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#![allow(improper_ctypes)] #[cfg(test)] mod tests; use crate::instance::Instance; use crate::val::{val_to_reg, val_to_stack, RegVal, UntypedRetVal, Val}; use nix; use nix::sys::signal; use std::arch::x86_64::{__m128, _mm_setzero_ps}; use std::ptr::NonNull; use std::{mem, ptr}; use thiserror::Error; /// Callee-saved general-purpose registers in the AMD64 ABI. /// /// # Layout /// /// `repr(C)` is required to preserve the ordering of members, which are read by the assembly at /// hard-coded offsets. /// /// # TODOs /// /// - Unlike the C code, this doesn't use the `packed` repr due to warnings in the Nomicon: /// <https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked>. Since the members are all /// `u64`, this should be fine? #[repr(C)] pub(crate) struct GpRegs { rbx: u64, pub(crate) rsp: u64, rbp: u64, pub(crate) rdi: u64, r12: u64, r13: u64, r14: u64, r15: u64, pub(crate) rsi: u64, } impl GpRegs { fn new() -> Self { GpRegs { rbx: 0, rsp: 0, rbp: 0, rdi: 0, r12: 0, r13: 0, r14: 0, r15: 0, rsi: 0, } } } /// Floating-point argument registers in the AMD64 ABI. /// /// # Layout /// /// `repr(C)` is required to preserve the ordering of members, which are read by the assembly at /// hard-coded offsets. /// /// # TODOs /// /// - Unlike the C code, this doesn't use the `packed` repr due to warnings in the Nomicon: /// <https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked>. Since the members are all /// `__m128`, this should be fine? #[repr(C)] struct FpRegs { xmm0: __m128, xmm1: __m128, xmm2: __m128, xmm3: __m128, xmm4: __m128, xmm5: __m128, xmm6: __m128, xmm7: __m128, } impl FpRegs { fn new() -> Self { let zero = unsafe { _mm_setzero_ps() }; FpRegs { xmm0: zero, xmm1: zero, xmm2: zero, xmm3: zero, xmm4: zero, xmm5: zero, xmm6: zero, xmm7: zero, } } } /// Everything we need to make a context switch: a signal mask, and the registers and return values /// that are manipulated directly by assembly code. /// /// A context also tracks which other context to swap back to if a child context's entrypoint function /// returns, and can optionally contain a callback function to be run just before that swap occurs. /// /// # Layout /// /// The `repr(C)` and order of fields in this struct are very important, as the assembly code reads /// and writes hard-coded offsets from the base of the struct. Without `repr(C)`, Rust is free to /// reorder the fields. /// /// Contexts are also `repr(align(64))` in order to align to cache lines and minimize contention /// when running multiple threads. /// /// # Movement /// /// `Context` values must not be moved once they've been initialized. Contexts contain a pointer to /// their stack, which in turn contains a pointer back to the context. If the context gets moved, /// that pointer becomes invalid, and the behavior of returning from that context becomes undefined. #[repr(C, align(64))] pub struct Context { pub(crate) gpr: GpRegs, fpr: FpRegs, retvals_gp: [u64; 2], retval_fp: __m128, parent_ctx: *mut Context, // TODO ACF 2019-10-23: make Instance into a generic parameter? backstop_callback: *const unsafe extern "C" fn(*mut Instance), backstop_data: *mut Instance, sigset: signal::SigSet, } impl Context { /// Create an all-zeroed `Context`. pub fn new() -> Self { Context { gpr: GpRegs::new(), fpr: FpRegs::new(), retvals_gp: [0; 2], retval_fp: unsafe { _mm_setzero_ps() }, parent_ctx: ptr::null_mut(), backstop_callback: Context::default_backstop_callback as *const _, backstop_data: ptr::null_mut(), sigset: signal::SigSet::empty(), } } } /// A wrapper around a `Context`, primarily meant for use in test code. /// /// Users of this library interact with contexts implicitly via `Instance` values, but for testing /// the context code independently, it is helpful to use contexts directly. /// /// # Movement of `ContextHandle` /// /// `ContextHandle` keeps a pointer to a `Context` rather than keeping all of the data directly as /// fields in order to have better control over where that data lives in memory. We always want that /// data to be heap-allocated, and to never move once it has been initialized. The `ContextHandle`, /// by contrast, should be treated like a normal Rust value with no such restrictions. /// /// Until the `Unpin` marker trait arrives in stable Rust, it is difficult to enforce this with the /// type system alone, so we use a bit of unsafety and (hopefully) clever API design to ensure that /// the data cannot be moved. /// /// We create the `Context` within a box to allocate it on the heap, then convert it into a raw /// pointer to relinquish ownership. When accessing the internal structure via the `DerefMut` trait, /// data must not be moved out of the `Context` with functions like `mem::replace`. /// /// # Layout /// /// Foreign code accesses the `internal` pointer in tests, so it is important that it is the first /// member, and that the struct is `repr(C)`. #[repr(C)] pub struct ContextHandle { internal: NonNull<Context>, } impl Drop for ContextHandle { fn drop(&mut self) { unsafe { // create a box from the pointer so that it'll get dropped // and we won't leak `Context`s Box::from_raw(self.internal.as_ptr()); } } } impl std::ops::Deref for ContextHandle { type Target = Context; fn deref(&self) -> &Self::Target { unsafe { self.internal.as_ref() } } } impl std::ops::DerefMut for ContextHandle { fn deref_mut(&mut self) -> &mut Self::Target { unsafe { self.internal.as_mut() } } } impl ContextHandle { /// Create an all-zeroed `ContextHandle`. pub fn new() -> Self { let internal = NonNull::new(Box::into_raw(Box::new(Context::new()))) .expect("Box::into_raw should never return NULL"); ContextHandle { internal } } pub fn create_and_init( stack: &mut [u64], fptr: usize, args: &[Val], ) -> Result<ContextHandle, Error> { let mut child = ContextHandle::new(); Context::init(stack, &mut child, fptr, args)?; Ok(child) } } struct CallStackBuilder<'a> { offset: usize, stack: &'a mut [u64], } impl<'a> CallStackBuilder<'a> { pub fn new(stack: &'a mut [u64]) -> Self { CallStackBuilder { offset: 0, stack } } fn push(&mut self, val: u64) { self.offset += 1; self.stack[self.stack.len() - self.offset] = val; } /// Stores `args` onto the stack such that when a return address is written after, the /// complete unit will be 16-byte aligned, as the x86_64 ABI requires. /// /// That is to say, `args` will be padded such that the current top of stack is 8-byte /// aligned. fn store_args(&mut self, args: &[u64]) { let items_end = args.len() + self.offset; if items_end % 2 == 1 { // we need to add one entry just before the arguments so that the arguments start on an // aligned address. self.push(0); } for arg in args.iter().rev() { self.push(*arg); } } fn offset(&self) -> usize { self.offset } fn into_inner(self) -> (&'a mut [u64], usize) { (self.stack, self.offset) } } impl Context { /// Initialize a new child context. /// /// - `stack`: The stack for the child; *must be 16-byte aligned*. /// /// - `child`: The context for the child. The fields of this structure will be overwritten by /// `init`. /// /// - `fptr`: A pointer to the entrypoint for the child. Note that while the type signature here /// is for a void function of no arguments (equivalent to `void (*fptr)(void)` in C), the /// entrypoint actually can be a function of any argument or return type that corresponds to a /// `val::Val` variant. /// /// - `args`: A slice of arguments for the `fptr` entrypoint. These must match the number and /// types of `fptr`'s actual arguments exactly, otherwise swapping to this context will cause /// undefined behavior. /// /// # Errors /// /// - `Error::UnalignedStack` if the _end_ of `stack` is not 16-byte aligned. /// /// # Examples /// /// ## C entrypoint /// /// This example initializes a context that will start in a C function `entrypoint` when first /// swapped to. /// /// ```c /// void entrypoint(uint64_t x, float y); /// ``` /// /// ```no_run /// # use lucet_runtime_internals::context::Context; /// # use lucet_runtime_internals::val::Val; /// extern "C" { fn entrypoint(x: u64, y: f32); } /// // allocating an even number of `u64`s seems to reliably yield /// // properly aligned stacks, but TODO do better /// let mut stack = vec![0u64; 1024].into_boxed_slice(); /// let mut child = Context::new(); /// let res = Context::init( /// &mut *stack, /// &mut child, /// entrypoint as usize, /// &[Val::U64(120), Val::F32(3.14)], /// ); /// assert!(res.is_ok()); /// ``` /// /// ## Rust entrypoint /// /// This example initializes a context that will start in a Rust function `entrypoint` when /// first swapped to. Note that we mark `entrypoint` as `extern "C"` to make sure it is compiled /// with C calling conventions. /// /// ```no_run /// # use lucet_runtime_internals::context::{Context, ContextHandle}; /// # use lucet_runtime_internals::val::Val; /// extern "C" fn entrypoint(x: u64, y: f32) { } /// // allocating an even number of `u64`s seems to reliably yield /// // properly aligned stacks, but TODO do better /// let mut stack = vec![0u64; 1024].into_boxed_slice(); /// let mut child = Context::new(); /// let res = Context::init( /// &mut *stack, /// &mut child, /// entrypoint as usize, /// &[Val::U64(120), Val::F32(3.14)], /// ); /// assert!(res.is_ok()); /// ``` /// /// # Implementation details /// /// This prepares a stack for the child context structured as follows, assuming an 0x1000 byte /// stack: /// ```text /// 0x1000: +-------------------------+ /// 0x0ff8: | NULL | // Null added if necessary for alignment. /// 0x0ff0: | spilled_arg_1 | // Guest arguments follow. /// 0x0fe8: | spilled_arg_2 | /// 0x0fe0: ~ spilled_arg_3 ~ // The three arguments here are just for show. /// 0x0fd8: | lucet_context_backstop | <-- This forms an ABI-matching call frame for fptr. /// 0x0fd0: | fptr | <-- The actual guest code we want to run. /// 0x0fc8: | lucet_context_bootstrap | <-- The guest stack pointer starts here. /// 0x0fc0: | | /// 0x0XXX: ~ ~ // Rest of the stack needs no preparation. /// 0x0000: | | /// +-------------------------+ /// ``` /// /// This packing of data on the stack is interwoven with noteworthy constraints on what the /// backstop may do: /// * The backstop must not return on the guest stack. /// - The next value will be a spilled argument or NULL. Neither are an intended address. /// * The backstop cannot have ABI-conforming spilled arguments. /// - No code runs between `fptr` and `lucet_context_backstop`, so nothing exists to /// clean up `fptr`'s arguments. `lucet_context_backstop` would have to adjust the /// stack pointer by a variable amount, and it does not, so `rsp` will continue to /// point to guest arguments. /// - This is why bootstrap recieves arguments via rbp, pointing elsewhere on the stack. /// /// The bootstrap function must be careful, but is less constrained since it can clean up /// and prepare a context for `fptr`. pub fn init( stack: &mut [u64], child: &mut Context, fptr: usize, args: &[Val], ) -> Result<(), Error> { Context::init_with_callback( stack, child, Context::default_backstop_callback, ptr::null_mut(), fptr, args, ) } /// The default backstop callback does nothing, and is just a marker. extern "C" fn default_backstop_callback(_: *mut Instance) {} /// Similar to `Context::init()`, but allows setting a callback function to be run when the /// guest entrypoint returns. /// /// After the entrypoint function returns, but before swapping back to the parent context, /// `backstop_callback` will be run with the single argument `backstop_data`. pub fn init_with_callback( stack: &mut [u64], child: &mut Context, backstop_callback: unsafe extern "C" fn(*mut Instance), backstop_data: *mut Instance, fptr: usize, args: &[Val], ) -> Result<(), Error> { if !stack_is_aligned(stack) { return Err(Error::UnalignedStack); } if backstop_callback != Context::default_backstop_callback { child.backstop_callback = backstop_callback as *const _; child.backstop_data = backstop_data; } let mut gp_args_ix = 0; let mut fp_args_ix = 0; let mut gp_regs_values = [0u64; 6]; let mut spilled_args = vec![]; for arg in args { match val_to_reg(arg) { RegVal::GpReg(v) => { if gp_args_ix >= 6 { spilled_args.push(val_to_stack(arg)); } else { gp_regs_values[gp_args_ix] = v; gp_args_ix += 1; } } RegVal::FpReg(v) => { if fp_args_ix >= 8 { spilled_args.push(val_to_stack(arg)); } else { child.bootstrap_fp_ix_arg(fp_args_ix, v); fp_args_ix += 1; } } } } // set up an initial call stack for guests to bootstrap into and execute let mut stack_builder = CallStackBuilder::new(stack); // we actually don't want to put an explicit pointer to these arguments anywhere. we'll // line up the rest of the stack such that these are in argument position when we jump to // `fptr`. stack_builder.store_args(spilled_args.as_slice()); // the stack must be aligned in the environment we'll execute `fptr` from - this is an ABI // requirement and can cause segfaults if not upheld. assert_eq!( stack_builder.offset() % 2, 0, "incorrect alignment for guest call frame" ); // we execute the guest code via returns, so we make a "call stack" of routines like: // -> lucet_context_backstop() // -> fptr() // -> lucet_context_bootstrap() // // with each address the start of the named function, so when the inner function // completes it returns to begin the next function up. stack_builder.push(lucet_context_backstop as u64); stack_builder.push(fptr as u64); // add all general purpose arguments for the guest to be bootstrapped for arg in gp_regs_values.iter() { stack_builder.push(*arg); } stack_builder.push(lucet_context_bootstrap as u64); let (stack, stack_start) = stack_builder.into_inner(); // RSP, RBP, and sigset still remain to be initialized. // Stack pointer: this points to the return address that will be used by `swap`, in place // of the original (eg, in the host) return address. The return address this points to is // the address of the first function to run on `swap`: `lucet_context_bootstrap`. child.gpr.rsp = &mut stack[stack.len() - stack_start] as *mut u64 as u64; child.gpr.rbp = child as *const Context as u64; // Read the mask to be restored if we ever need to jump out of a signal handler. If this // isn't possible, die. signal::pthread_sigmask( signal::SigmaskHow::SIG_SETMASK, None, Some(&mut child.sigset), ) .expect("pthread_sigmask could not be retrieved"); Ok(()) } /// Save the current context, and swap to another context. /// /// - `from`: the current context is written here /// - `to`: the context to read from and swap to /// /// The current registers, including the stack pointer, are saved to `from`. The current stack /// pointer is then replaced by the value saved in `to.gpr.rsp`, so when `swap` returns, it will /// return to the pointer saved in `to`'s stack. /// /// If `to` was freshly initialized by passing it as the `child` argument to `init`, `swap` will /// return to the function that bootstraps arguments and then calls the entrypoint that was /// passed to `init`. /// /// If `to` was previously passed as the `from` argument to another call to `swap`, the program /// will return as if from that _first_ call to `swap`. /// /// The address of `from` will be saved as `to.parent_ctx`. If `to` was initialized by `init`, /// it will swap back to the `from` context when the entrypoint function returns via /// `lucet_context_backstop`. /// /// # Safety /// /// The value in `to.gpr.rsp` must be a valid pointer into the stack that was originally passed /// to `init` when the `to` context was initialized, or to the original stack created implicitly /// by Rust. /// /// The registers saved in the `to` context must match the arguments expected by the entrypoint /// of the function passed to `init`, or be unaltered from when they were previously written by /// `swap`. /// /// If `to` was initialized by `init`, the `from` context must not be moved, dropped, or /// otherwise invalidated while in the `to` context unless `to`'s entrypoint function never /// returns. /// /// If `from` is never returned to, `swap`ped to, or `set` to, resources could leak due to /// implicit `drop`s never being called: /// /// ```no_run /// # use lucet_runtime_internals::context::Context; /// fn f(x: Box<u64>, child: &mut Context) { /// let mut xs = vec![187; 410757864530]; /// xs[0] += *x; /// /// // manually drop here to avoid leaks /// drop(x); /// drop(xs); /// /// let mut parent = Context::new(); /// unsafe { Context::swap(&mut parent, child); } /// // implicit `drop(x)` and `drop(xs)` here never get called unless we swap back /// } /// ``` /// /// # Examples /// /// The typical case is to initialize a new child context, and then swap to it from a zeroed /// parent context. /// /// ```no_run /// # use lucet_runtime_internals::context::Context; /// # extern "C" fn entrypoint() {} /// # let mut stack = vec![0u64; 1024].into_boxed_slice(); /// let mut parent = Context::new(); /// let mut child = Context::new(); /// Context::init( /// &mut stack, /// &mut child, /// entrypoint as usize, /// &[], /// ).unwrap(); /// /// unsafe { Context::swap(&mut parent, &mut child); } /// ``` #[inline] pub unsafe fn swap(from: &mut Context, to: &mut Context) { to.parent_ctx = from; lucet_context_swap(from as *mut _, to as *mut _); } /// Swap to another context without saving the current context. /// /// - `to`: the context to read from and swap to /// /// The current registers, including the stack pointer, are discarded. The current stack pointer /// is then replaced by the value saved in `to.gpr.rsp`, so when `swap` returns, it will return /// to the pointer saved in `to`'s stack. /// /// If `to` was freshly initialized by passing it as the child to `init`, `swap` will return to /// the function that bootstraps arguments and then calls the entrypoint that was passed to /// `init`. /// /// If `to` was previously passed as the `from` argument to another call to `swap`, the program /// will return as if from the call to `swap`. /// /// # Safety /// /// ## Stack and registers /// /// The value in `to.gpr.rsp` must be a valid pointer into the stack that was originally passed /// to `init` when the context was initialized, or to the original stack created implicitly by /// Rust. /// /// The registers saved in `to` must match the arguments expected by the entrypoint of the /// function passed to `init`, or be unaltered from when they were previously written by `swap`. /// /// ## Returning /// /// If `to` is a context freshly initialized by `init` (as opposed to a context populated only /// by `swap`, such as a host context), at least one of the following must be true, otherwise /// the program will return to a context with uninitialized registers: /// /// - The `fptr` argument to `init` is a function that never returns /// /// - A valid context must have been passed as the `from` argument to `swap` when entering the /// current context before this call to `set` /// /// ## Resource leaks /// /// Since control flow will not return to the calling context, care must be taken to ensure that /// any resources owned by the calling context are manually dropped. The implicit `drop`s /// inserted by Rust at the end of the calling scope will not be reached: /// /// ```no_run /// # use lucet_runtime_internals::context::Context; /// fn f(x: Box<u64>, child: &Context) { /// let mut xs = vec![187; 410757864530]; /// xs[0] += *x; /// /// // manually drop here to avoid leaks /// drop(x); /// drop(xs); /// /// unsafe { Context::set(child); } /// // implicit `drop(x)` and `drop(xs)` here never get called /// } /// ``` #[inline] pub unsafe fn set(to: &Context) -> ! { lucet_context_set(to as *const Context); } /// Like `set`, but also manages the return from a signal handler. /// /// TODO: the return type of this function should really be `Result<!, nix::Error>`, but using /// `!` as a type like that is currently experimental. #[inline] pub unsafe fn set_from_signal(to: &Context) -> Result<(), nix::Error> { signal::pthread_sigmask(signal::SigmaskHow::SIG_SETMASK, Some(&to.sigset), None)?; Context::set(to) } /// Clear (zero) return values. pub fn clear_retvals(&mut self) { self.retvals_gp = [0; 2]; let zero = unsafe { _mm_setzero_ps() }; self.retval_fp = zero; } /// Get the general-purpose return value at index `idx`. /// /// If this method is called before the context has returned from its original entrypoint, the /// result will be `0`. pub fn get_retval_gp(&self, idx: usize) -> u64 { self.retvals_gp[idx] } /// Get the floating point return value. /// /// If this method is called before the context has returned from its original entrypoint, the /// result will be `0.0`. pub fn get_retval_fp(&self) -> __m128 { self.retval_fp } /// Get the return value as an `UntypedRetVal`. /// /// This combines the 0th general-purpose return value, and the single floating-point return value. pub fn get_untyped_retval(&self) -> UntypedRetVal { let gp = self.get_retval_gp(0); let fp = self.get_retval_fp(); UntypedRetVal::new(gp, fp) } /// Put one of the first 8 floating-point arguments into a `Context` register. /// /// - `ix`: ABI floating-point argument number /// - `arg`: argument value fn bootstrap_fp_ix_arg(&mut self, ix: usize, arg: __m128) { match ix { 0 => self.fpr.xmm0 = arg, 1 => self.fpr.xmm1 = arg, 2 => self.fpr.xmm2 = arg, 3 => self.fpr.xmm3 = arg, 4 => self.fpr.xmm4 = arg, 5 => self.fpr.xmm5 = arg, 6 => self.fpr.xmm6 = arg, 7 => self.fpr.xmm7 = arg, _ => panic!("unexpected fp register index {}", ix), } } } /// Errors that may arise when working with contexts. #[derive(Debug, Error)] pub enum Error { /// Raised when the bottom of the stack provided to `Context::init` is not 16-byte aligned #[error("context initialized with unaligned stack")] UnalignedStack, } /// Check whether the bottom (highest address) of the stack is 16-byte aligned, as required by the /// ABI. fn stack_is_aligned(stack: &[u64]) -> bool { let size = stack.len(); let last_elt_addr = &stack[size - 1] as *const u64 as usize; let bottom_addr = last_elt_addr + mem::size_of::<u64>(); bottom_addr % 16 == 0 } extern "C" { /// Bootstraps arguments and calls the entrypoint via returning; implemented in assembly. /// /// Loads general-purpose arguments from the callee-saved registers in a `Context` to the /// appropriate argument registers for the AMD64 ABI, and then returns to the entrypoint. fn lucet_context_bootstrap(); /// Stores return values into the parent context, and then swaps to it; implemented in assembly. /// /// This is where the entrypoint function returns to, so that we swap back to the parent on /// return. fn lucet_context_backstop(); /// Saves the current context and performs the context switch. Implemented in assembly. fn lucet_context_swap(from: *mut Context, to: *mut Context); /// Performs the context switch; implemented in assembly. /// /// Never returns because the current context is discarded. fn lucet_context_set(to: *const Context) -> !; /// Enables termination for the instance, after performing a context switch. /// /// Takes the guest return address as an argument as a consequence of implementation details, /// see `Instance::swap_and_return` for more. pub(crate) fn lucet_context_activate(); }