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//! Low-level abstraction for allocating and managing zero-filled pages
//! of memory.
use crate::sys::mmap;
use anyhow::{Context, Result};
use std::fs::File;
use std::ops::Range;
use std::path::Path;
use std::sync::Arc;
/// A simple struct consisting of a page-aligned pointer to page-aligned
/// and initially-zeroed memory and a length.
#[derive(Debug)]
pub struct Mmap {
sys: mmap::Mmap,
file: Option<Arc<File>>,
}
impl Mmap {
/// Create a new `Mmap` pointing to at least `size` bytes of page-aligned
/// accessible memory.
pub fn with_at_least(size: usize) -> Result<Self> {
let page_size = crate::page_size();
let rounded_size = (size + (page_size - 1)) & !(page_size - 1);
Self::accessible_reserved(rounded_size, rounded_size)
}
/// Creates a new `Mmap` by opening the file located at `path` and mapping
/// it into memory.
///
/// The memory is mapped in read-only mode for the entire file. If portions
/// of the file need to be modified then the `region` crate can be use to
/// alter permissions of each page.
///
/// The memory mapping and the length of the file within the mapping are
/// returned.
pub fn from_file(path: &Path) -> Result<Self> {
let (sys, file) = mmap::Mmap::from_file(path)?;
Ok(Mmap {
sys,
file: Some(Arc::new(file)),
})
}
/// Create a new `Mmap` pointing to `accessible_size` bytes of page-aligned
/// accessible memory, within a reserved mapping of `mapping_size` bytes.
/// `accessible_size` and `mapping_size` must be native page-size multiples.
///
/// # Panics
///
/// This function will panic if `accessible_size` is greater than
/// `mapping_size` or if either of them are not page-aligned.
pub fn accessible_reserved(accessible_size: usize, mapping_size: usize) -> Result<Self> {
let page_size = crate::page_size();
assert!(accessible_size <= mapping_size);
assert_eq!(mapping_size & (page_size - 1), 0);
assert_eq!(accessible_size & (page_size - 1), 0);
if mapping_size == 0 {
Ok(Mmap {
sys: mmap::Mmap::new_empty(),
file: None,
})
} else if accessible_size == mapping_size {
Ok(Mmap {
sys: mmap::Mmap::new(mapping_size)
.context(format!("mmap failed to allocate {mapping_size:#x} bytes"))?,
file: None,
})
} else {
let mut result = Mmap {
sys: mmap::Mmap::reserve(mapping_size)
.context(format!("mmap failed to reserve {mapping_size:#x} bytes"))?,
file: None,
};
if accessible_size > 0 {
result.make_accessible(0, accessible_size).context(format!(
"mmap failed to allocate {accessible_size:#x} bytes"
))?;
}
Ok(result)
}
}
/// Make the memory starting at `start` and extending for `len` bytes
/// accessible. `start` and `len` must be native page-size multiples and
/// describe a range within `self`'s reserved memory.
///
/// # Panics
///
/// This function will panic if `start` or `len` is not page aligned or if
/// either are outside the bounds of this mapping.
pub fn make_accessible(&mut self, start: usize, len: usize) -> Result<()> {
let page_size = crate::page_size();
assert_eq!(start & (page_size - 1), 0);
assert_eq!(len & (page_size - 1), 0);
assert!(len <= self.len());
assert!(start <= self.len() - len);
self.sys.make_accessible(start, len)
}
/// Return the allocated memory as a slice of u8.
///
/// # Safety
///
/// The caller must ensure that the range of bytes is accessible to the
/// program and additionally has previously been initialized.
///
/// # Panics
///
/// Panics of the `range` provided is outside of the limits of this mmap.
#[inline]
pub unsafe fn slice(&self, range: Range<usize>) -> &[u8] {
assert!(range.start <= range.end);
assert!(range.end <= self.len());
std::slice::from_raw_parts(self.as_ptr().add(range.start), range.end - range.start)
}
/// Return the allocated memory as a mutable slice of u8.
///
/// # Safety
///
/// The caller must ensure that the range of bytes is accessible to the
/// program and additionally has previously been initialized.
///
/// # Panics
///
/// Panics of the `range` provided is outside of the limits of this mmap.
pub unsafe fn slice_mut(&mut self, range: Range<usize>) -> &mut [u8] {
assert!(range.start <= range.end);
assert!(range.end <= self.len());
std::slice::from_raw_parts_mut(self.as_mut_ptr().add(range.start), range.end - range.start)
}
/// Return the allocated memory as a pointer to u8.
#[inline]
pub fn as_ptr(&self) -> *const u8 {
self.sys.as_ptr()
}
/// Return the allocated memory as a mutable pointer to u8.
#[inline]
pub fn as_mut_ptr(&mut self) -> *mut u8 {
self.sys.as_mut_ptr()
}
/// Return the length of the allocated memory.
///
/// This is the byte length of this entire mapping which includes both
/// addressible and non-addressible memory.
#[inline]
pub fn len(&self) -> usize {
self.sys.len()
}
/// Return whether any memory has been allocated or reserved.
pub fn is_empty(&self) -> bool {
self.len() == 0
}
/// Makes the specified `range` within this `Mmap` to be read/execute.
///
/// # Unsafety
///
/// This method is unsafe as it's generally not valid to simply make memory
/// executable, so it's up to the caller to ensure that everything is in
/// order and this doesn't overlap with other memory that should only be
/// read or only read/write.
///
/// # Panics
///
/// Panics of `range` is out-of-bounds or not page-aligned.
pub unsafe fn make_executable(
&self,
range: Range<usize>,
enable_branch_protection: bool,
) -> Result<()> {
assert!(range.start <= self.len());
assert!(range.end <= self.len());
assert!(range.start <= range.end);
assert!(
range.start % crate::page_size() == 0,
"changing of protections isn't page-aligned",
);
self.sys
.make_executable(range, enable_branch_protection)
.context("failed to make memory executable")
}
/// Makes the specified `range` within this `Mmap` to be readonly.
pub unsafe fn make_readonly(&self, range: Range<usize>) -> Result<()> {
assert!(range.start <= self.len());
assert!(range.end <= self.len());
assert!(range.start <= range.end);
assert!(
range.start % crate::page_size() == 0,
"changing of protections isn't page-aligned",
);
self.sys
.make_readonly(range)
.context("failed to make memory readonly")
}
/// Returns the underlying file that this mmap is mapping, if present.
pub fn original_file(&self) -> Option<&Arc<File>> {
self.file.as_ref()
}
}
fn _assert() {
fn _assert_send_sync<T: Send + Sync>() {}
_assert_send_sync::<Mmap>();
}