path: root/src/unix.rs

extern crate libc;

use std::fs::File;
use std::mem::ManuallyDrop;
use std::os::unix::io::{FromRawFd, RawFd};
use std::sync::atomic::{AtomicUsize, Ordering};
use std::{io, ptr};

use crate::advice::Advice;

#[cfg(any(
    all(target_os = "linux", not(target_arch = "mips")),
    target_os = "freebsd",
    target_os = "android"
))]
const MAP_STACK: libc::c_int = libc::MAP_STACK;

#[cfg(not(any(
    all(target_os = "linux", not(target_arch = "mips")),
    target_os = "freebsd",
    target_os = "android"
)))]
const MAP_STACK: libc::c_int = 0;

#[cfg(any(target_os = "linux", target_os = "android"))]
const MAP_POPULATE: libc::c_int = libc::MAP_POPULATE;

#[cfg(not(any(target_os = "linux", target_os = "android")))]
const MAP_POPULATE: libc::c_int = 0;

#[cfg(any(
    target_os = "android",
    all(target_os = "linux", not(target_env = "musl"))
))]
use libc::{mmap64 as mmap, off64_t as off_t};

#[cfg(not(any(
    target_os = "android",
    all(target_os = "linux", not(target_env = "musl"))
)))]
use libc::{mmap, off_t};

pub struct MmapInner {
    ptr: *mut libc::c_void,
    len: usize,
}

impl MmapInner {
    /// Creates a new `MmapInner`.
    ///
    /// This is a thin wrapper around the `mmap` system call.
    fn new(
        len: usize,
        prot: libc::c_int,
        flags: libc::c_int,
        file: RawFd,
        offset: u64,
    ) -> io::Result<MmapInner> {
        let alignment = offset % page_size() as u64;
        let aligned_offset = offset - alignment;

        let (map_len, map_offset) = Self::adjust_mmap_params(len, alignment as usize)?;

        unsafe {
            let ptr = mmap(
                ptr::null_mut(),
                map_len as libc::size_t,
                prot,
                flags,
                file,
                aligned_offset as off_t,
            );

            if ptr == libc::MAP_FAILED {
                Err(io::Error::last_os_error())
            } else {
                Ok(Self::from_raw_parts(ptr, len, map_offset))
            }
        }
    }

    fn adjust_mmap_params(len: usize, alignment: usize) -> io::Result<(usize, usize)> {
        use std::isize;

        // Rust's slice cannot be larger than isize::MAX.
        // See https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html
        //
        // This is not a problem on 64-bit targets, but on 32-bit one
        // having a file or an anonymous mapping larger than 2GB is quite normal
        // and we have to prevent it.
        //
        // The code below is essentially the same as in Rust's std:
        // https://github.com/rust-lang/rust/blob/db78ab70a88a0a5e89031d7ee4eccec835dcdbde/library/alloc/src/raw_vec.rs#L495
        if std::mem::size_of::<usize>() < 8 && len > isize::MAX as usize {
            return Err(io::Error::new(
                io::ErrorKind::InvalidData,
                "memory map length overflows isize",
            ));
        }

        let map_len = len + alignment;
        let map_offset = alignment;

        // `libc::mmap` does not support zero-size mappings. POSIX defines:
        //
        // https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html
        // > If `len` is zero, `mmap()` shall fail and no mapping shall be established.
        //
        // So if we would create such a mapping, crate a one-byte mapping instead:
        let map_len = map_len.max(1);

        // Note that in that case `MmapInner::len` is still set to zero,
        // and `Mmap` will still dereferences to an empty slice.
        //
        // If this mapping is backed by an empty file, we create a mapping larger than the file.
        // This is unusual but well-defined. On the same man page, POSIX further defines:
        //
        // > The `mmap()` function can be used to map a region of memory that is larger
        // > than the current size of the object.
        //
        // (The object here is the file.)
        //
        // > Memory access within the mapping but beyond the current end of the underlying
        // > objects may result in SIGBUS signals being sent to the process. The reason for this
        // > is that the size of the object can be manipulated by other processes and can change
        // > at any moment. The implementation should tell the application that a memory reference
        // > is outside the object where this can be detected; otherwise, written data may be lost
        // > and read data may not reflect actual data in the object.
        //
        // Because `MmapInner::len` is not incremented, this increment of `aligned_len`
        // will not allow accesses past the end of the file and will not cause SIGBUS.
        //
        // (SIGBUS is still possible by mapping a non-empty file and then truncating it
        // to a shorter size, but that is unrelated to this handling of empty files.)
        Ok((map_len, map_offset))
    }

    /// Get the current memory mapping as a `(ptr, map_len, offset)` tuple.
    ///
    /// Note that `map_len` is the length of the memory mapping itself and
    /// _not_ the one that would be passed to `from_raw_parts`.
    fn as_mmap_params(&self) -> (*mut libc::c_void, usize, usize) {
        let offset = self.ptr as usize % page_size();
        let len = self.len + offset;

        // There are two possible memory layouts we could have, depending on
        // the length and offset passed when constructing this instance:
        //
        // 1. The "normal" memory layout looks like this:
        //
        //         |<------------------>|<---------------------->|
        //     mmap ptr    offset      ptr     public slice
        //
        //    That is, we have
        //    - The start of the page-aligned memory mapping returned by mmap,
        //      followed by,
        //    - Some number of bytes that are memory mapped but ignored since
        //      they are before the byte offset requested by the user, followed
        //      by,
        //    - The actual memory mapped slice requested by the user.
        //
        //    This maps cleanly to a (ptr, len, offset) tuple.
        //
        // 2. Then, we have the case where the user requested a zero-length
        //    memory mapping. mmap(2) does not support zero-length mappings so
        //    this crate works around that by actually making a mapping of
        //    length one. This means that we have
        //    - A length zero slice, followed by,
        //    - A single memory mapped byte
        //
        //    Note that this only happens if the offset within the page is also
        //    zero. Otherwise, we have a memory map of offset bytes and not a
        //    zero-length memory map.
        //
        //    This doesn't fit cleanly into a (ptr, len, offset) tuple. Instead,
        //    we fudge it slightly: a zero-length memory map turns into a
        //    mapping of length one and can't be told apart outside of this
        //    method without knowing the original length.
        if len == 0 {
            (self.ptr, 1, 0)
        } else {
            (unsafe { self.ptr.offset(-(offset as isize)) }, len, offset)
        }
    }

    /// Construct this `MmapInner` from its raw components
    ///
    /// # Safety
    ///
    /// - `ptr` must point to the start of memory mapping that can be freed
    ///   using `munmap(2)` (i.e. returned by `mmap(2)` or `mremap(2)`)
    /// - The memory mapping at `ptr` must have a length of `len + offset`.
    /// - If `len + offset == 0` then the memory mapping must be of length 1.
    /// - `offset` must be less than the current page size.
    unsafe fn from_raw_parts(ptr: *mut libc::c_void, len: usize, offset: usize) -> Self {
        debug_assert_eq!(ptr as usize % page_size(), 0, "ptr not page-aligned");
        debug_assert!(offset < page_size(), "offset larger than page size");

        Self {
            ptr: ptr.add(offset),
            len,
        }
    }

    pub fn map(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ,
            libc::MAP_SHARED | populate,
            file,
            offset,
        )
    }

    pub fn map_exec(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_EXEC,
            libc::MAP_SHARED | populate,
            file,
            offset,
        )
    }

    pub fn map_mut(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_SHARED | populate,
            file,
            offset,
        )
    }

    pub fn map_copy(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_PRIVATE | populate,
            file,
            offset,
        )
    }

    pub fn map_copy_read_only(
        len: usize,
        file: RawFd,
        offset: u64,
        populate: bool,
    ) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ,
            libc::MAP_PRIVATE | populate,
            file,
            offset,
        )
    }

    /// Open an anonymous memory map.
    pub fn map_anon(len: usize, stack: bool, populate: bool) -> io::Result<MmapInner> {
        let stack = if stack { MAP_STACK } else { 0 };
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_PRIVATE | libc::MAP_ANON | stack | populate,
            -1,
            0,
        )
    }

    pub fn flush(&self, offset: usize, len: usize) -> io::Result<()> {
        let alignment = (self.ptr as usize + offset) % page_size();
        let offset = offset as isize - alignment as isize;
        let len = len + alignment;
        let result =
            unsafe { libc::msync(self.ptr.offset(offset), len as libc::size_t, libc::MS_SYNC) };
        if result == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    pub fn flush_async(&self, offset: usize, len: usize) -> io::Result<()> {
        let alignment = (self.ptr as usize + offset) % page_size();
        let offset = offset as isize - alignment as isize;
        let len = len + alignment;
        let result =
            unsafe { libc::msync(self.ptr.offset(offset), len as libc::size_t, libc::MS_ASYNC) };
        if result == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn mprotect(&mut self, prot: libc::c_int) -> io::Result<()> {
        unsafe {
            let alignment = self.ptr as usize % page_size();
            let ptr = self.ptr.offset(-(alignment as isize));
            let len = self.len + alignment;
            let len = len.max(1);
            if libc::mprotect(ptr, len, prot) == 0 {
                Ok(())
            } else {
                Err(io::Error::last_os_error())
            }
        }
    }

    pub fn make_read_only(&mut self) -> io::Result<()> {
        self.mprotect(libc::PROT_READ)
    }

    pub fn make_exec(&mut self) -> io::Result<()> {
        self.mprotect(libc::PROT_READ | libc::PROT_EXEC)
    }

    pub fn make_mut(&mut self) -> io::Result<()> {
        self.mprotect(libc::PROT_READ | libc::PROT_WRITE)
    }

    #[inline]
    pub fn ptr(&self) -> *const u8 {
        self.ptr as *const u8
    }

    #[inline]
    pub fn mut_ptr(&mut self) -> *mut u8 {
        self.ptr as *mut u8
    }

    #[inline]
    pub fn len(&self) -> usize {
        self.len
    }

    pub fn advise(&self, advice: Advice, offset: usize, len: usize) -> io::Result<()> {
        let alignment = (self.ptr as usize + offset) % page_size();
        let offset = offset as isize - alignment as isize;
        let len = len + alignment;
        unsafe {
            if libc::madvise(self.ptr.offset(offset), len, advice.0) != 0 {
                Err(io::Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }

    #[cfg(target_os = "linux")]
    pub fn remap(&mut self, new_len: usize, options: crate::RemapOptions) -> io::Result<()> {
        let (old_ptr, old_len, offset) = self.as_mmap_params();
        let (map_len, offset) = Self::adjust_mmap_params(new_len, offset)?;

        unsafe {
            let new_ptr = libc::mremap(old_ptr, old_len, map_len, options.into_flags());

            if new_ptr == libc::MAP_FAILED {
                Err(io::Error::last_os_error())
            } else {
                // We explicitly don't drop self since the pointer within is no longer valid.
                ptr::write(self, Self::from_raw_parts(new_ptr, new_len, offset));
                Ok(())
            }
        }
    }

    pub fn lock(&self) -> io::Result<()> {
        unsafe {
            if libc::mlock(self.ptr, self.len) != 0 {
                Err(io::Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }

    pub fn unlock(&self) -> io::Result<()> {
        unsafe {
            if libc::munlock(self.ptr, self.len) != 0 {
                Err(io::Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }
}

impl Drop for MmapInner {
    fn drop(&mut self) {
        let (ptr, len, _) = self.as_mmap_params();

        // Any errors during unmapping/closing are ignored as the only way
        // to report them would be through panicking which is highly discouraged
        // in Drop impls, c.f. https://github.com/rust-lang/lang-team/issues/97
        unsafe { libc::munmap(ptr, len as libc::size_t) };
    }
}

unsafe impl Sync for MmapInner {}
unsafe impl Send for MmapInner {}

fn page_size() -> usize {
    static PAGE_SIZE: AtomicUsize = AtomicUsize::new(0);

    match PAGE_SIZE.load(Ordering::Relaxed) {
        0 => {
            let page_size = unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize };

            PAGE_SIZE.store(page_size, Ordering::Relaxed);

            page_size
        }
        page_size => page_size,
    }
}

pub fn file_len(file: RawFd) -> io::Result<u64> {
    // SAFETY: We must not close the passed-in fd by dropping the File we create,
    // we ensure this by immediately wrapping it in a ManuallyDrop.
    unsafe {
        let file = ManuallyDrop::new(File::from_raw_fd(file));
        Ok(file.metadata()?.len())
    }
}