wasmi/src/memory/mmap_bytebuf.rs

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//! An implementation of a `ByteBuf` based on virtual memory.
//!
//! This implementation uses `mmap` on POSIX systems (and should use `VirtualAlloc` on windows).
//! There are possibilities to improve the performance for the reallocating case by reserving
//! memory up to maximum. This might be a problem for systems that don't have a lot of virtual
//! memory (i.e. 32-bit platforms).
use std::ptr::{self, NonNull};
use std::slice;
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use super::{MemoryBackend, ByteBuf};
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struct Mmap {
/// The pointer that points to the start of the mapping.
///
/// This value doesn't change after creation.
ptr: NonNull<u8>,
/// The length of this mapping.
///
/// Cannot be more than `isize::max_value()`. This value doesn't change after creation.
len: usize,
}
impl Mmap {
/// Create a new mmap mapping
///
/// Returns `Err` if:
/// - `len` should not exceed `isize::max_value()`
/// - `len` should be greater than 0.
/// - `mmap` returns an error (almost certainly means out of memory).
fn new(len: usize) -> Result<Self, &'static str> {
if len > isize::max_value() as usize {
return Err("`len` should not exceed `isize::max_value()`");
}
if len == 0 {
return Err("`len` should be greater than 0");
}
let ptr_or_err = unsafe {
// Safety Proof:
// There are not specific safety proofs are required for this call, since the call
// by itself can't invoke any safety problems (however, misusing its result can).
libc::mmap(
// `addr` - let the system to choose the address at which to create the mapping.
ptr::null_mut(),
// the length of the mapping in bytes.
len,
// `prot` - protection flags: READ WRITE !EXECUTE
libc::PROT_READ | libc::PROT_WRITE,
// `flags`
// `MAP_ANON` - mapping is not backed by any file and initial contents are
// initialized to zero.
// `MAP_PRIVATE` - the mapping is private to this process.
libc::MAP_ANON | libc::MAP_PRIVATE,
// `fildes` - a file descriptor. Pass -1 as this is required for some platforms
// when the `MAP_ANON` is passed.
-1,
// `offset` - offset from the file.
0,
)
};
match ptr_or_err {
// With the current parameters, the error can only be returned in case of insufficient
// memory.
libc::MAP_FAILED => Err("mmap returned an error"),
_ => {
let ptr = NonNull::new(ptr_or_err as *mut u8).ok_or("mmap returned 0")?;
Ok(Self { ptr, len })
}
}
}
fn as_slice(&self) -> &[u8] {
unsafe {
// Safety Proof:
// - Aliasing guarantees of `self.ptr` are not violated since `self` is the only owner.
// - This pointer was allocated for `self.len` bytes and thus is a valid slice.
// - `self.len` doesn't change throughout the lifetime of `self`.
// - The value is returned valid for the duration of lifetime of `self`.
// `self` cannot be destroyed while the returned slice is alive.
// - `self.ptr` is of `NonNull` type and thus `.as_ptr()` can never return NULL.
// - `self.len` cannot be larger than `isize::max_value()`.
slice::from_raw_parts(self.ptr.as_ptr(), self.len)
}
}
fn as_slice_mut(&mut self) -> &mut [u8] {
unsafe {
// Safety Proof:
// - See the proof for `Self::as_slice`
// - Additionally, it is not possible to obtain two mutable references for `self.ptr`
slice::from_raw_parts_mut(self.ptr.as_ptr(), self.len)
}
}
}
impl Drop for Mmap {
fn drop(&mut self) {
let ret_val = unsafe {
// Safety proof:
// - `self.ptr` was allocated by a call to `mmap`.
// - `self.len` was saved at the same time and it doesn't change throughout the lifetime
// of `self`.
libc::munmap(self.ptr.as_ptr() as *mut libc::c_void, self.len)
};
// There is no reason for `munmap` to fail to deallocate a private annonymous mapping
// allocated by `mmap`.
// However, for the cases when it actually fails prefer to fail, in order to not leak
// and exhaust the virtual memory.
assert_eq!(ret_val, 0, "munmap failed");
}
}
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pub struct MmapByteBuf {
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mmap: Option<Mmap>,
}
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impl MmapByteBuf {
pub fn empty() -> Self {
MmapByteBuf { mmap: None }
}
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pub fn new(len: usize) -> Result<Self, &'static str> {
let mmap = if len == 0 {
None
} else {
Some(Mmap::new(len)?)
};
Ok(Self { mmap })
}
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}
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impl MemoryBackend for MmapByteBuf {
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fn alloc(&mut self, initial: usize, _maximum: Option<usize>) -> Result<ByteBuf, &'static str> {
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self.realloc(initial)
}
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fn realloc(&mut self, new_len: usize) -> Result<ByteBuf, &'static str> {
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let new_mmap = if new_len == 0 {
None
} else {
let mut new_mmap = Mmap::new(new_len)?;
if let Some(cur_mmap) = self.mmap.take() {
let src = cur_mmap.as_slice();
let dst = new_mmap.as_slice_mut();
let amount = src.len().min(dst.len());
dst[..amount].copy_from_slice(&src[..amount]);
}
Some(new_mmap)
};
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let bytebuf = ByteBuf {
ptr: new_mmap.as_ref().map(|m| m.ptr.as_ptr()).unwrap_or(NonNull::dangling().as_ptr()),
len: new_mmap.as_ref().map(|m| m.len).unwrap_or(0),
};
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self.mmap = new_mmap;
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Ok(bytebuf)
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}
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fn erase(&mut self) -> Result<(), &'static str> {
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let len = self.mmap.as_ref().map(|m| m.len).unwrap_or(0);
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if len > 0 {
// The order is important.
//
// 1. First we clear, and thus drop, the current mmap if any.
// 2. And then we create a new one.
//
// Otherwise we double the peak memory consumption.
self.mmap = None;
self.mmap = Some(Mmap::new(len)?);
}
Ok(())
}
}
#[cfg(test)]
mod tests {
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use super::{MmapByteBuf, MemoryBackend};
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const PAGE_SIZE: usize = 4096;
// This is not required since wasm memories can only grow but nice to have.
#[test]
fn byte_buf_shrink() {
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let mut byte_buf = MmapByteBuf::new(PAGE_SIZE * 3).unwrap();
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byte_buf.realloc(PAGE_SIZE * 2).unwrap();
}
}