malcontent-dns-parental-controls/src/helpers.rs

// SPDX-FileCopyrightText: 2022 Matteo Settenvini <matteo.settenvini@montecristosoftware.eu>
// SPDX-License-Identifier: GPL-3.0-or-later

use {
    crate::nss_bindings::gaih_addrtuple,
    anyhow::{bail, Result},
    libc::{c_char, hostent, AF_INET, AF_INET6},
    nix::errno::Errno,
    once_cell::sync::Lazy,
    std::ffi::CString,
    std::mem::{align_of, discriminant, size_of},
    trust_dns_proto::rr::resource::Record,
    trust_dns_proto::rr::RData,
    trust_dns_resolver::lookup::Lookup,
};

static RUNTIME: Lazy<std::io::Result<tokio::runtime::Runtime>> = Lazy::new(|| {
    // The runtime should remain single-threaded, some
    // programs depend on it (e.g. programs calling unshare())
    let rt = tokio::runtime::Builder::new_current_thread()
        .enable_time()
        .enable_io()
        .build()?;
    Ok(rt)
});

// TODO: error handling codes chosen a bit sloppily
pub unsafe fn records_to_gaih_addr(
    lookup: Lookup,
    mut buf: &mut [u8],
) -> std::io::Result<*mut gaih_addrtuple> {
    const GAIH_ADDRTUPLE_SZ: usize = size_of::<gaih_addrtuple>();

    let mut ret = std::ptr::null_mut();
    let mut prev_link: *mut *mut gaih_addrtuple = std::ptr::null_mut();

    let mut name_dest;
    for record in lookup.record_iter() {
        // First add the name to the buffer
        (buf, name_dest) = write_record_name_to_buf(record, buf)?;

        // Then add the tuple with the address
        let offset = buf.as_ptr().align_offset(align_of::<gaih_addrtuple>());
        let l = GAIH_ADDRTUPLE_SZ;
        if buf.len() < offset + l {
            return Err(Errno::ERANGE.into());
        }

        let tuple = &mut *(buf.as_mut_ptr().add(offset) as *mut gaih_addrtuple);
        tuple.next = std::ptr::null_mut();
        tuple.name = name_dest;
        tuple.scopeid = 0; // how to set tuple.scopeid ????
        set_if_valid(prev_link, &mut *tuple); // link from previous tuple to this tuple
        prev_link = &mut (*tuple).next;

        match record.data() {
            Some(RData::A(addr)) => {
                tuple.family = AF_INET;
                tuple.addr[0] = std::mem::transmute_copy(&addr.octets());
            }
            Some(RData::AAAA(addr)) => {
                tuple.family = AF_INET6;
                tuple.addr = std::mem::transmute_copy(&addr.octets());
            }
            Some(_) => return Err(Errno::EBADMSG.into()),
            None => return Err(Errno::ENODATA.into()),
        }

        if ret == std::ptr::null_mut() {
            ret = tuple;
        }

        buf = &mut buf[(offset + l)..];
    }

    Ok(ret)
}

pub enum DnsResult {
    NxDomain,
    Found,
}

// TODO: refactor to be less ugly
pub unsafe fn records_to_hostent(
    lookup: Lookup,
    host: &mut hostent,
    mut buf: &mut [u8],
) -> std::io::Result<DnsResult> {
    // In C struct hostent:
    //
    // - for the type of queries we perform, we can assume h_aliases
    //   is an empty array.
    // - hostent is limited to just one address type for all addresses
    //   in the list. We pick the type of first result, and only
    //   append addresses of the same type.

    let first_record = match lookup.record_iter().peekable().peek() {
        Some(record) => *record,
        None => return Ok(DnsResult::NxDomain),
    };

    let first_rdata = match first_record.data() {
        Some(rdata) => rdata,
        None => return Err(Errno::ENODATA.into()),
    };

    // First put the name in the buffer
    let name_dest;
    (buf, name_dest) = write_record_name_to_buf(first_record, buf)?;

    // Then the empty aliases array (just a null pointer)
    let offset = buf.as_ptr().align_offset(align_of::<*mut c_char>());
    let l = size_of::<*mut c_char>();
    if buf.len() < offset + l {
        return Err(Errno::ERANGE.into());
    }
    let aliases_ptr = buf[offset..].as_mut_ptr().cast::<*mut c_char>();
    aliases_ptr.write(std::ptr::null_mut());
    buf = &mut buf[(offset + l)..];

    // Then the address list
    let offset = buf.as_ptr().align_offset(align_of::<*mut c_char>());
    buf = &mut buf[offset..];

    let records = lookup
        .record_iter()
        .flat_map(Record::data)
        .filter(|r| discriminant(*r) == discriminant(first_rdata));

    let mut addresses = vec![];
    for record in records {
        let offset = buf.as_ptr().align_offset(align_of::<*mut c_char>());
        buf = &mut buf[offset..];
        addresses.push(buf.as_ptr());

        let l = match record {
            RData::A(addr) => {
                let octets = addr.octets();
                let l = octets.len();
                if buf.len() < l {
                    return Err(Errno::ERANGE.into());
                }
                buf[..l].copy_from_slice(&octets);
                l
            }
            RData::AAAA(addr) => {
                let octets = addr.octets();
                let l = octets.len();
                if buf.len() < l {
                    return Err(Errno::ERANGE.into());
                }
                buf[..l].copy_from_slice(&octets);
                l
            }
            _ => unreachable!(),
        };

        buf = &mut buf[l..];
    }

    addresses.push(std::ptr::null_mut());

    let offset = buf.as_ptr().align_offset(align_of::<*mut *mut c_char>());
    let l = size_of::<*mut c_char>() * addresses.len();
    if buf.len() < offset + l {
        return Err(Errno::ERANGE.into());
    }
    let addr_list = buf[offset..].as_mut_ptr().cast::<*mut c_char>();
    std::ptr::copy_nonoverlapping(addresses.as_ptr() as *const *mut c_char, addr_list, l);

    // Finally, populate the hostent structure
    host.h_name = name_dest;
    host.h_aliases = aliases_ptr;
    host.h_addr_list = addr_list;
    match first_rdata {
        RData::A(_) => {
            host.h_addrtype = AF_INET;
            host.h_length = 4;
        }
        RData::AAAA(_) => {
            host.h_addrtype = AF_INET6;
            host.h_length = 16;
        }
        _ => return Err(Errno::EBADMSG.into()),
    }

    Ok(DnsResult::Found)
}

unsafe fn write_record_name_to_buf<'a>(
    record: &Record,
    buf: &'a mut [u8],
) -> std::io::Result<(&'a mut [u8], *mut c_char)> {
    use std::io::{Error, ErrorKind};
    let name = CString::new(record.name().to_utf8())
        .map_err(|e| Error::new(ErrorKind::InvalidData, e.to_string()))?;

    let offset = buf.as_ptr().align_offset(align_of::<libc::c_char>());
    let name_src = name.as_bytes_with_nul();
    let l = name_src.len();
    if buf.len() < offset + l {
        return Err(Errno::ERANGE.into());
    }

    let name_dest = buf.as_mut_ptr().add(offset);
    std::ptr::copy_nonoverlapping(name_src.as_ptr(), name_dest, l);
    Ok((&mut buf[(offset + l)..], name_dest as *mut c_char))
}

pub fn set_if_valid<T>(ptr: *mut T, val: T) {
    if !ptr.is_null() {
        unsafe { *ptr = val };
    }
}

pub fn block_on<F>(f: F) -> Result<F::Output>
where
    F: std::future::Future,
{
    // TODO: use std::panic::catch_unwind() to be resilient
    // to OOM problems

    use std::ops::Deref;
    match RUNTIME.deref() {
        Ok(rt) => Ok(rt.block_on(async { f.await })),
        Err(e) => bail!("Unable to start tokio runtime: {}", e),
    }
}