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//! Incremental decoder for `Transfer-Encoding: chunked` request bodies.
/// Longest accepted chunk-size line or trailer line.
const MAX_LINE: usize = 4096;
#[derive(Debug)]
enum State {
/// Reading a `size[;ext]\r\n` line.
Size,
/// Reading chunk data; this many bytes are left.
Data(usize),
/// Expecting the `\r\n` after chunk data.
DataEnd,
/// Reading trailer lines until the empty one.
Trailers,
}
#[derive(Debug, PartialEq, Eq)]
pub(crate) enum Progress {
/// The body is complete; the encoded form took this many bytes in total.
Done(usize),
NeedMore,
}
/// Decodes a chunked body as its bytes arrive. Each byte is looked at once, no
/// matter how the input is split up.
#[derive(Debug)]
pub(crate) struct Decoder {
state: State,
/// Encoded bytes consumed so far.
pos: usize,
trailer_bytes: usize,
pub(crate) body: Vec<u8>,
max: usize,
}
impl Decoder {
pub(crate) fn new(max: usize) -> Self {
Self {
state: State::Size,
pos: 0,
trailer_bytes: 0,
body: Vec::new(),
max,
}
}
/// `data` is everything received after the head so far. Returns the status
/// code to answer with on failure.
pub(crate) fn feed(&mut self, data: &[u8]) -> Result<Progress, u16> {
loop {
let rest = &data[self.pos..];
match self.state {
State::Size => {
let Some(nl) = rest.iter().position(|&b| b == b'\n') else {
return if rest.len() > MAX_LINE {
Err(400)
} else {
Ok(Progress::NeedMore)
};
};
if nl == 0 || rest[nl - 1] != b'\r' || nl > MAX_LINE {
return Err(400);
}
let line = &rest[..nl - 1];
let digits = line.iter().position(|&b| b == b';').unwrap_or(line.len());
let hex = &line[..digits];
if hex.is_empty() || hex.len() > 16 || !hex.iter().all(u8::is_ascii_hexdigit) {
return Err(400);
}
// Chunk extensions are ignored, but must not hide controls.
if !crate::parse::is_field_value(&line[digits..]) {
return Err(400);
}
let size = hex.iter().fold(0u64, |acc, &b| {
acc * 16 + (b as char).to_digit(16).unwrap_or(0) as u64
});
if size > (self.max - self.body.len()) as u64 {
return Err(413);
}
self.pos += nl + 1;
self.state = if size == 0 {
State::Trailers
} else {
State::Data(size as usize)
};
}
State::Data(left) => {
if rest.is_empty() {
return Ok(Progress::NeedMore);
}
let n = left.min(rest.len());
self.body.extend_from_slice(&rest[..n]);
self.pos += n;
self.state = if n == left {
State::DataEnd
} else {
State::Data(left - n)
};
}
State::DataEnd => {
if rest.len() < 2 {
return Ok(Progress::NeedMore);
}
if &rest[..2] != b"\r\n" {
return Err(400);
}
self.pos += 2;
self.state = State::Size;
}
State::Trailers => {
let Some(nl) = rest.iter().position(|&b| b == b'\n') else {
return if rest.len() > MAX_LINE {
Err(400)
} else {
Ok(Progress::NeedMore)
};
};
if nl == 0 || rest[nl - 1] != b'\r' {
return Err(400);
}
self.pos += nl + 1;
if nl == 1 {
return Ok(Progress::Done(self.pos));
}
// Trailer fields are read and dropped, within a budget.
self.trailer_bytes += nl + 1;
if nl > MAX_LINE || self.trailer_bytes > 4 * MAX_LINE {
return Err(400);
}
}
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
fn decode(input: &[u8], max: usize) -> Result<(Vec<u8>, usize), u16> {
let mut d = Decoder::new(max);
match d.feed(input)? {
Progress::Done(n) => Ok((d.body, n)),
Progress::NeedMore => Err(0),
}
}
#[test]
fn decodes_chunks_extensions_and_trailers() {
let input = b"5\r\nhello\r\n6;ext=1\r\n world\r\n0\r\nX-Trailer: 1\r\n\r\nNEXT";
let (body, used) = decode(input, 1024).unwrap();
assert_eq!(body, b"hello world");
assert_eq!(&input[used..], b"NEXT");
assert_eq!(decode(b"0\r\n\r\n", 0).unwrap(), (vec![], 5));
assert_eq!(
decode(b"A\r\n0123456789\r\n0\r\n\r\n", 10).unwrap().0,
b"0123456789"
);
}
#[test]
fn gives_the_same_answer_however_the_input_is_split() {
let input = b"5\r\nhello\r\n6;ext=1\r\n world\r\n0\r\nX-Trailer: 1\r\n\r\n";
for step in 1..input.len() {
let mut d = Decoder::new(1024);
let mut end = 0;
let done = loop {
end = (end + step).min(input.len());
match d.feed(&input[..end]).unwrap() {
Progress::Done(n) => break n,
Progress::NeedMore => assert!(end < input.len(), "step {step}"),
}
};
assert_eq!(done, input.len());
assert_eq!(d.body, b"hello world");
}
}
#[test]
fn rejects_bad_framing_and_oversized_bodies() {
for bad in [
&b"5\nhello\r\n0\r\n\r\n"[..],
b"\r\n",
b"g\r\n",
b"-1\r\n",
b"0x5\r\n",
b"5 \r\nhello\r\n0\r\n\r\n",
b"5\r\nhelloXX0\r\n\r\n",
b"11111111111111111\r\n",
b"1;a\0b\r\nx\r\n0\r\n\r\n",
b"0\r\nbad\n\r\n",
] {
assert_eq!(
decode(bad, 1 << 20),
Err(400),
"{:?}",
String::from_utf8_lossy(bad)
);
}
assert_eq!(decode(b"5\r\nhello\r\n1\r\n!\r\n0\r\n\r\n", 5), Err(413));
assert_eq!(decode(b"ffffffffffffffff\r\n", 1 << 20), Err(413));
// Incomplete input is not an error.
assert_eq!(decode(b"5\r\nhel", 1024), Err(0));
// An endless size line or trailer section is.
assert_eq!(decode(&vec![b'1'; MAX_LINE + 2], 1 << 20), Err(400));
let mut trailers = b"0\r\n".to_vec();
for _ in 0..2000 {
trailers.extend_from_slice(b"X-Trailer: aaaaaaaaaaaaaaaa\r\n");
}
assert_eq!(decode(&trailers, 1 << 20), Err(400));
}
}