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// This file contains code from external sources.
// Attributions: https://github.com/wasmerio/wasmer/blob/master/ATTRIBUTIONS.md
//! WebAssembly trap handling, which is built on top of the lower-level
//! signalhandling mechanisms.
use super::trapcode::TrapCode;
use crate::vmcontext::{VMFunctionBody, VMFunctionEnvironment, VMTrampoline};
use backtrace::Backtrace;
use std::any::Any;
use std::cell::{Cell, UnsafeCell};
use std::error::Error;
use std::io;
use std::mem::{self, MaybeUninit};
use std::ptr;
use std::sync::Once;
pub use tls::TlsRestore;
cfg_if::cfg_if! {
if #[cfg(unix)] {
/// Function which may handle custom signals while processing traps.
pub type TrapHandlerFn = dyn Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool;
} else if #[cfg(target_os = "windows")] {
/// Function which may handle custom signals while processing traps.
pub type TrapHandlerFn = dyn Fn(winapi::um::winnt::PEXCEPTION_POINTERS) -> bool;
}
}
extern "C" {
fn wasmer_register_setjmp(
jmp_buf: *mut *const u8,
callback: extern "C" fn(*mut u8),
payload: *mut u8,
) -> i32;
fn wasmer_unwind(jmp_buf: *const u8) -> !;
}
cfg_if::cfg_if! {
if #[cfg(unix)] {
static mut PREV_SIGSEGV: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
static mut PREV_SIGBUS: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
static mut PREV_SIGILL: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
static mut PREV_SIGFPE: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
unsafe fn platform_init() {
let register = |slot: &mut MaybeUninit<libc::sigaction>, signal: i32| {
let mut handler: libc::sigaction = mem::zeroed();
// The flags here are relatively careful, and they are...
//
// SA_SIGINFO gives us access to information like the program
// counter from where the fault happened.
//
// SA_ONSTACK allows us to handle signals on an alternate stack,
// so that the handler can run in response to running out of
// stack space on the main stack. Rust installs an alternate
// stack with sigaltstack, so we rely on that.
//
// SA_NODEFER allows us to reenter the signal handler if we
// crash while handling the signal, and fall through to the
// Breakpad handler by testing handlingSegFault.
handler.sa_flags = libc::SA_SIGINFO | libc::SA_NODEFER | libc::SA_ONSTACK;
handler.sa_sigaction = trap_handler as usize;
libc::sigemptyset(&mut handler.sa_mask);
if libc::sigaction(signal, &handler, slot.as_mut_ptr()) != 0 {
panic!(
"unable to install signal handler: {}",
io::Error::last_os_error(),
);
}
};
// Allow handling OOB with signals on all architectures
register(&mut PREV_SIGSEGV, libc::SIGSEGV);
// Handle `unreachable` instructions which execute `ud2` right now
register(&mut PREV_SIGILL, libc::SIGILL);
// x86 uses SIGFPE to report division by zero
if cfg!(target_arch = "x86") || cfg!(target_arch = "x86_64") {
register(&mut PREV_SIGFPE, libc::SIGFPE);
}
// On ARM, handle Unaligned Accesses.
// On Darwin, guard page accesses are raised as SIGBUS.
if cfg!(target_arch = "arm") || cfg!(target_vendor = "apple") {
register(&mut PREV_SIGBUS, libc::SIGBUS);
}
}
#[cfg(target_vendor = "apple")]
unsafe fn thread_stack() -> (usize, usize) {
let this_thread = libc::pthread_self();
let stackaddr = libc::pthread_get_stackaddr_np(this_thread);
let stacksize = libc::pthread_get_stacksize_np(this_thread);
(stackaddr as usize - stacksize, stacksize)
}
#[cfg(not(target_vendor = "apple"))]
unsafe fn thread_stack() -> (usize, usize) {
let this_thread = libc::pthread_self();
let mut thread_attrs: libc::pthread_attr_t = mem::zeroed();
let mut stackaddr: *mut libc::c_void = ptr::null_mut();
let mut stacksize: libc::size_t = 0;
#[cfg(not(target_os = "freebsd"))]
let ok = libc::pthread_getattr_np(this_thread, &mut thread_attrs);
#[cfg(target_os = "freebsd")]
let ok = libc::pthread_attr_get_np(this_thread, &mut thread_attrs);
if ok == 0 {
libc::pthread_attr_getstack(&thread_attrs, &mut stackaddr, &mut stacksize);
libc::pthread_attr_destroy(&mut thread_attrs);
}
(stackaddr as usize, stacksize)
}
unsafe extern "C" fn trap_handler(
signum: libc::c_int,
siginfo: *mut libc::siginfo_t,
context: *mut libc::c_void,
) {
let previous = match signum {
libc::SIGSEGV => &PREV_SIGSEGV,
libc::SIGBUS => &PREV_SIGBUS,
libc::SIGFPE => &PREV_SIGFPE,
libc::SIGILL => &PREV_SIGILL,
_ => panic!("unknown signal: {}", signum),
};
// We try to get the Code trap associated to this signal
let maybe_signal_trap = match signum {
libc::SIGSEGV | libc::SIGBUS => {
let addr = (*siginfo).si_addr() as usize;
let (stackaddr, stacksize) = thread_stack();
// The stack and its guard page covers the
// range [stackaddr - guard pages .. stackaddr + stacksize).
// We assume the guard page is 1 page, and pages are 4KiB (or 16KiB in Apple Silicon)
if stackaddr - region::page::size() <= addr && addr < stackaddr + stacksize {
Some(TrapCode::StackOverflow)
} else {
Some(TrapCode::HeapAccessOutOfBounds)
}
}
_ => None,
};
let handled = tls::with(|info| {
// If no wasm code is executing, we don't handle this as a wasm
// trap.
let info = match info {
Some(info) => info,
None => return false,
};
// If we hit an exception while handling a previous trap, that's
// quite bad, so bail out and let the system handle this
// recursive segfault.
//
// Otherwise flag ourselves as handling a trap, do the trap
// handling, and reset our trap handling flag. Then we figure
// out what to do based on the result of the trap handling.
let jmp_buf = info.handle_trap(
get_pc(context),
false,
maybe_signal_trap,
|handler| handler(signum, siginfo, context),
);
// Figure out what to do based on the result of this handling of
// the trap. Note that our sentinel value of 1 means that the
// exception was handled by a custom exception handler, so we
// keep executing.
if jmp_buf.is_null() {
false
} else if jmp_buf as usize == 1 {
true
} else {
wasmer_unwind(jmp_buf)
}
});
if handled {
return;
}
// This signal is not for any compiled wasm code we expect, so we
// need to forward the signal to the next handler. If there is no
// next handler (SIG_IGN or SIG_DFL), then it's time to crash. To do
// this, we set the signal back to its original disposition and
// return. This will cause the faulting op to be re-executed which
// will crash in the normal way. If there is a next handler, call
// it. It will either crash synchronously, fix up the instruction
// so that execution can continue and return, or trigger a crash by
// returning the signal to it's original disposition and returning.
let previous = &*previous.as_ptr();
if previous.sa_flags & libc::SA_SIGINFO != 0 {
mem::transmute::<
usize,
extern "C" fn(libc::c_int, *mut libc::siginfo_t, *mut libc::c_void),
>(previous.sa_sigaction)(signum, siginfo, context)
} else if previous.sa_sigaction == libc::SIG_DFL ||
previous.sa_sigaction == libc::SIG_IGN
{
libc::sigaction(signum, previous, ptr::null_mut());
} else {
mem::transmute::<usize, extern "C" fn(libc::c_int)>(
previous.sa_sigaction
)(signum)
}
}
unsafe fn get_pc(cx: *mut libc::c_void) -> *const u8 {
cfg_if::cfg_if! {
if #[cfg(all(target_os = "linux", target_arch = "x86_64"))] {
let cx = &*(cx as *const libc::ucontext_t);
cx.uc_mcontext.gregs[libc::REG_RIP as usize] as *const u8
} else if #[cfg(all(target_os = "linux", target_arch = "x86"))] {
let cx = &*(cx as *const libc::ucontext_t);
cx.uc_mcontext.gregs[libc::REG_EIP as usize] as *const u8
} else if #[cfg(all(target_os = "android", target_arch = "x86"))] {
let cx = &*(cx as *const libc::ucontext_t);
cx.uc_mcontext.gregs[libc::REG_EIP as usize] as *const u8
} else if #[cfg(all(target_os = "linux", target_arch = "aarch64"))] {
let cx = &*(cx as *const libc::ucontext_t);
cx.uc_mcontext.pc as *const u8
} else if #[cfg(all(target_os = "android", target_arch = "aarch64"))] {
let cx = &*(cx as *const libc::ucontext_t);
cx.uc_mcontext.pc as *const u8
} else if #[cfg(all(target_vendor = "apple", target_arch = "x86_64"))] {
let cx = &*(cx as *const libc::ucontext_t);
(*cx.uc_mcontext).__ss.__rip as *const u8
} else if #[cfg(all(target_vendor = "apple", target_arch = "aarch64"))] {
use std::mem;
// TODO: This should be integrated into rust/libc
// Related issue: https://github.com/rust-lang/libc/issues/1977
#[allow(non_camel_case_types)]
pub struct __darwin_arm_thread_state64 {
pub __x: [u64; 29], /* General purpose registers x0-x28 */
pub __fp: u64, /* Frame pointer x29 */
pub __lr: u64, /* Link register x30 */
pub __sp: u64, /* Stack pointer x31 */
pub __pc: u64, /* Program counter */
pub __cpsr: u32, /* Current program status register */
pub __pad: u32, /* Same size for 32-bit or 64-bit clients */
}
let cx = &*(cx as *const libc::ucontext_t);
let uc_mcontext = mem::transmute::<_, *const __darwin_arm_thread_state64>(&(*cx.uc_mcontext).__ss);
(*uc_mcontext).__pc as *const u8
} else if #[cfg(all(target_os = "freebsd", target_arch = "x86_64"))] {
let cx = &*(cx as *const libc::ucontext_t);
cx.uc_mcontext.mc_rip as *const u8
} else if #[cfg(all(target_os = "freebsd", target_arch = "aarch64"))] {
#[repr(align(16))]
#[allow(non_camel_case_types)]
pub struct gpregs {
pub gp_x: [libc::register_t; 30],
pub gp_lr: libc::register_t,
pub gp_sp: libc::register_t,
pub gp_elr: libc::register_t,
pub gp_spsr: u32,
pub gp_pad: libc::c_int,
};
#[repr(align(16))]
#[allow(non_camel_case_types)]
pub struct fpregs {
pub fp_q: [u128; 32],
pub fp_sr: u32,
pub fp_cr: u32,
pub fp_flags: libc::c_int,
pub fp_pad: libc::c_int,
};
#[repr(align(16))]
#[allow(non_camel_case_types)]
pub struct mcontext_t {
pub mc_gpregs: gpregs,
pub mc_fpregs: fpregs,
pub mc_flags: libc::c_int,
pub mc_pad: libc::c_int,
pub mc_spare: [u64; 8],
};
#[repr(align(16))]
#[allow(non_camel_case_types)]
pub struct ucontext_t {
pub uc_sigmask: libc::sigset_t,
pub uc_mcontext: mcontext_t,
pub uc_link: *mut ucontext_t,
pub uc_stack: libc::stack_t,
pub uc_flags: libc::c_int,
__spare__: [libc::c_int; 4],
}
let cx = &*(cx as *const ucontext_t);
cx.uc_mcontext.mc_gpregs.gp_elr as *const u8
} else {
compile_error!("unsupported platform");
}
}
}
} else if #[cfg(target_os = "windows")] {
use winapi::um::errhandlingapi::*;
use winapi::um::winnt::*;
use winapi::um::minwinbase::*;
use winapi::vc::excpt::*;
unsafe fn platform_init() {
// our trap handler needs to go first, so that we can recover from
// wasm faults and continue execution, so pass `1` as a true value
// here.
if AddVectoredExceptionHandler(1, Some(exception_handler)).is_null() {
panic!("failed to add exception handler: {}", io::Error::last_os_error());
}
}
unsafe extern "system" fn exception_handler(
exception_info: PEXCEPTION_POINTERS
) -> LONG {
// Check the kind of exception, since we only handle a subset within
// wasm code. If anything else happens we want to defer to whatever
// the rest of the system wants to do for this exception.
let record = &*(*exception_info).ExceptionRecord;
if record.ExceptionCode != EXCEPTION_ACCESS_VIOLATION &&
record.ExceptionCode != EXCEPTION_ILLEGAL_INSTRUCTION &&
record.ExceptionCode != EXCEPTION_STACK_OVERFLOW &&
record.ExceptionCode != EXCEPTION_INT_DIVIDE_BY_ZERO &&
record.ExceptionCode != EXCEPTION_INT_OVERFLOW
{
return EXCEPTION_CONTINUE_SEARCH;
}
// FIXME: this is what the previous C++ did to make sure that TLS
// works by the time we execute this trap handling code. This isn't
// exactly super easy to call from Rust though and it's not clear we
// necessarily need to do so. Leaving this here in case we need this
// in the future, but for now we can probably wait until we see a
// strange fault before figuring out how to reimplement this in
// Rust.
//
// if (!NtCurrentTeb()->Reserved1[sThreadLocalArrayPointerIndex]) {
// return EXCEPTION_CONTINUE_SEARCH;
// }
// This is basically the same as the unix version above, only with a
// few parameters tweaked here and there.
tls::with(|info| {
let info = match info {
Some(info) => info,
None => return EXCEPTION_CONTINUE_SEARCH,
};
#[cfg(target_pointer_width = "32")]
let pc = (*(*exception_info).ContextRecord).Eip as *const u8;
#[cfg(target_pointer_width = "64")]
let pc = (*(*exception_info).ContextRecord).Rip as *const u8;
let jmp_buf = info.handle_trap(
pc,
record.ExceptionCode == EXCEPTION_STACK_OVERFLOW,
// TODO: fix the signal trap associated to memory access in Windows
None,
|handler| handler(exception_info),
);
if jmp_buf.is_null() {
EXCEPTION_CONTINUE_SEARCH
} else if jmp_buf as usize == 1 {
EXCEPTION_CONTINUE_EXECUTION
} else {
wasmer_unwind(jmp_buf)
}
})
}
}
}
/// Globally-set callback to determine whether a program counter is actually a
/// wasm trap.
///
/// This is initialized during `init_traps` below. The definition lives within
/// `wasmer` currently.
static mut IS_WASM_PC: fn(usize) -> bool = |_| false;
/// This function is required to be called before any WebAssembly is entered.
/// This will configure global state such as signal handlers to prepare the
/// process to receive wasm traps.
///
/// This function must not only be called globally once before entering
/// WebAssembly but it must also be called once-per-thread that enters
/// WebAssembly. Currently in wasmer's integration this function is called on
/// creation of a `Store`.
///
/// The `is_wasm_pc` argument is used when a trap happens to determine if a
/// program counter is the pc of an actual wasm trap or not. This is then used
/// to disambiguate faults that happen due to wasm and faults that happen due to
/// bugs in Rust or elsewhere.
pub fn init_traps(is_wasm_pc: fn(usize) -> bool) {
static INIT: Once = Once::new();
INIT.call_once(|| unsafe {
IS_WASM_PC = is_wasm_pc;
platform_init();
});
}
/// Raises a user-defined trap immediately.
///
/// This function performs as-if a wasm trap was just executed, only the trap
/// has a dynamic payload associated with it which is user-provided. This trap
/// payload is then returned from `catch_traps` below.
///
/// # Safety
///
/// Only safe to call when wasm code is on the stack, aka `catch_traps` must
/// have been previous called and not yet returned.
/// Additionally no Rust destructors may be on the stack.
/// They will be skipped and not executed.
pub unsafe fn raise_user_trap(data: Box<dyn Error + Send + Sync>) -> ! {
tls::with(|info| info.unwrap().unwind_with(UnwindReason::UserTrap(data)))
}
/// Raises a trap from inside library code immediately.
///
/// This function performs as-if a wasm trap was just executed. This trap
/// payload is then returned from `catch_traps` below.
///
/// # Safety
///
/// Only safe to call when wasm code is on the stack, aka `catch_traps` must
/// have been previous called and not yet returned.
/// Additionally no Rust destructors may be on the stack.
/// They will be skipped and not executed.
pub unsafe fn raise_lib_trap(trap: Trap) -> ! {
tls::with(|info| info.unwrap().unwind_with(UnwindReason::LibTrap(trap)))
}
/// Carries a Rust panic across wasm code and resumes the panic on the other
/// side.
///
/// # Safety
///
/// Only safe to call when wasm code is on the stack, aka `catch_traps` must
/// have been previously called and not returned. Additionally no Rust destructors may be on the
/// stack. They will be skipped and not executed.
pub unsafe fn resume_panic(payload: Box<dyn Any + Send>) -> ! {
tls::with(|info| info.unwrap().unwind_with(UnwindReason::Panic(payload)))
}
#[cfg(target_os = "windows")]
fn reset_guard_page() {
extern "C" {
fn _resetstkoflw() -> winapi::ctypes::c_int;
}
// We need to restore guard page under stack to handle future stack overflows properly.
// https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/resetstkoflw?view=vs-2019
if unsafe { _resetstkoflw() } == 0 {
panic!("failed to restore stack guard page");
}
}
#[cfg(not(target_os = "windows"))]
fn reset_guard_page() {}
/// Stores trace message with backtrace.
#[derive(Debug)]
pub enum Trap {
/// A user-raised trap through `raise_user_trap`.
User(Box<dyn Error + Send + Sync>),
/// A trap raised from the Wasm generated code
///
/// Note: this trap is deterministic (assuming a deterministic host implementation)
Wasm {
/// The program counter in generated code where this trap happened.
pc: usize,
/// Native stack backtrace at the time the trap occurred
backtrace: Backtrace,
/// Optional trapcode associated to the signal that caused the trap
signal_trap: Option<TrapCode>,
},
/// A trap raised from a wasm libcall
///
/// Note: this trap is deterministic (assuming a deterministic host implementation)
Lib {
/// Code of the trap.
trap_code: TrapCode,
/// Native stack backtrace at the time the trap occurred
backtrace: Backtrace,
},
/// A trap indicating that the runtime was unable to allocate sufficient memory.
///
/// Note: this trap is nondeterministic, since it depends on the host system.
OOM {
/// Native stack backtrace at the time the OOM occurred
backtrace: Backtrace,
},
}
impl Trap {
/// Construct a new Wasm trap with the given source location and backtrace.
///
/// Internally saves a backtrace when constructed.
pub fn wasm(pc: usize, backtrace: Backtrace, signal_trap: Option<TrapCode>) -> Self {
Trap::Wasm {
pc,
backtrace,
signal_trap,
}
}
/// Construct a new Wasm trap with the given trap code.
///
/// Internally saves a backtrace when constructed.
pub fn lib(trap_code: TrapCode) -> Self {
let backtrace = Backtrace::new_unresolved();
Trap::Lib {
trap_code,
backtrace,
}
}
/// Construct a new OOM trap with the given source location and trap code.
///
/// Internally saves a backtrace when constructed.
pub fn oom() -> Self {
let backtrace = Backtrace::new_unresolved();
Trap::OOM { backtrace }
}
}
/// Call the wasm function pointed to by `callee`.
///
/// * `vmctx` - the callee vmctx argument
/// * `caller_vmctx` - the caller vmctx argument
/// * `trampoline` - the jit-generated trampoline whose ABI takes 4 values, the
/// callee vmctx, the caller vmctx, the `callee` argument below, and then the
/// `values_vec` argument.
/// * `callee` - the third argument to the `trampoline` function
/// * `values_vec` - points to a buffer which holds the incoming arguments, and to
/// which the outgoing return values will be written.
///
/// # Safety
///
/// Wildly unsafe because it calls raw function pointers and reads/writes raw
/// function pointers.
pub unsafe fn wasmer_call_trampoline(
trap_handler: &impl TrapHandler,
vmctx: VMFunctionEnvironment,
trampoline: VMTrampoline,
callee: *const VMFunctionBody,
values_vec: *mut u8,
) -> Result<(), Trap> {
catch_traps(trap_handler, || {
mem::transmute::<_, extern "C" fn(VMFunctionEnvironment, *const VMFunctionBody, *mut u8)>(
trampoline,
)(vmctx, callee, values_vec);
})
}
/// Catches any wasm traps that happen within the execution of `closure`,
/// returning them as a `Result`.
///
/// Highly unsafe since `closure` won't have any dtors run.
pub unsafe fn catch_traps<F>(trap_handler: &dyn TrapHandler, mut closure: F) -> Result<(), Trap>
where
F: FnMut(),
{
return CallThreadState::new(trap_handler).with(|cx| {
wasmer_register_setjmp(
cx.jmp_buf.as_ptr(),
call_closure::<F>,
&mut closure as *mut F as *mut u8,
)
});
extern "C" fn call_closure<F>(payload: *mut u8)
where
F: FnMut(),
{
unsafe { (*(payload as *mut F))() }
}
}
/// Catches any wasm traps that happen within the execution of `closure`,
/// returning them as a `Result`, with the closure contents.
///
/// The main difference from this method and `catch_traps`, is that is able
/// to return the results from the closure.
///
/// # Safety
///
/// Check [`catch_traps`].
pub unsafe fn catch_traps_with_result<F, R>(
trap_handler: &dyn TrapHandler,
mut closure: F,
) -> Result<R, Trap>
where
F: FnMut() -> R,
{
let mut global_results = MaybeUninit::<R>::uninit();
catch_traps(trap_handler, || {
global_results.as_mut_ptr().write(closure());
})?;
Ok(global_results.assume_init())
}
/// Temporary state stored on the stack which is registered in the `tls` module
/// below for calls into wasm.
pub struct CallThreadState<'a> {
unwind: UnsafeCell<MaybeUninit<UnwindReason>>,
jmp_buf: Cell<*const u8>,
reset_guard_page: Cell<bool>,
prev: Cell<tls::Ptr>,
trap_handler: &'a (dyn TrapHandler + 'a),
handling_trap: Cell<bool>,
}
/// A package of functionality needed by `catch_traps` to figure out what to do
/// when handling a trap.
///
/// Note that this is an `unsafe` trait at least because it's being run in the
/// context of a synchronous signal handler, so it needs to be careful to not
/// access too much state in answering these queries.
pub unsafe trait TrapHandler {
/// Converts this object into an `Any` to dynamically check its type.
fn as_any(&self) -> &dyn Any;
/// Uses `call` to call a custom signal handler, if one is specified.
///
/// Returns `true` if `call` returns true, otherwise returns `false`.
fn custom_trap_handler(&self, call: &dyn Fn(&TrapHandlerFn) -> bool) -> bool;
}
enum UnwindReason {
/// A panic caused by the host
Panic(Box<dyn Any + Send>),
/// A custom error triggered by the user
UserTrap(Box<dyn Error + Send + Sync>),
/// A Trap triggered by a wasm libcall
LibTrap(Trap),
/// A trap caused by the Wasm generated code
WasmTrap {
backtrace: Backtrace,
pc: usize,
signal_trap: Option<TrapCode>,
},
}
impl<'a> CallThreadState<'a> {
#[inline]
fn new(trap_handler: &'a (dyn TrapHandler + 'a)) -> CallThreadState<'a> {
Self {
unwind: UnsafeCell::new(MaybeUninit::uninit()),
jmp_buf: Cell::new(ptr::null()),
reset_guard_page: Cell::new(false),
prev: Cell::new(ptr::null()),
trap_handler,
handling_trap: Cell::new(false),
}
}
fn with(self, closure: impl FnOnce(&CallThreadState) -> i32) -> Result<(), Trap> {
let ret = tls::set(&self, || closure(&self))?;
if ret != 0 {
return Ok(());
}
// We will only reach this path if ret == 0. And that will
// only happen if a trap did happen. As such, it's safe to
// assume that the `unwind` field is already initialized
// at this moment.
match unsafe { (*self.unwind.get()).as_ptr().read() } {
UnwindReason::UserTrap(data) => Err(Trap::User(data)),
UnwindReason::LibTrap(trap) => Err(trap),
UnwindReason::WasmTrap {
backtrace,
pc,
signal_trap,
} => Err(Trap::wasm(pc, backtrace, signal_trap)),
UnwindReason::Panic(panic) => std::panic::resume_unwind(panic),
}
}
fn unwind_with(&self, reason: UnwindReason) -> ! {
unsafe {
(*self.unwind.get()).as_mut_ptr().write(reason);
wasmer_unwind(self.jmp_buf.get());
}
}
/// Trap handler using our thread-local state.
///
/// * `pc` - the program counter the trap happened at
/// * `reset_guard_page` - whether or not to reset the guard page,
/// currently Windows specific
/// * `call_handler` - a closure used to invoke the platform-specific
/// signal handler for each instance, if available.
///
/// Attempts to handle the trap if it's a wasm trap. Returns a few
/// different things:
///
/// * null - the trap didn't look like a wasm trap and should continue as a
/// trap
/// * 1 as a pointer - the trap was handled by a custom trap handler on an
/// instance, and the trap handler should quickly return.
/// * a different pointer - a jmp_buf buffer to longjmp to, meaning that
/// the wasm trap was succesfully handled.
fn handle_trap(
&self,
pc: *const u8,
reset_guard_page: bool,
signal_trap: Option<TrapCode>,
call_handler: impl Fn(&TrapHandlerFn) -> bool,
) -> *const u8 {
// If we hit a fault while handling a previous trap, that's quite bad,
// so bail out and let the system handle this recursive segfault.
//
// Otherwise flag ourselves as handling a trap, do the trap handling,
// and reset our trap handling flag.
if self.handling_trap.replace(true) {
return ptr::null();
}
// First up see if we have a custom trap handler,
// in which case run it. If anything handles the trap then we
// return that the trap was handled.
if self.trap_handler.custom_trap_handler(&call_handler) {
return 1 as *const _;
}
// If this fault wasn't in wasm code, then it's not our problem
// except if it's a StackOverflow (see below)
if unsafe { !IS_WASM_PC(pc as _) } && signal_trap != Some(TrapCode::StackOverflow) {
return ptr::null();
}
// TODO: stack overflow can happen at any random time (i.e. in malloc()
// in memory.grow) and it's really hard to determine if the cause was
// stack overflow and if it happened in WebAssembly module.
//
// So, let's assume that any untrusted code called from WebAssembly
// doesn't trap. Then, if we have called some WebAssembly code, it
// means the trap is stack overflow.
if self.jmp_buf.get().is_null() {
self.handling_trap.set(false);
return ptr::null();
}
let backtrace = Backtrace::new_unresolved();
self.reset_guard_page.set(reset_guard_page);
unsafe {
(*self.unwind.get())
.as_mut_ptr()
.write(UnwindReason::WasmTrap {
backtrace,
signal_trap,
pc: pc as usize,
});
}
self.handling_trap.set(false);
self.jmp_buf.get()
}
}
impl<'a> Drop for CallThreadState<'a> {
fn drop(&mut self) {
if self.reset_guard_page.get() {
reset_guard_page();
}
}
}
// A private inner module for managing the TLS state that we require across
// calls in wasm. The WebAssembly code is called from C++ and then a trap may
// happen which requires us to read some contextual state to figure out what to
// do with the trap. This `tls` module is used to persist that information from
// the caller to the trap site.
mod tls {
use super::CallThreadState;
use crate::Trap;
use std::mem;
use std::ptr;
pub use raw::Ptr;
// An even *more* inner module for dealing with TLS. This actually has the
// thread local variable and has functions to access the variable.
//
// Note that this is specially done to fully encapsulate that the accessors
// for tls must not be inlined. Wasmer's async support will employ stack
// switching which can resume execution on different OS threads. This means
// that borrows of our TLS pointer must never live across accesses because
// otherwise the access may be split across two threads and cause unsafety.
//
// This also means that extra care is taken by the runtime to save/restore
// these TLS values when the runtime may have crossed threads.
mod raw {
use super::CallThreadState;
use crate::Trap;
use std::cell::Cell;
use std::ptr;
pub type Ptr = *const CallThreadState<'static>;
// The first entry here is the `Ptr` which is what's used as part of the
// public interface of this module. The second entry is a boolean which
// allows the runtime to perform per-thread initialization if necessary
// for handling traps (e.g. setting up ports on macOS and sigaltstack on
// Unix).
thread_local!(static PTR: Cell<(Ptr, bool)> = Cell::new((ptr::null(), false)));
#[inline(never)] // see module docs for why this is here
pub fn replace(val: Ptr) -> Result<Ptr, Trap> {
PTR.with(|p| {
// When a new value is configured that means that we may be
// entering WebAssembly so check to see if this thread has
// performed per-thread initialization for traps.
let (prev, mut initialized) = p.get();
if !initialized {
super::super::lazy_per_thread_init()?;
initialized = true;
}
p.set((val, initialized));
Ok(prev)
})
}
#[inline(never)] // see module docs for why this is here
pub fn get() -> Ptr {
PTR.with(|p| p.get().0)
}
}
/// Opaque state used to help control TLS state across stack switches for
/// async support.
pub struct TlsRestore(raw::Ptr);
impl TlsRestore {
/// Takes the TLS state that is currently configured and returns a
/// token that is used to replace it later.
///
/// # Safety
///
/// This is not a safe operation since it's intended to only be used
/// with stack switching found with fibers and async wasmer.
pub unsafe fn take() -> Result<TlsRestore, Trap> {
// Our tls pointer must be set at this time, and it must not be
// null. We need to restore the previous pointer since we're
// removing ourselves from the call-stack, and in the process we
// null out our own previous field for safety in case it's
// accidentally used later.
let raw = raw::get();
assert!(!raw.is_null());
let prev = (*raw).prev.replace(ptr::null());
raw::replace(prev)?;
Ok(TlsRestore(raw))
}
/// Restores a previous tls state back into this thread's TLS.
///
/// # Safety
///
/// This is unsafe because it's intended to only be used within the
/// context of stack switching within wasmer.
pub unsafe fn replace(self) -> Result<(), super::Trap> {
// We need to configure our previous TLS pointer to whatever is in
// TLS at this time, and then we set the current state to ourselves.
let prev = raw::get();
assert!((*self.0).prev.get().is_null());
(*self.0).prev.set(prev);
raw::replace(self.0)?;
Ok(())
}
}
/// Configures thread local state such that for the duration of the
/// execution of `closure` any call to `with` will yield `ptr`, unless this
/// is recursively called again.
pub fn set<R>(state: &CallThreadState<'_>, closure: impl FnOnce() -> R) -> Result<R, Trap> {
struct Reset<'a, 'b>(&'a CallThreadState<'b>);
impl Drop for Reset<'_, '_> {
#[inline]
fn drop(&mut self) {
raw::replace(self.0.prev.replace(ptr::null()))
.expect("tls should be previously initialized");
}
}
// Note that this extension of the lifetime to `'static` should be
// safe because we only ever access it below with an anonymous
// lifetime, meaning `'static` never leaks out of this module.
let ptr = unsafe { mem::transmute::<*const CallThreadState<'_>, _>(state) };
let prev = raw::replace(ptr)?;
state.prev.set(prev);
let _reset = Reset(state);
Ok(closure())
}
/// Returns the last pointer configured with `set` above. Panics if `set`
/// has not been previously called and not returned.
pub fn with<R>(closure: impl FnOnce(Option<&CallThreadState<'_>>) -> R) -> R {
let p = raw::get();
unsafe { closure(if p.is_null() { None } else { Some(&*p) }) }
}
}
#[cfg(not(unix))]
pub fn lazy_per_thread_init() -> Result<(), Trap> {
// Unused on Windows
Ok(())
}
/// A module for registering a custom alternate signal stack (sigaltstack).
///
/// Rust's libstd installs an alternate stack with size `SIGSTKSZ`, which is not
/// always large enough for our signal handling code. Override it by creating
/// and registering our own alternate stack that is large enough and has a guard
/// page.
#[cfg(unix)]
pub fn lazy_per_thread_init() -> Result<(), Trap> {
use std::cell::RefCell;
use std::ptr::null_mut;
thread_local! {
/// Thread-local state is lazy-initialized on the first time it's used,
/// and dropped when the thread exits.
static TLS: RefCell<Tls> = RefCell::new(Tls::None);
}
/// The size of the sigaltstack (not including the guard, which will be
/// added). Make this large enough to run our signal handlers.
const MIN_STACK_SIZE: usize = 16 * 4096;
enum Tls {
None,
Allocated {
mmap_ptr: *mut libc::c_void,
mmap_size: usize,
},
BigEnough,
}
return TLS.with(|slot| unsafe {
let mut slot = slot.borrow_mut();
match *slot {
Tls::None => {}
// already checked
_ => return Ok(()),
}
// Check to see if the existing sigaltstack, if it exists, is big
// enough. If so we don't need to allocate our own.
let mut old_stack = mem::zeroed();
let r = libc::sigaltstack(ptr::null(), &mut old_stack);
assert_eq!(r, 0, "learning about sigaltstack failed");
if old_stack.ss_flags & libc::SS_DISABLE == 0 && old_stack.ss_size >= MIN_STACK_SIZE {
*slot = Tls::BigEnough;
return Ok(());
}
// ... but failing that we need to allocate our own, so do all that
// here.
let page_size: usize = region::page::size();
let guard_size = page_size;
let alloc_size = guard_size + MIN_STACK_SIZE;
let ptr = libc::mmap(
null_mut(),
alloc_size,
libc::PROT_NONE,
libc::MAP_PRIVATE | libc::MAP_ANON,
-1,
0,
);
if ptr == libc::MAP_FAILED {
return Err(Trap::oom());
}
// Prepare the stack with readable/writable memory and then register it
// with `sigaltstack`.
let stack_ptr = (ptr as usize + guard_size) as *mut libc::c_void;
let r = libc::mprotect(
stack_ptr,
MIN_STACK_SIZE,
libc::PROT_READ | libc::PROT_WRITE,
);
assert_eq!(r, 0, "mprotect to configure memory for sigaltstack failed");
let new_stack = libc::stack_t {
ss_sp: stack_ptr,
ss_flags: 0,
ss_size: MIN_STACK_SIZE,
};
let r = libc::sigaltstack(&new_stack, ptr::null_mut());
assert_eq!(r, 0, "registering new sigaltstack failed");
*slot = Tls::Allocated {
mmap_ptr: ptr,
mmap_size: alloc_size,
};
Ok(())
});
impl Drop for Tls {
fn drop(&mut self) {
let (ptr, size) = match self {
Self::Allocated {
mmap_ptr,
mmap_size,
} => (*mmap_ptr, *mmap_size),
_ => return,
};
unsafe {
// Deallocate the stack memory.
let r = libc::munmap(ptr, size);
debug_assert_eq!(r, 0, "munmap failed during thread shutdown");
}
}
}
}