tor_cell/chancell/
codec.rs

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1//! Implementation for encoding and decoding of ChanCells.
2
3use super::{CELL_DATA_LEN, ChanCell};
4use crate::Error;
5use crate::chancell::{ChanCmd, ChanMsg, CircId};
6use tor_bytes::{self, Reader, Writer};
7use tor_error::internal;
8
9use bytes::BytesMut;
10
11/// This object can be used to encode and decode channel cells.
12///
13/// NOTE: only link protocol versions 3 and higher are supported.
14/// VERSIONS cells are not supported via the encoder/decoder, since
15/// VERSIONS cells always use a two-byte circuit-ID for backwards
16/// compatibility with protocol versions < 4.
17///
18/// The implemented format is one of the following:
19///
20/// Variable-length cells (since protocol versions 2 and 3 respectively):
21/// ```ignore
22///     u32 circid;
23///     u8 command;
24///     u16 len;
25///     u8 body[len];
26/// ```
27///
28/// Fixed-width cells (since protocol version 1 and 4 respectively):
29/// ```ignore
30///     u32 circid;
31///     u8 command;
32///     u8 body[509];
33/// ```
34pub struct ChannelCodec {
35    #[allow(dead_code)] // We don't support any link versions where this matters
36    /// The link protocol version being used for this channel.
37    ///
38    /// (We don't currently support any versions of the link protocol
39    /// where this version matters, but for protocol versions below 4, it would
40    /// affect the length of the circuit ID.)
41    link_version: u16,
42}
43
44impl ChannelCodec {
45    /// Create a new ChannelCodec with a given link protocol version
46    pub fn new(link_version: u16) -> Self {
47        ChannelCodec { link_version }
48    }
49
50    /// Return the link protocol version of this codec.
51    pub fn link_version(&self) -> u16 {
52        self.link_version
53    }
54
55    /// Write the given cell into the provided BytesMut object.
56    pub fn write_cell<M: ChanMsg>(
57        &mut self,
58        item: ChanCell<M>,
59        dst: &mut BytesMut,
60    ) -> crate::Result<()> {
61        let ChanCell { circid, msg } = item;
62        let cmd = msg.cmd();
63        dst.write_u32(CircId::get_or_zero(circid));
64        dst.write_u8(cmd.into());
65
66        // this is typically 5, but not always
67        // (for example if we were given a non-empty `dst`)
68        let pos = dst.len();
69
70        // now write the cell body and handle the length.
71        if cmd.is_var_cell() {
72            dst.write_u16(0);
73            msg.encode_onto(dst)?;
74            let len = dst.len() - pos - 2;
75            if len > u16::MAX as usize {
76                return Err(Error::Internal(internal!("ran out of space for varcell")));
77            }
78            // go back and set the length.
79            *(<&mut [u8; 2]>::try_from(&mut dst[pos..pos + 2])
80                .expect("two-byte slice was not two bytes!?")) = (len as u16).to_be_bytes();
81        } else {
82            msg.encode_onto(dst)?;
83            let len = dst.len() - pos;
84            if len > CELL_DATA_LEN {
85                return Err(Error::Internal(internal!("ran out of space for cell")));
86            }
87            // pad to end of fixed-length cell
88            dst.write_zeros(CELL_DATA_LEN - len);
89        }
90        Ok(())
91    }
92
93    /// Try to decode a cell from the provided BytesMut object.
94    ///
95    /// On a definite decoding error, return Err(_).  On a cell that might
96    /// just be truncated, return Ok(None).
97    pub fn decode_cell<M: ChanMsg>(
98        &mut self,
99        src: &mut BytesMut,
100    ) -> crate::Result<Option<ChanCell<M>>> {
101        /// Wrap `be` as an appropriate type.
102        fn wrap_err(be: tor_bytes::Error) -> crate::Error {
103            crate::Error::BytesErr {
104                err: be,
105                parsed: "channel cell",
106            }
107        }
108
109        if src.len() < 7 {
110            // Smallest possible command: varcell with len 0
111            return Ok(None);
112        }
113        let cmd: ChanCmd = src[4].into();
114        let varcell = cmd.is_var_cell();
115        let cell_len: usize = if varcell {
116            let msg_len = u16::from_be_bytes(
117                src[5..7]
118                    .try_into()
119                    .expect("Two-byte slice was not two bytes long!?"),
120            );
121            msg_len as usize + 7
122        } else {
123            514
124        };
125        if src.len() < cell_len {
126            return Ok(None);
127        }
128
129        let cell = src.split_to(cell_len).freeze();
130        //trace!("{:?} cell body ({}) is {:?}", cmd, cell.len(), &cell[..]);
131        let mut r = Reader::from_bytes(&cell);
132        let circid: Option<CircId> = CircId::new(r.take_u32().map_err(wrap_err)?);
133        r.advance(if varcell { 3 } else { 1 }).map_err(wrap_err)?;
134        let msg = M::decode_from_reader(cmd, &mut r).map_err(wrap_err)?;
135
136        if !cmd.accepts_circid_val(circid) {
137            return Err(Error::ChanProto(format!(
138                "Invalid circuit ID {} for cell command {}",
139                CircId::get_or_zero(circid),
140                cmd
141            )));
142        }
143        Ok(Some(ChanCell { circid, msg }))
144    }
145}
tor_cell/chancell/codec.rs

tor_cell/chancell/
codec.rs
