util: assert compatibility between LengthDelimitedCodec options

tokio-util/src/codec/length_delimited.rs

@@ -386,6 +386,10 @@ use std::{cmp, fmt, mem};
 /// `Builder` enables constructing configured length delimited codecs. Note
 /// that not all configuration settings apply to both encoding and decoding. See
 /// the documentation for specific methods for more detail.
+///
+/// Note that the if the value of [`Builder::max_frame_length`] becomes larger than
+/// what can actually fit in [`Builder::length_field_length`], it will be clipped to
+/// the maximum value that can fit.
 #[derive(Debug, Clone, Copy)]
 pub struct Builder {
     // Maximum frame length
@@ -935,8 +939,12 @@ impl Builder {
     /// # }
     /// ```
     pub fn new_codec(&self) -> LengthDelimitedCodec {
+        let mut builder = *self;
+
+        builder.adjust_max_frame_len();
+
         LengthDelimitedCodec {
-            builder: *self,
+            builder,
             state: DecodeState::Head,
         }
     }
@@ -1018,6 +1026,35 @@ impl Builder {
         self.num_skip
             .unwrap_or(self.length_field_offset + self.length_field_len)
     }
+
+    fn adjust_max_frame_len(&mut self) {
+        // This function is basically `std::u64::saturating_add_signed`. Since it
+        // requires MSRV 1.66, its implementation is copied here.
+        //
+        // TODO: use the method from std when MSRV becomes >= 1.66
+        fn saturating_add_signed(num: u64, rhs: i64) -> u64 {
+            let (res, overflow) = num.overflowing_add(rhs as u64);
+            if overflow == (rhs < 0) {
+                res
+            } else if overflow {
+                u64::MAX
+            } else {
+                0
+            }
+        }
+
+        // Calculate the maximum number that can be represented using `length_field_len` bytes.
+        let max_number = match 1u64.checked_shl((8 * self.length_field_len) as u32) {
+            Some(shl) => shl - 1,
+            None => u64::MAX,
+        };
+
+        let max_allowed_len = saturating_add_signed(max_number, self.length_adjustment as i64);
+
+        if self.max_frame_len as u64 > max_allowed_len {
+            self.max_frame_len = usize::try_from(max_allowed_len).unwrap_or(usize::MAX);
+        }
+    }
 }
 
 impl Default for Builder {

tokio-util/tests/length_delimited.rs

@@ -689,6 +689,66 @@ fn encode_overflow() {
     codec.encode(Bytes::from("hello"), &mut buf).unwrap();
 }
 
+#[test]
+fn frame_does_not_fit() {
+    let codec = LengthDelimitedCodec::builder()
+        .length_field_length(1)
+        .max_frame_length(256)
+        .new_codec();
+
+    assert_eq!(codec.max_frame_length(), 255);
+}
+
+#[test]
+fn neg_adjusted_frame_does_not_fit() {
+    let codec = LengthDelimitedCodec::builder()
+        .length_field_length(1)
+        .length_adjustment(-1)
+        .new_codec();
+
+    assert_eq!(codec.max_frame_length(), 254);
+}
+
+#[test]
+fn pos_adjusted_frame_does_not_fit() {
+    let codec = LengthDelimitedCodec::builder()
+        .length_field_length(1)
+        .length_adjustment(1)
+        .new_codec();
+
+    assert_eq!(codec.max_frame_length(), 256);
+}
+
+#[test]
+fn max_allowed_frame_fits() {
+    let codec = LengthDelimitedCodec::builder()
+        .length_field_length(std::mem::size_of::<usize>())
+        .max_frame_length(usize::MAX)
+        .new_codec();
+
+    assert_eq!(codec.max_frame_length(), usize::MAX);
+}
+
+#[test]
+fn smaller_frame_len_not_adjusted() {
+    let codec = LengthDelimitedCodec::builder()
+        .max_frame_length(10)
+        .length_field_length(std::mem::size_of::<usize>())
+        .new_codec();
+
+    assert_eq!(codec.max_frame_length(), 10);
+}
+
+#[test]
+fn max_allowed_length_field() {
+    let codec = LengthDelimitedCodec::builder()
+        .length_field_length(8)
+        .max_frame_length(usize::MAX)
+        .new_codec();
+
+    assert_eq!(codec.max_frame_length(), usize::MAX);
+}
+
 // ===== Test utils =====
 
 struct Mock {