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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
//! Linux implementation for memory mapping abstractions.
#![cfg(unix)]
use pal::unix::SyscallResult;
use std::ffi::c_void;
use std::fs::File;
use std::io;
use std::io::Error;
use std::os::unix::prelude::*;
use std::ptr::null_mut;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering;
pub(crate) fn page_size() -> usize {
static PAGE_SIZE: AtomicUsize = AtomicUsize::new(0);
let s = PAGE_SIZE.load(Ordering::Relaxed);
if s != 0 {
s
} else {
let s = unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize };
PAGE_SIZE.store(s, Ordering::Relaxed);
s
}
}
/// A reserved virtual address range that may be partially populated with memory
/// mappings.
#[derive(Debug)]
pub struct SparseMapping {
address: *mut c_void,
len: usize,
}
/// An owned handle to an OS object that can be mapped into a [`SparseMapping`].
///
/// On Windows, this is a section handle. On Linux, it is a file descriptor.
pub type Mappable = OwnedFd;
/// An object that can be mapped into a `SparseMapping`.
///
/// On Windows, this is a section handle. On Linux, it is a file descriptor.
pub use std::os::unix::io::AsFd as AsMappableRef;
/// A reference to an object that can be mapped into a [`SparseMapping`].
///
/// On Windows, this is a section handle. On Linux, it is a file descriptor.
pub type MappableRef<'a> = BorrowedFd<'a>;
/// Creates a new mappable from a file.
///
/// N.B. `writable` and `executable` have no effect on Linux.
pub fn new_mappable_from_file(
file: &File,
_writable: bool,
_executable: bool,
) -> io::Result<Mappable> {
file.as_fd().try_clone_to_owned()
}
// SAFETY: SparseMapping's internal pointer represents an owned virtual address
// range. There is no safety issue accessing this pointer across threads.
unsafe impl Send for SparseMapping {}
// SAFETY: See above comment
unsafe impl Sync for SparseMapping {}
unsafe fn mmap(
addr: *mut c_void,
len: usize,
prot: i32,
flags: i32,
fd: i32,
offset: i64,
) -> Result<*mut c_void, Error> {
let address = unsafe { libc::mmap(addr, len, prot, flags, fd, offset) };
if address == libc::MAP_FAILED {
return Err(Error::last_os_error());
}
Ok(address)
}
unsafe fn munmap(addr: *mut c_void, len: usize) -> Result<(), Error> {
if unsafe { libc::munmap(addr, len) } < 0 {
return Err(Error::last_os_error());
}
Ok(())
}
impl SparseMapping {
/// Reserves a sparse mapping range with the given size.
///
/// The range will be aligned to the largest system page size that's smaller
/// or equal to `len`.
pub fn new(len: usize) -> Result<Self, Error> {
super::initialize_try_copy();
// Length of 0 return an OS error, so we need to handle it explicitly.
if len == 0 {
return Err(Error::new(
io::ErrorKind::InvalidInput,
"length must be greater than 0",
));
}
let size_4k = 4096;
let size_2m = 0x200000;
let size_1g = 0x40000000;
let alignment = if len < size_2m {
size_4k
} else if len < size_1g {
size_2m
} else {
size_1g
};
let len = len
.checked_add(alignment - 1)
.map(|temp| temp & !(alignment - 1))
.ok_or_else(|| {
Error::new(
io::ErrorKind::InvalidInput,
"length and alignment combination causes overflow",
)
})?;
let alloc_len = len
.checked_add(alignment)
.map(|temp| temp - size_4k)
.ok_or_else(|| {
Error::new(
io::ErrorKind::InvalidInput,
"length and alignment combination causes overflow",
)
})?;
// SAFETY: calling mmap to allocate a new range.
let address = unsafe {
mmap(
null_mut(),
alloc_len,
libc::PROT_NONE,
libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
-1,
0,
)? as usize
};
let aligned_address = (address + alignment - 1) & !(alignment - 1);
let end = address + alloc_len;
let aligned_end = aligned_address + len;
assert!(aligned_end <= end);
if address != aligned_address {
// SAFETY: freeing VA just allocated above.
unsafe { munmap(address as *mut _, aligned_address - address).unwrap() };
}
if aligned_end != end {
// SAFETY: freeing VA just allocated above.
unsafe { munmap(aligned_end as *mut _, end - aligned_end).unwrap() };
}
Ok(Self {
address: aligned_address as *mut _,
len,
})
}
/// Returns true if the mapping is local to the current process.
pub fn is_local(&self) -> bool {
true
}
/// Returns the pointer to the beginning of the sparse mapping.
pub fn as_ptr(&self) -> *mut c_void {
self.address
}
/// Returns the length of the mapping, in bytes.
pub fn len(&self) -> usize {
self.len
}
fn validate_offset_len(&self, offset: usize, len: usize) -> io::Result<usize> {
let end = offset.checked_add(len).ok_or(io::ErrorKind::InvalidInput)?;
let page_size = page_size();
if offset % page_size != 0 || end % page_size != 0 || end > self.len {
return Err(io::ErrorKind::InvalidInput.into());
}
Ok(end)
}
/// Allocates private, writable memory at the given offset within the mapping.
pub fn alloc(&self, offset: usize, len: usize) -> Result<(), Error> {
// SAFETY: The flags passed in are guaranteed to be valid
unsafe {
self.mmap_anonymous(
offset,
len,
libc::PROT_READ | libc::PROT_WRITE,
libc::MAP_PRIVATE,
)
}
}
/// Maps read-only zero pages at the given offset within the mapping.
pub fn map_zero(&self, offset: usize, len: usize) -> Result<(), Error> {
// SAFETY: The flags passed in are guaranteed to be valid
unsafe { self.mmap_anonymous(offset, len, libc::PROT_READ, libc::MAP_PRIVATE) }
}
/// Maps a portion of a file mapping at `offset`.
pub fn map_file(
&self,
offset: usize,
len: usize,
file_mapping: impl AsFd,
file_offset: u64,
writable: bool,
) -> Result<(), Error> {
let prot = if writable {
libc::PROT_READ | libc::PROT_WRITE
} else {
libc::PROT_READ
};
// SAFETY: The flags passed in are guaranteed to be valid
unsafe {
self.mmap(
offset,
len,
prot,
libc::MAP_SHARED,
file_mapping.as_fd(),
file_offset as i64,
)
}
}
/// Maps memory into the mapping, passing parameters through to the mmap
/// syscall.
///
/// # Safety
///
/// This routine is safe to use as long as the caller ensures `map_flags` excludes
/// any flags that render the memory region non-unmappable (e.g., `MAP_LOCKED`).
/// Misuse may lead to system resource issues, such as falsely perceived out-of-memory
/// conditions.
pub unsafe fn mmap(
&self,
offset: usize,
len: usize,
prot: i32,
map_flags: i32,
fd: impl AsFd,
file_offset: i64,
) -> Result<(), Error> {
let _ = self.validate_offset_len(offset, len)?;
// SAFETY: guaranteed by caller and offset + len checks above
unsafe {
let address = self.address.add(offset);
let mapped_address = mmap(
address,
len,
prot,
map_flags | libc::MAP_FIXED,
fd.as_fd().as_raw_fd(),
file_offset,
)?;
assert_eq!(mapped_address, address);
}
Ok(())
}
/// Maps anonymous memory into the mapping, with parameters for the mmap syscall.
///
/// # Safety
///
/// This routine is safe to use as long as the caller ensures `map_flags` excludes
/// any flags that render the memory region non-unmappable (e.g., `MAP_LOCKED`).
/// Misuse may lead to system resource issues, such as falsely perceived out-of-memory
/// conditions.
pub unsafe fn mmap_anonymous(
&self,
offset: usize,
len: usize,
prot: i32,
map_flags: i32,
) -> io::Result<()> {
let _ = self.validate_offset_len(offset, len)?;
// SAFETY: guaranteed by caller and offset + len checks above
unsafe {
let address = self.address.add(offset);
let mapped_address = mmap(
address,
len,
prot,
map_flags | libc::MAP_ANONYMOUS | libc::MAP_FIXED,
-1,
0,
)?;
assert_eq!(mapped_address, address);
}
Ok(())
}
/// Unmaps memory from the mapping.
pub fn unmap(&self, offset: usize, len: usize) -> io::Result<()> {
let _ = self.validate_offset_len(offset, len)?;
// Skipping this check would result in the "expect" below
if len == 0 {
return Err(io::ErrorKind::InvalidInput.into());
}
// Remap to PROT_NONE to preserve the reservation.
// SAFETY: guaranteed by caller and offset + len checks above
unsafe {
let address = self.address.add(offset);
let mapped_address = mmap(
address,
len,
libc::PROT_NONE,
libc::MAP_PRIVATE | libc::MAP_ANONYMOUS | libc::MAP_FIXED,
-1,
0,
)
.expect("remap to PROT_NONE should not fail (except for low resources)");
assert_eq!(mapped_address, address);
}
Ok(())
}
}
impl Drop for SparseMapping {
fn drop(&mut self) {
unsafe {
libc::munmap(self.address, self.len)
.syscall_result()
.expect("unmap should not fail");
}
}
}
#[cfg(target_os = "linux")]
fn new_memfd() -> io::Result<File> {
// SAFETY: creating and truncating a new file descriptor according to
// the documented contract.
unsafe {
let fd = libc::memfd_create(c"mem".as_ptr(), libc::MFD_CLOEXEC).syscall_result()?;
Ok(File::from_raw_fd(fd))
}
}
#[cfg(not(target_os = "linux"))]
fn new_memfd() -> io::Result<File> {
let mut name = [0; 16];
getrandom::getrandom(&mut name).unwrap();
let mut name = format!("{:x}", u128::from_ne_bytes(name));
// macOS limits the name length to 31 bytes, which is sufficient to ensure uniqueness.
name.truncate(31);
let name = std::ffi::CString::new(name).unwrap();
unsafe {
// Create a new shared memory object.
let fd = libc::shm_open(name.as_ptr(), libc::O_RDWR | libc::O_EXCL | libc::O_CREAT)
.syscall_result()?;
// Unlink it to make it anonymous.
let _ = libc::shm_unlink(name.as_ptr());
Ok(File::from_raw_fd(fd))
}
}
/// Allocates a mappable shared memory object of `size` bytes.
pub fn alloc_shared_memory(size: usize) -> io::Result<OwnedFd> {
let fd = new_memfd()?;
fd.set_len(size as u64)?;
Ok(fd.into())
}