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Merged
merged 3 commits into from
Mar 18, 2025
Merged

treat hermit like wasm32 #109

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b219b65
treat hermit like wasm32
m-mueller678 Dec 11, 2024
93247a2
move page rounding logic into `StackRestoreGuard` and split mmap and …
m-mueller678 Mar 17, 2025
5472d9e
Apply suggestions from code review
nagisa Mar 18, 2025
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47 changes: 47 additions & 0 deletions src/alloc_stack_restore_guard.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,47 @@
use crate::{get_stack_limit, set_stack_limit};

pub struct StackRestoreGuard {
new_stack: *mut u8,
stack_bytes: usize,
old_stack_limit: Option<usize>,
}

const ALIGNMENT: usize = 16;

impl StackRestoreGuard {
pub fn new(stack_bytes: usize) -> StackRestoreGuard {
// On these platforms we do not use stack guards. this is very unfortunate,
// but there is not much we can do about it without OS support.
// We simply allocate the requested size from the global allocator with a suitable
// alignment.
let stack_bytes = stack_bytes
.checked_add(ALIGNMENT - 1)
.expect("unreasonably large stack requested")
/ ALIGNMENT
* ALIGNMENT;
let layout = std::alloc::Layout::from_size_align(stack_bytes, ALIGNMENT).unwrap();
let ptr = unsafe { std::alloc::alloc(layout) };
assert!(!ptr.is_null(), "unable to allocate stack");
StackRestoreGuard {
new_stack: ptr,
stack_bytes,
old_stack_limit: get_stack_limit(),
}
}

pub fn stack_area(&self) -> (*mut u8, usize) {
(self.new_stack, self.stack_bytes)
}
}

impl Drop for StackRestoreGuard {
fn drop(&mut self) {
unsafe {
std::alloc::dealloc(
self.new_stack,
std::alloc::Layout::from_size_align_unchecked(self.stack_bytes, ALIGNMENT),
);
}
set_stack_limit(self.old_stack_limit);
}
}
139 changes: 19 additions & 120 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -135,123 +135,30 @@ 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 {
#[cfg(target_arch = "wasm32")]
unsafe fn new(stack_bytes: usize, _page_size: usize) -> StackRestoreGuard {
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(),
}
}

#[cfg(not(target_arch = "wasm32"))]
unsafe fn new(stack_bytes: usize, page_size: usize) -> StackRestoreGuard {
let new_stack = libc::mmap(
std::ptr::null_mut(),
stack_bytes,
libc::PROT_NONE,
libc::MAP_PRIVATE |
libc::MAP_ANON,
-1, // Some implementations assert fd = -1 if MAP_ANON is specified
0
);
assert_ne!(
new_stack,
libc::MAP_FAILED,
"mmap failed to allocate stack: {}",
std::io::Error::last_os_error()
);
let guard = StackRestoreGuard {
new_stack,
stack_bytes,
old_stack_limit: get_stack_limit(),
};
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
);
#[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 {
0
} else {
-1
};
assert_ne!(
result,
-1,
"mprotect/mmap failed: {}",
std::io::Error::last_os_error()
);
guard
}
}
#[cfg(not(any(target_arch = "wasm32",target_os = "hermit")))]
#[path = "mmap_stack_restore_guard.rs"]
mod stack_restore_guard;

impl Drop for StackRestoreGuard {
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),
);
}
#[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);
}
set_stack_limit(self.old_stack_limit);
}
}
#[cfg(any(target_arch = "wasm32",target_os = "hermit"))]
#[path = "alloc_stack_restore_guard.rs"]
mod stack_restore_guard;

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 requested");
use stack_restore_guard::StackRestoreGuard;

// 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).
//
fn _grow(requested_stack_size: usize, callback: &mut dyn FnMut()) {
// 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 || {
// 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).
// `StackRestoreGuard` allocates a memory area with suitable size and alignment.
// It also sets up stack guards if supported on target.
let guard = StackRestoreGuard::new(requested_stack_size);
let (stack_base, allocated_stack_size) = guard.stack_area();
debug_assert!(allocated_stack_size >= requested_stack_size);
set_stack_limit(Some(stack_base as usize));
// TODO should we not pass `allocated_stack_size` here?
let panic = psm::on_stack(stack_base, requested_stack_size, move || {
std::panic::catch_unwind(std::panic::AssertUnwindSafe(callback)).err()
});
drop(guard);
Expand All @@ -260,14 +167,6 @@ psm_stack_manipulation! {
}
}
}

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 {
Expand Down
105 changes: 105 additions & 0 deletions src/mmap_stack_restore_guard.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,105 @@
use crate::{get_stack_limit, set_stack_limit};

pub struct StackRestoreGuard {
mapping: *mut u8,
size_with_guard: usize,
page_size: usize,
old_stack_limit: Option<usize>,
}

impl StackRestoreGuard {
pub fn new(requested_size: usize) -> StackRestoreGuard {
// 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. It is natural to use mmap to allocate
// these pages. 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 = requested_size
.checked_add(page_size - 1)
.expect("unreasonably large stack requested")
/ page_size;
let page_count_with_guard = std::cmp::max(1, requested_pages) + 2;
let size_with_guard = page_count_with_guard
.checked_mul(page_size)
.expect("unreasonably large stack requested");

unsafe {
let new_stack = libc::mmap(
std::ptr::null_mut(),
size_with_guard,
libc::PROT_NONE,
libc::MAP_PRIVATE | libc::MAP_ANON,
-1, // Some implementations assert fd = -1 if MAP_ANON is specified
0,
);
assert_ne!(
new_stack,
libc::MAP_FAILED,
"mmap failed to allocate stack: {}",
std::io::Error::last_os_error()
);
let guard = StackRestoreGuard {
mapping: new_stack as *mut u8,
page_size,
size_with_guard,
old_stack_limit: get_stack_limit(),
};
// We leave a guard page without read/write access in our allocation.
// There is one guard page below the stack and another above it.
let above_guard_page = new_stack.add(page_size);
#[cfg(not(target_os = "openbsd"))]
let result = libc::mprotect(
above_guard_page,
size_with_guard - page_size,
libc::PROT_READ | libc::PROT_WRITE,
);
#[cfg(target_os = "openbsd")]
let result = if libc::mmap(
above_guard_page,
size_with_guard - 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
};
assert_ne!(
result,
-1,
"mprotect/mmap failed: {}",
std::io::Error::last_os_error()
);
guard
}
}

// TODO this should return a *mut [u8], but pointer slices only got proper support with Rust 1.79.
pub fn stack_area(&self) -> (*mut u8, usize) {
unsafe {
(
self.mapping.add(self.page_size),
self.size_with_guard - self.page_size,
)
}
}
}

impl Drop for StackRestoreGuard {
fn drop(&mut self) {
unsafe {
// FIXME: check the error code and decide what to do with it.
// Perhaps a debug_assertion?
libc::munmap(self.mapping as *mut std::ffi::c_void, self.size_with_guard);
}
set_stack_limit(self.old_stack_limit);
}
}

fn page_size() -> usize {
// FIXME: consider caching the page size.
unsafe { libc::sysconf(libc::_SC_PAGE_SIZE) as usize }
}
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