!("Failed to open perf map file {}", &name));
PerfMap { file }
}
/// Add an entry to the perf map file.
pub(super) fn add_entry(&mut self, start: usize, len: usize) {
// Each line has the following format, fields separated with spaces:
// START SIZE NAME
//
// START and SIZE are hex numbers without 0x.
// NAME is the rest of the line, so it could contain special characters.
writeln!(self.file, "{:x} {:x} jitfn_{:x}", start, len, start)
.expect("Failed to write PerfMap entry");
}
}
}
/// A simple `mmap`ed runtime with executable pages.
pub struct Runtime {
buf: *mut u8,
len: usize,
idx: usize,
perf: Option<perf::PerfMap>,
}
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,
perf: None,
}
}
/// Create a new [Runtime] which also generates static perf metat data.
///
/// For each function added to the [Runtime], an entry will be generated in the
/// `/tmp/perf-<PID>.map` file, which `perf report` uses to symbolicate unknown addresses.
/// This is applicable for static runtimes only.
///
/// # Panics
///
/// Panics if the `mmap` call fails.
pub fn with_profile() -> Runtime {
let mut rt = Runtime::new();
rt.perf = Some(perf::PerfMap::new());
rt
}
/// 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();
// Add perf map entry.
if let Some(map) = &mut self.perf {
map.add_entry(fn_start as usize, 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);
}
}
}