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Reusable Stack #74

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192 changes: 118 additions & 74 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -134,143 +134,187 @@ fn set_stack_limit(l: Option<usize>) {
psm_stack_manipulation! {
yes {
struct StackRestoreGuard {
new_stack: *mut std::ffi::c_void,
stack_bytes: usize,
old_stack_limit: Option<usize>,
}

impl StackRestoreGuard {
fn new() -> StackRestoreGuard {
StackRestoreGuard {
old_stack_limit: get_stack_limit(),
}
}
}

impl Drop for StackRestoreGuard {
fn drop(&mut self) {
set_stack_limit(self.old_stack_limit);
}
}

fn _grow(stack_size: usize, callback: &mut dyn FnMut()) {
unsafe {
let stack = Stack::new(stack_size);
stack.call(callback);
}
}

fn page_size() -> usize {
// FIXME: consider caching the page size.
#[cfg(not(target_arch = "wasm32"))]
unsafe {
libc::sysconf(libc::_SC_PAGE_SIZE) as usize
}
#[cfg(target_arch = "wasm32")]
unsafe fn new(stack_bytes: usize, _page_size: usize) -> StackRestoreGuard {
{
65536
}
}

fn calculate_stack_size(stack_size: usize) -> usize {
// Calculate a number of pages we want to allocate for the new stack.
// For maximum portability we want to produce a stack that is aligned to a page and has
// a size that’s a multiple of page size. Furthermore we want to allocate two extras pages
// for the stack guard. To achieve that we do our calculations in number of pages and
// convert to bytes last.
let page_size = page_size();
let requested_pages = stack_size
.checked_add(page_size - 1)
.expect("unreasonably large stack requested")
/ page_size;
let stack_pages = std::cmp::max(1, requested_pages) + 2;
let stack_bytes = stack_pages
.checked_mul(page_size)
.expect("unreasonably large stack requesteed");

// Next, there are a couple of approaches to how we allocate the new stack. We take the
// most obvious path and use `mmap`. We also `mprotect` a guard page into our
// allocation.
stack_bytes
}

pub struct Stack {
ptr: *mut std::ffi::c_void,
size: usize,
requested_size: usize
}

impl Stack {

#[cfg(target_arch = "wasm32")]
pub unsafe fn new(stack_size: usize) -> Stack {
let stack_bytes = calculate_stack_size();
let layout = std::alloc::Layout::from_size_align(stack_bytes, 16).unwrap();
let ptr = std::alloc::alloc(layout);
assert!(!ptr.is_null(), "unable to allocate stack");
StackRestoreGuard {
new_stack: ptr as *mut _,
stack_bytes,
old_stack_limit: get_stack_limit(),
Stack {
ptr: ptr as *mut _,
size: stack_bytes,
requested_size: stack_size
}
}

#[cfg(not(target_arch = "wasm32"))]
unsafe fn new(stack_bytes: usize, page_size: usize) -> StackRestoreGuard {
pub unsafe fn new(stack_size: usize) -> Stack {
let stack_bytes = calculate_stack_size(stack_size);
let new_stack = libc::mmap(
std::ptr::null_mut(),
stack_bytes,
libc::PROT_NONE,
libc::MAP_PRIVATE |
libc::MAP_ANON,
libc::MAP_PRIVATE | libc::MAP_ANON,
-1, // Some implementations assert fd = -1 if MAP_ANON is specified
0
0,
);
let page_size = page_size();
if new_stack == libc::MAP_FAILED {
let error = std::io::Error::last_os_error();
panic!("allocating stack failed with: {}", error)
}
let guard = StackRestoreGuard {
new_stack,
stack_bytes,
old_stack_limit: get_stack_limit(),
let stack = Stack {
ptr: new_stack,
size: stack_bytes,
requested_size: stack_size
};
let above_guard_page = new_stack.add(page_size);
#[cfg(not(target_os = "openbsd"))]
let result = libc::mprotect(
above_guard_page,
stack_bytes - page_size,
libc::PROT_READ | libc::PROT_WRITE
libc::PROT_READ | libc::PROT_WRITE,
);
#[cfg(target_os = "openbsd")]
let result = if libc::mmap(
above_guard_page,
stack_bytes - page_size,
libc::PROT_READ | libc::PROT_WRITE,
libc::MAP_FIXED | libc::MAP_PRIVATE | libc::MAP_ANON | libc::MAP_STACK,
-1,
0) == above_guard_page {
above_guard_page,
stack_bytes - page_size,
libc::PROT_READ | libc::PROT_WRITE,
libc::MAP_FIXED | libc::MAP_PRIVATE | libc::MAP_ANON | libc::MAP_STACK,
-1,
0,
) == above_guard_page
{
0
} else {
-1
};
if result == -1 {
let error = std::io::Error::last_os_error();
drop(guard);
drop(stack);
panic!("setting stack permissions failed with: {}", error)
}
guard
stack
}

pub fn call(&self, callback: &mut dyn FnMut()) {
unsafe {
let guard = StackRestoreGuard::new();
let above_guard_page = self.ptr.add(page_size());
set_stack_limit(Some(above_guard_page as usize));
let panic = psm::on_stack(above_guard_page as *mut _, self.requested_size, move || {
std::panic::catch_unwind(std::panic::AssertUnwindSafe(callback)).err()
});
drop(guard);
if let Some(p) = panic {
std::panic::resume_unwind(p);
}
}
}
}

impl Drop for StackRestoreGuard {
impl Drop for Stack {
fn drop(&mut self) {
#[cfg(target_arch = "wasm32")]
unsafe {
std::alloc::dealloc(
self.new_stack as *mut u8,
std::alloc::Layout::from_size_align_unchecked(self.stack_bytes, 16),
std::alloc::Layout::from_size_align_unchecked(self.size, 16),
);
}
#[cfg(not(target_arch = "wasm32"))]
unsafe {
// FIXME: check the error code and decide what to do with it.
// Perhaps a debug_assertion?
libc::munmap(self.new_stack, self.stack_bytes);
libc::munmap(self.ptr, self.size);
}
set_stack_limit(self.old_stack_limit);
}
}

fn _grow(stack_size: usize, callback: &mut dyn FnMut()) {
// Calculate a number of pages we want to allocate for the new stack.
// For maximum portability we want to produce a stack that is aligned to a page and has
// a size that’s a multiple of page size. Furthermore we want to allocate two extras pages
// for the stack guard. To achieve that we do our calculations in number of pages and
// convert to bytes last.
let page_size = page_size();
let requested_pages = stack_size
.checked_add(page_size - 1)
.expect("unreasonably large stack requested") / page_size;
let stack_pages = std::cmp::max(1, requested_pages) + 2;
let stack_bytes = stack_pages.checked_mul(page_size)
.expect("unreasonably large stack requesteed");

// Next, there are a couple of approaches to how we allocate the new stack. We take the
// most obvious path and use `mmap`. We also `mprotect` a guard page into our
// allocation.
//
// We use a guard pattern to ensure we deallocate the allocated stack when we leave
// this function and also try to uphold various safety invariants required by `psm`
// (such as not unwinding from the callback we pass to it).
//
// Other than that this code has no meaningful gotchas.
unsafe {
let guard = StackRestoreGuard::new(stack_bytes, page_size);
let above_guard_page = guard.new_stack.add(page_size);
set_stack_limit(Some(above_guard_page as usize));
let panic = psm::on_stack(above_guard_page as *mut _, stack_size, move || {
std::panic::catch_unwind(std::panic::AssertUnwindSafe(callback)).err()
});
drop(guard);
if let Some(p) = panic {
std::panic::resume_unwind(p);
}
}
}

fn page_size() -> usize {
// FIXME: consider caching the page size.
#[cfg(not(target_arch = "wasm32"))]
unsafe { libc::sysconf(libc::_SC_PAGE_SIZE) as usize }
#[cfg(target_arch = "wasm32")]
{ 65536 }
}
}

no {
#[cfg(not(windows))]
fn _grow(stack_size: usize, callback: &mut dyn FnMut()) {
drop(stack_size);
callback();
}

pub struct Stack;

impl Stack {
pub unsafe fn new(_stack_bytes: usize, _page_size: usize) -> Stack {
Stack
}

pub call(&self, callback: &mut dyn FnMut()) {
callback();
}
}
}
}

Expand Down