wasmtime/crates/runtime/src/traphandlers/unix.rs

use crate::traphandlers::{tls, wasmtime_longjmp};
use std::cell::RefCell;
use std::io;
use std::mem::{self, MaybeUninit};
use std::ptr::{self, null_mut};

/// Function which may handle custom signals while processing traps.
pub type SignalHandler<'a> =
    dyn Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool + Send + Sync + 'a;

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();

pub 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 and s390x use SIGFPE to report division by zero
    if cfg!(target_arch = "x86") || cfg!(target_arch = "x86_64") || cfg!(target_arch = "s390x") {
        register(&mut PREV_SIGFPE, libc::SIGFPE);
    }

    // Sometimes we need to handle SIGBUS too:
    // - On ARM, handle Unaligned Accesses.
    // - On Darwin, guard page accesses are raised as SIGBUS.
    if cfg!(target_arch = "arm") || cfg!(target_os = "macos") || cfg!(target_os = "freebsd") {
        register(&mut PREV_SIGBUS, libc::SIGBUS);
    }
}

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),
    };
    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 pc = get_pc(context, signum);
        let jmp_buf = info.jmp_buf_if_trap(pc, |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() {
            return false;
        }
        if jmp_buf as usize == 1 {
            return true;
        }
        info.capture_backtrace(pc);
        // On macOS this is a bit special, unfortunately. If we were to
        // `siglongjmp` out of the signal handler that notably does
        // *not* reset the sigaltstack state of our signal handler. This
        // seems to trick the kernel into thinking that the sigaltstack
        // is still in use upon delivery of the next signal, meaning
        // that the sigaltstack is not ever used again if we immediately
        // call `wasmtime_longjmp` here.
        //
        // Note that if we use `longjmp` instead of `siglongjmp` then
        // the problem is fixed. The problem with that, however, is that
        // `setjmp` is much slower than `sigsetjmp` due to the
        // preservation of the proceses signal mask. The reason
        // `longjmp` appears to work is that it seems to call a function
        // (according to published macOS sources) called
        // `_sigunaltstack` which updates the kernel to say the
        // sigaltstack is no longer in use. We ideally want to call that
        // here but I don't think there's a stable way for us to call
        // that.
        //
        // Given all that, on macOS only, we do the next best thing. We
        // return from the signal handler after updating the register
        // context. This will cause control to return to our shim
        // function defined here which will perform the
        // `wasmtime_longjmp` (`siglongjmp`) for us. The reason this
        // works is that by returning from the signal handler we'll
        // trigger all the normal machinery for "the signal handler is
        // done running" which will clear the sigaltstack flag and allow
        // reusing it for the next signal. Then upon resuming in our custom
        // code we blow away the stack anyway with a longjmp.
        if cfg!(target_os = "macos") {
            unsafe extern "C" fn wasmtime_longjmp_shim(jmp_buf: *const u8) {
                wasmtime_longjmp(jmp_buf)
            }
            set_pc(context, wasmtime_longjmp_shim as usize, jmp_buf as usize);
            return true;
        }
        wasmtime_longjmp(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, _signum: libc::c_int) -> *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(any(target_os = "linux", 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_os = "linux", target_arch = "s390x"))] {
            // On s390x, SIGILL and SIGFPE are delivered with the PSW address
            // pointing *after* the faulting instruction, while SIGSEGV and
            // SIGBUS are delivered with the PSW address pointing *to* the
            // faulting instruction.  To handle this, the code generator registers
            // any trap that results in one of "late" signals on the last byte
            // of the instruction, and any trap that results in one of the "early"
            // signals on the first byte of the instruction (as usual).  This
            // means we simply need to decrement the reported PSW address by
            // one in the case of a "late" signal here to ensure we always
            // correctly find the associated trap handler.
            let trap_offset = match _signum {
                libc::SIGILL | libc::SIGFPE => 1,
                _ => 0,
            };
            let cx = &*(cx as *const libc::ucontext_t);
            (cx.uc_mcontext.psw.addr - trap_offset) as *const u8
        } else if #[cfg(all(target_os = "macos", 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_os = "macos", target_arch = "x86"))] {
            let cx = &*(cx as *const libc::ucontext_t);
            (*cx.uc_mcontext).__ss.__eip as *const u8
        } else if #[cfg(all(target_os = "macos", target_arch = "aarch64"))] {
            let cx = &*(cx as *const libc::ucontext_t);
            (*cx.uc_mcontext).__ss.__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 {
            compile_error!("unsupported platform");
        }
    }
}

// This is only used on macOS targets for calling an unwinding shim
// function to ensure that we return from the signal handler.
//
// See more comments above where this is called for what it's doing.
unsafe fn set_pc(cx: *mut libc::c_void, pc: usize, arg1: usize) {
    cfg_if::cfg_if! {
        if #[cfg(not(target_os = "macos"))] {
            drop((cx, pc, arg1));
            unreachable!(); // not used on these platforms
        } else if #[cfg(target_arch = "x86_64")] {
            let cx = &mut *(cx as *mut libc::ucontext_t);
            (*cx.uc_mcontext).__ss.__rip = pc as u64;
            (*cx.uc_mcontext).__ss.__rdi = arg1 as u64;
            // We're simulating a "pseudo-call" so we need to ensure
            // stack alignment is properly respected, notably that on a
            // `call` instruction the stack is 8/16-byte aligned, then
            // the function adjusts itself to be 16-byte aligned.
            //
            // Most of the time the stack pointer is 16-byte aligned at
            // the time of the trap but for more robust-ness with JIT
            // code where it may ud2 in a prologue check before the
            // stack is aligned we double-check here.
            if (*cx.uc_mcontext).__ss.__rsp % 16 == 0 {
                (*cx.uc_mcontext).__ss.__rsp -= 8;
            }
        } else if #[cfg(target_arch = "aarch64")] {
            let cx = &mut *(cx as *mut libc::ucontext_t);
            (*cx.uc_mcontext).__ss.__pc = pc as u64;
            (*cx.uc_mcontext).__ss.__x[0] = arg1 as u64;
        } else {
            compile_error!("unsupported macos target architecture");
        }
    }
}

/// A function 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.
#[cold]
pub fn lazy_per_thread_init() {
    // This thread local is purely used to register a `Stack` to get deallocated
    // when the thread exists. Otherwise this function is only ever called at
    // most once per-thread.
    thread_local! {
        static STACK: RefCell<Option<Stack>> = const { RefCell::new(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;

    struct Stack {
        mmap_ptr: *mut libc::c_void,
        mmap_size: usize,
    }

    return STACK.with(|s| {
        *s.borrow_mut() = unsafe { allocate_sigaltstack() };
    });

    unsafe fn allocate_sigaltstack() -> Option<Stack> {
        // 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: {}",
            io::Error::last_os_error()
        );
        if old_stack.ss_flags & libc::SS_DISABLE == 0 && old_stack.ss_size >= MIN_STACK_SIZE {
            return None;
        }

        // ... but failing that we need to allocate our own, so do all that
        // here.
        let page_size = crate::page_size();
        let guard_size = page_size;
        let alloc_size = guard_size + MIN_STACK_SIZE;

        let ptr = rustix::mm::mmap_anonymous(
            null_mut(),
            alloc_size,
            rustix::mm::ProtFlags::empty(),
            rustix::mm::MapFlags::PRIVATE,
        )
        .expect("failed to allocate memory for sigaltstack");

        // Prepare the stack with readable/writable memory and then register it
        // with `sigaltstack`.
        let stack_ptr = (ptr as usize + guard_size) as *mut std::ffi::c_void;
        rustix::mm::mprotect(
            stack_ptr,
            MIN_STACK_SIZE,
            rustix::mm::MprotectFlags::READ | rustix::mm::MprotectFlags::WRITE,
        )
        .expect("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: {}",
            io::Error::last_os_error()
        );

        Some(Stack {
            mmap_ptr: ptr,
            mmap_size: alloc_size,
        })
    }

    impl Drop for Stack {
        fn drop(&mut self) {
            unsafe {
                // Deallocate the stack memory.
                let r = rustix::mm::munmap(self.mmap_ptr, self.mmap_size);
                debug_assert!(r.is_ok(), "munmap failed during thread shutdown");
            }
        }
    }
}