read_dir.rs

// Copyright 2020 The ChromiumOS Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use std::ffi::CStr;
use std::io;
use std::mem::size_of;
use std::ops::Deref;
use std::ops::DerefMut;

use base::AsRawDescriptor;
use fuse::filesystem::DirEntry;
use fuse::filesystem::DirectoryIterator;
use zerocopy::FromBytes;
use zerocopy::Immutable;
use zerocopy::IntoBytes;
use zerocopy::KnownLayout;

#[repr(C, packed)]
#[derive(Clone, Copy, FromBytes, Immutable, IntoBytes, KnownLayout)]
struct LinuxDirent64 {
    d_ino: libc::ino64_t,
    d_off: libc::off64_t,
    d_reclen: libc::c_ushort,
    d_ty: libc::c_uchar,
}

pub struct ReadDir<P> {
    buf: P,
    current: usize,
    end: usize,
}

impl<P: DerefMut<Target = [u8]>> ReadDir<P> {
    pub fn new<D: AsRawDescriptor>(dir: &D, offset: libc::off64_t, mut buf: P) -> io::Result<Self> {
        // SAFETY:
        // Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe { libc::lseek64(dir.as_raw_descriptor(), offset, libc::SEEK_SET) };
        if res < 0 {
            return Err(io::Error::last_os_error());
        }

        // SAFETY:
        // Safe because the kernel guarantees that it will only write to `buf` and we check the
        // return value.
        let res = unsafe {
            libc::syscall(
                libc::SYS_getdents64,
                dir.as_raw_descriptor(),
                buf.as_mut_ptr() as *mut LinuxDirent64,
                buf.len() as libc::c_int,
            )
        };
        if res < 0 {
            return Err(io::Error::last_os_error());
        }

        Ok(ReadDir {
            buf,
            current: 0,
            end: res as usize,
        })
    }
}

impl<P> ReadDir<P> {
    /// Returns the number of bytes from the internal buffer that have not yet been consumed.
    pub fn remaining(&self) -> usize {
        self.end.saturating_sub(self.current)
    }
}

impl<P: Deref<Target = [u8]>> DirectoryIterator for ReadDir<P> {
    fn next(&mut self) -> Option<DirEntry> {
        let rem = &self.buf[self.current..self.end];
        if rem.is_empty() {
            return None;
        }

        let (dirent64, back) =
            LinuxDirent64::read_from_prefix(rem).expect("unable to get LinuxDirent64 from slice");

        let namelen = dirent64.d_reclen as usize - size_of::<LinuxDirent64>();
        debug_assert!(namelen <= back.len(), "back is smaller than `namelen`");

        // The kernel will pad the name with additional nul bytes until it is 8-byte aligned so
        // we need to strip those off here.
        let name = strip_padding(&back[..namelen]);
        let entry = DirEntry {
            ino: dirent64.d_ino,
            offset: dirent64.d_off as u64,
            type_: dirent64.d_ty as u32,
            name,
        };

        debug_assert!(
            rem.len() >= dirent64.d_reclen as usize,
            "rem is smaller than `d_reclen`"
        );
        self.current += dirent64.d_reclen as usize;
        Some(entry)
    }
}

// Like `CStr::from_bytes_with_nul` but strips any bytes after the first '\0'-byte. Panics if `b`
// doesn't contain any '\0' bytes.
fn strip_padding(b: &[u8]) -> &CStr {
    // It would be nice if we could use memchr here but that's locked behind an unstable gate.
    let pos = b
        .iter()
        .position(|&c| c == 0)
        .expect("`b` doesn't contain any nul bytes");

    // SAFETY:
    // Safe because we are creating this string with the first nul-byte we found so we can
    // guarantee that it is nul-terminated and doesn't contain any interior nuls.
    unsafe { CStr::from_bytes_with_nul_unchecked(&b[..pos + 1]) }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn padded_cstrings() {
        assert_eq!(strip_padding(b".\0\0\0\0\0\0\0").to_bytes(), b".");
        assert_eq!(strip_padding(b"..\0\0\0\0\0\0").to_bytes(), b"..");
        assert_eq!(
            strip_padding(b"normal cstring\0").to_bytes(),
            b"normal cstring"
        );
        assert_eq!(strip_padding(b"\0\0\0\0").to_bytes(), b"");
        assert_eq!(
            strip_padding(b"interior\0nul bytes\0\0\0").to_bytes(),
            b"interior"
        );
    }

    #[test]
    #[should_panic(expected = "`b` doesn't contain any nul bytes")]
    fn no_nul_byte() {
        strip_padding(b"no nul bytes in string");
    }
}