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//! Memory management for linear memories.
//!
//! `RuntimeLinearMemory` is to WebAssembly linear memories what `Table` is to WebAssembly tables.
use crate::mmap::Mmap;
use crate::vmcontext::VMMemoryDefinition;
use crate::ResourceLimiter;
use anyhow::{bail, format_err, Error, Result};
use more_asserts::{assert_ge, assert_le};
use std::convert::TryFrom;
use wasmtime_environ::{MemoryPlan, MemoryStyle, WASM32_MAX_PAGES, WASM64_MAX_PAGES};
const WASM_PAGE_SIZE: usize = wasmtime_environ::WASM_PAGE_SIZE as usize;
const WASM_PAGE_SIZE_U64: u64 = wasmtime_environ::WASM_PAGE_SIZE as u64;
/// A memory allocator
pub trait RuntimeMemoryCreator: Send + Sync {
/// Create new RuntimeLinearMemory
fn new_memory(
&self,
plan: &MemoryPlan,
minimum: usize,
maximum: Option<usize>,
) -> Result<Box<dyn RuntimeLinearMemory>>;
}
/// A default memory allocator used by Wasmtime
pub struct DefaultMemoryCreator;
impl RuntimeMemoryCreator for DefaultMemoryCreator {
/// Create new MmapMemory
fn new_memory(
&self,
plan: &MemoryPlan,
minimum: usize,
maximum: Option<usize>,
) -> Result<Box<dyn RuntimeLinearMemory>> {
Ok(Box::new(MmapMemory::new(plan, minimum, maximum)?))
}
}
/// A linear memory
pub trait RuntimeLinearMemory: Send + Sync {
/// Returns the number of allocated bytes.
fn byte_size(&self) -> usize;
/// Returns the maximum number of bytes the memory can grow to.
/// Returns `None` if the memory is unbounded.
fn maximum_byte_size(&self) -> Option<usize>;
/// Grow memory to the specified amount of bytes.
///
/// Returns an error if memory can't be grown by the specified amount
/// of bytes.
fn grow_to(&mut self, size: usize) -> Result<()>;
/// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm
/// code.
fn vmmemory(&self) -> VMMemoryDefinition;
}
/// A linear memory instance.
#[derive(Debug)]
pub struct MmapMemory {
// The underlying allocation.
mmap: Mmap,
// The number of bytes that are accessible in `mmap` and available for
// reading and writing.
//
// This region starts at `pre_guard_size` offset from the base of `mmap`.
accessible: usize,
// The optional maximum accessible size, in bytes, for this linear memory.
//
// Note that this maximum does not factor in guard pages, so this isn't the
// maximum size of the linear address space reservation for this memory.
maximum: Option<usize>,
// The amount of extra bytes to reserve whenever memory grows. This is
// specified so that the cost of repeated growth is amortized.
extra_to_reserve_on_growth: usize,
// Size in bytes of extra guard pages before the start and after the end to
// optimize loads and stores with constant offsets.
pre_guard_size: usize,
offset_guard_size: usize,
}
impl MmapMemory {
/// Create a new linear memory instance with specified minimum and maximum number of wasm pages.
pub fn new(plan: &MemoryPlan, minimum: usize, maximum: Option<usize>) -> Result<Self> {
// It's a programmer error for these two configuration values to exceed
// the host available address space, so panic if such a configuration is
// found (mostly an issue for hypothetical 32-bit hosts).
let offset_guard_bytes = usize::try_from(plan.offset_guard_size).unwrap();
let pre_guard_bytes = usize::try_from(plan.pre_guard_size).unwrap();
let (alloc_bytes, extra_to_reserve_on_growth) = match plan.style {
MemoryStyle::Dynamic { reserve } => (minimum, usize::try_from(reserve).unwrap()),
MemoryStyle::Static { bound } => {
assert_ge!(bound, plan.memory.minimum);
(
usize::try_from(bound.checked_mul(WASM_PAGE_SIZE_U64).unwrap()).unwrap(),
0,
)
}
};
let request_bytes = pre_guard_bytes
.checked_add(alloc_bytes)
.and_then(|i| i.checked_add(extra_to_reserve_on_growth))
.and_then(|i| i.checked_add(offset_guard_bytes))
.ok_or_else(|| format_err!("cannot allocate {} with guard regions", minimum))?;
let mut mmap = Mmap::accessible_reserved(0, request_bytes)?;
if minimum > 0 {
mmap.make_accessible(pre_guard_bytes, minimum)?;
}
Ok(Self {
mmap,
accessible: minimum,
maximum,
pre_guard_size: pre_guard_bytes,
offset_guard_size: offset_guard_bytes,
extra_to_reserve_on_growth,
})
}
}
impl RuntimeLinearMemory for MmapMemory {
fn byte_size(&self) -> usize {
self.accessible
}
fn maximum_byte_size(&self) -> Option<usize> {
self.maximum
}
fn grow_to(&mut self, new_size: usize) -> Result<()> {
if new_size > self.mmap.len() - self.offset_guard_size - self.pre_guard_size {
// If the new size of this heap exceeds the current size of the
// allocation we have, then this must be a dynamic heap. Use
// `new_size` to calculate a new size of an allocation, allocate it,
// and then copy over the memory from before.
let request_bytes = self
.pre_guard_size
.checked_add(new_size)
.and_then(|s| s.checked_add(self.extra_to_reserve_on_growth))
.and_then(|s| s.checked_add(self.offset_guard_size))
.ok_or_else(|| format_err!("overflow calculating size of memory allocation"))?;
let mut new_mmap = Mmap::accessible_reserved(0, request_bytes)?;
new_mmap.make_accessible(self.pre_guard_size, new_size)?;
new_mmap.as_mut_slice()[self.pre_guard_size..][..self.accessible]
.copy_from_slice(&self.mmap.as_slice()[self.pre_guard_size..][..self.accessible]);
self.mmap = new_mmap;
} else {
// If the new size of this heap fits within the existing allocation
// then all we need to do is to make the new pages accessible. This
// can happen either for "static" heaps which always hit this case,
// or "dynamic" heaps which have some space reserved after the
// initial allocation to grow into before the heap is moved in
// memory.
assert!(new_size > self.accessible);
self.mmap.make_accessible(
self.pre_guard_size + self.accessible,
new_size - self.accessible,
)?;
}
self.accessible = new_size;
Ok(())
}
fn vmmemory(&self) -> VMMemoryDefinition {
VMMemoryDefinition {
base: unsafe { self.mmap.as_mut_ptr().add(self.pre_guard_size) },
current_length: self.accessible,
}
}
}
/// Representation of a runtime wasm linear memory.
pub enum Memory {
/// A "static" memory where the lifetime of the backing memory is managed
/// elsewhere. Currently used with the pooling allocator.
Static {
/// The memory in the host for this wasm memory. The length of this
/// slice is the maximum size of the memory that can be grown to.
base: &'static mut [u8],
/// The current size, in bytes, of this memory.
size: usize,
/// A callback which makes portions of `base` accessible for when memory
/// is grown. Otherwise it's expected that accesses to `base` will
/// fault.
make_accessible: fn(*mut u8, usize) -> Result<()>,
/// Stores the pages in the linear memory that have faulted as guard pages when using the `uffd` feature.
/// These pages need their protection level reset before the memory can grow.
#[cfg(all(feature = "uffd", target_os = "linux"))]
guard_page_faults: Vec<(usize, usize, fn(*mut u8, usize) -> Result<()>)>,
},
/// A "dynamic" memory whose data is managed at runtime and lifetime is tied
/// to this instance.
Dynamic(Box<dyn RuntimeLinearMemory>),
}
fn memory_growing(
limiter: &mut Option<&mut dyn ResourceLimiter>,
current: usize,
desired: usize,
maximum: Option<usize>,
) -> bool {
match limiter {
Some(ref mut l) => l.memory_growing(current, desired, maximum),
None => true,
}
}
fn memory_grow_failed(limiter: &mut Option<&mut dyn ResourceLimiter>, error: &Error) {
match limiter {
Some(l) => l.memory_grow_failed(error),
None => {}
}
}
impl Memory {
/// Create a new dynamic (movable) memory instance for the specified plan.
pub fn new_dynamic(
plan: &MemoryPlan,
creator: &dyn RuntimeMemoryCreator,
limiter: Option<&mut dyn ResourceLimiter>,
) -> Result<Self> {
let (minimum, maximum) = Self::limit_new(plan, limiter)?;
Ok(Memory::Dynamic(creator.new_memory(plan, minimum, maximum)?))
}
/// Create a new static (immovable) memory instance for the specified plan.
pub fn new_static(
plan: &MemoryPlan,
base: &'static mut [u8],
make_accessible: fn(*mut u8, usize) -> Result<()>,
limiter: Option<&mut dyn ResourceLimiter>,
) -> Result<Self> {
let (minimum, maximum) = Self::limit_new(plan, limiter)?;
let base = match maximum {
Some(max) if max < base.len() => &mut base[..max],
_ => base,
};
if minimum > 0 {
make_accessible(base.as_mut_ptr(), minimum)?;
}
Ok(Memory::Static {
base,
size: minimum,
make_accessible,
#[cfg(all(feature = "uffd", target_os = "linux"))]
guard_page_faults: Vec::new(),
})
}
/// Calls the `limiter`, if specified, to optionally prevent a memory from
/// being allocated.
///
/// Returns the minimum size and optional maximum size of the memory, in
/// bytes.
fn limit_new(
plan: &MemoryPlan,
mut limiter: Option<&mut dyn ResourceLimiter>,
) -> Result<(usize, Option<usize>)> {
// Sanity-check what should already be true from wasm module validation.
let absolute_max = if plan.memory.memory64 {
WASM64_MAX_PAGES
} else {
WASM32_MAX_PAGES
};
assert_le!(plan.memory.minimum, absolute_max);
assert!(plan.memory.maximum.is_none() || plan.memory.maximum.unwrap() <= absolute_max);
// This is the absolute possible maximum that the module can try to
// allocate, which is our entire address space minus a wasm page. That
// shouldn't ever actually work in terms of an allocation because
// presumably the kernel wants *something* for itself, but this is used
// to pass to the `limiter` specified, if present, for a requested size
// to approximate the scale of the request that the wasm module is
// making. This is necessary because the limiter works on `usize` bytes
// whereas we're working with possibly-overflowing `u64` calculations
// here. To actually faithfully represent the byte requests of modules
// we'd have to represent things as `u128`, but that's kinda
// overkill for this purpose.
let absolute_max = 0usize.wrapping_sub(WASM_PAGE_SIZE);
// If the minimum memory size overflows the size of our own address
// space, then we can't satisfy this request, but defer the error to
// later so the `limiter` can be informed that an effective oom is
// happening.
let minimum = plan
.memory
.minimum
.checked_mul(WASM_PAGE_SIZE_U64)
.and_then(|m| usize::try_from(m).ok());
// The plan stores the maximum size in units of wasm pages, but we
// use units of bytes. Unlike for the `minimum` size we silently clamp
// the effective maximum size to `absolute_max` above if the maximum is
// too large. This should be ok since as a wasm runtime we get to
// arbitrarily decide the actual maximum size of memory, regardless of
// what's actually listed on the memory itself.
let mut maximum = plan.memory.maximum.map(|max| {
usize::try_from(max)
.ok()
.and_then(|m| m.checked_mul(WASM_PAGE_SIZE))
.unwrap_or(absolute_max)
});
// If this is a 32-bit memory and no maximum is otherwise listed then we
// need to still specify a maximum size of 4GB. If the host platform is
// 32-bit then there's no need to limit the maximum this way since no
// allocation of 4GB can succeed, but for 64-bit platforms this is
// required to limit memories to 4GB.
if !plan.memory.memory64 && maximum.is_none() {
maximum = usize::try_from(1u64 << 32).ok();
}
// Inform the limiter what's about to happen. This will let the limiter
// reject anything if necessary, and this also guarantees that we should
// call the limiter for all requested memories, even if our `minimum`
// calculation overflowed. This means that the `minimum` we're informing
// the limiter is lossy and may not be 100% accurate, but for now the
// expected uses of `limiter` means that's ok.
if !memory_growing(&mut limiter, 0, minimum.unwrap_or(absolute_max), maximum) {
bail!(
"memory minimum size of {} pages exceeds memory limits",
plan.memory.minimum
);
}
// At this point we need to actually handle overflows, so bail out with
// an error if we made it this far.
let minimum = minimum.ok_or_else(|| {
format_err!(
"memory minimum size of {} pages exceeds memory limits",
plan.memory.minimum
)
})?;
Ok((minimum, maximum))
}
/// Returns the number of allocated wasm pages.
pub fn byte_size(&self) -> usize {
match self {
Memory::Static { size, .. } => *size,
Memory::Dynamic(mem) => mem.byte_size(),
}
}
/// Returns the maximum number of pages the memory can grow to at runtime.
///
/// Returns `None` if the memory is unbounded.
///
/// The runtime maximum may not be equal to the maximum from the linear memory's
/// Wasm type when it is being constrained by an instance allocator.
pub fn maximum_byte_size(&self) -> Option<usize> {
match self {
Memory::Static { base, .. } => Some(base.len()),
Memory::Dynamic(mem) => mem.maximum_byte_size(),
}
}
/// Returns whether or not the underlying storage of the memory is "static".
pub(crate) fn is_static(&self) -> bool {
if let Memory::Static { .. } = self {
true
} else {
false
}
}
/// Grow memory by the specified amount of wasm pages.
///
/// Returns `None` if memory can't be grown by the specified amount
/// of wasm pages. Returns `Some` with the old size of memory, in bytes, on
/// successful growth.
///
/// # Safety
///
/// Resizing the memory can reallocate the memory buffer for dynamic memories.
/// An instance's `VMContext` may have pointers to the memory's base and will
/// need to be fixed up after growing the memory.
///
/// Generally, prefer using `InstanceHandle::memory_grow`, which encapsulates
/// this unsafety.
pub unsafe fn grow(
&mut self,
delta_pages: u64,
mut limiter: Option<&mut dyn ResourceLimiter>,
) -> Option<usize> {
let old_byte_size = self.byte_size();
// Wasm spec: when growing by 0 pages, always return the current size.
if delta_pages == 0 {
return Some(old_byte_size);
}
// largest wasm-page-aligned region of memory it is possible to
// represent in a usize. This will be impossible for the system to
// actually allocate.
let absolute_max = 0usize.wrapping_sub(WASM_PAGE_SIZE);
// calculate byte size of the new allocation. Let it overflow up to
// usize::MAX, then clamp it down to absolute_max.
let new_byte_size = usize::try_from(delta_pages)
.unwrap_or(usize::MAX)
.saturating_mul(WASM_PAGE_SIZE)
.saturating_add(old_byte_size);
let new_byte_size = if new_byte_size > absolute_max {
absolute_max
} else {
new_byte_size
};
let maximum = self.maximum_byte_size();
// Limiter gets first chance to reject memory_growing.
if !memory_growing(&mut limiter, old_byte_size, new_byte_size, maximum) {
return None;
}
// Never exceed maximum, even if limiter permitted it.
if let Some(max) = maximum {
if new_byte_size > max {
memory_grow_failed(&mut limiter, &format_err!("Memory maximum size exceeded"));
return None;
}
}
#[cfg(all(feature = "uffd", target_os = "linux"))]
{
if self.is_static() {
// Reset any faulted guard pages before growing the memory.
self.reset_guard_pages().ok()?;
}
}
match self {
Memory::Static {
base,
size,
make_accessible,
..
} => {
// Never exceed static memory size
if new_byte_size > base.len() {
memory_grow_failed(&mut limiter, &format_err!("static memory size exceeded"));
return None;
}
// Operating system can fail to make memory accessible
let r = make_accessible(
base.as_mut_ptr().add(old_byte_size),
new_byte_size - old_byte_size,
);
r.map_err(|e| memory_grow_failed(&mut limiter, &e)).ok()?;
*size = new_byte_size;
}
Memory::Dynamic(mem) => {
let r = mem.grow_to(new_byte_size);
r.map_err(|e| memory_grow_failed(&mut limiter, &e)).ok()?;
}
}
Some(old_byte_size)
}
/// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm code.
pub fn vmmemory(&self) -> VMMemoryDefinition {
match self {
Memory::Static { base, size, .. } => VMMemoryDefinition {
base: base.as_ptr() as *mut _,
current_length: *size,
},
Memory::Dynamic(mem) => mem.vmmemory(),
}
}
/// Records a faulted guard page in a static memory.
///
/// This is used to track faulted guard pages that need to be reset for the uffd feature.
///
/// This function will panic if called on a dynamic memory.
#[cfg(all(feature = "uffd", target_os = "linux"))]
pub(crate) fn record_guard_page_fault(
&mut self,
page_addr: *mut u8,
size: usize,
reset: fn(*mut u8, usize) -> Result<()>,
) {
match self {
Memory::Static {
guard_page_faults, ..
} => {
guard_page_faults.push((page_addr as usize, size, reset));
}
Memory::Dynamic(_) => {
unreachable!("dynamic memories should not have guard page faults")
}
}
}
/// Resets the previously faulted guard pages of a static memory.
///
/// This is used to reset the protection of any guard pages that were previously faulted.
///
/// This function will panic if called on a dynamic memory.
#[cfg(all(feature = "uffd", target_os = "linux"))]
pub(crate) fn reset_guard_pages(&mut self) -> Result<()> {
match self {
Memory::Static {
guard_page_faults, ..
} => {
for (addr, len, reset) in guard_page_faults.drain(..) {
reset(addr as *mut u8, len)?;
}
}
Memory::Dynamic(_) => {
unreachable!("dynamic memories should not have guard page faults")
}
}
Ok(())
}
}
// The default memory representation is an empty memory that cannot grow.
impl Default for Memory {
fn default() -> Self {
Memory::Static {
base: &mut [],
size: 0,
make_accessible: |_, _| unreachable!(),
#[cfg(all(feature = "uffd", target_os = "linux"))]
guard_page_faults: Vec::new(),
}
}
}