From 7f86b2ace52b737db68513edbe94e3086ff06224 Mon Sep 17 00:00:00 2001
From: Minh Vu <vuhoangminh97@gmail.com>
Date: Sun, 9 Aug 2026 16:39:18 +0200
Subject: [PATCH] codec: clamp runtime frame length to field width (#8275)

---
 tokio-util/src/codec/length_delimited.rs | 29 +++++++++++++++++-------
 tokio-util/tests/length_delimited.rs     | 27 ++++++++++++++++++++++
 2 files changed, 48 insertions(+), 8 deletions(-)

diff --git a/tokio-util/src/codec/length_delimited.rs b/tokio-util/src/codec/length_delimited.rs
index 31799fdeb..8a7efbbdc 100644
--- a/tokio-util/src/codec/length_delimited.rs
+++ b/tokio-util/src/codec/length_delimited.rs
@@ -493,8 +493,12 @@ impl LengthDelimitedCodec {
     /// words, if a frame is currently in process of being decoded with a frame
     /// size greater than `val` but less than the max frame length in effect
     /// before calling this function, then the frame will be allowed.
+    ///
+    /// If `val` is larger than what the length field can represent, it is
+    /// clipped to the maximum representable value.
     pub fn set_max_frame_length(&mut self, val: usize) {
         self.builder.max_frame_length(val);
+        self.builder.adjust_max_frame_len();
     }
 
     fn decode_head(&mut self, src: &mut BytesMut) -> io::Result<Option<usize>> {
@@ -1049,16 +1053,25 @@ impl Builder {
     }
 
     fn adjust_max_frame_len(&mut self) {
-        // 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) {
+        let max_allowed_len = self.max_allowed_frame_len();
+
+        if self.max_frame_len > max_allowed_len {
+            self.max_frame_len = max_allowed_len;
+        }
+    }
+
+    fn max_allowed_frame_len(&self) -> usize {
+        let max_allowed_len = self
+            .max_length_field_value()
+            .saturating_add_signed(self.length_adjustment as i64);
+
+        usize::try_from(max_allowed_len).unwrap_or(usize::MAX)
+    }
+
+    fn max_length_field_value(&self) -> u64 {
+        match 1u64.checked_shl((8 * self.length_field_len) as u32) {
             Some(shl) => shl - 1,
             None => u64::MAX,
-        };
-
-        let max_allowed_len = max_number.saturating_add_signed(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);
         }
     }
 }
diff --git a/tokio-util/tests/length_delimited.rs b/tokio-util/tests/length_delimited.rs
index 14d4ab521..2ff60d13e 100644
--- a/tokio-util/tests/length_delimited.rs
+++ b/tokio-util/tests/length_delimited.rs
@@ -699,6 +699,33 @@ fn frame_does_not_fit() {
     assert_eq!(codec.max_frame_length(), 255);
 }
 
+#[test]
+fn runtime_max_frame_len_respects_length_field() {
+    for (adjustment, max_frame_len) in [(-1, 254), (0, 255), (1, 256)] {
+        let mut codec = LengthDelimitedCodec::builder()
+            .length_field_length(1)
+            .length_adjustment(adjustment)
+            .new_codec();
+
+        codec.set_max_frame_length(1_000);
+        assert_eq!(codec.max_frame_length(), max_frame_len);
+
+        let mut dst = BytesMut::new();
+        codec
+            .encode(Bytes::from(vec![0; max_frame_len]), &mut dst)
+            .unwrap();
+        assert_eq!(dst[0], u8::MAX);
+
+        let mut dst = BytesMut::from(&b"prefix"[..]);
+        let original = dst.clone();
+        let result = codec.encode(Bytes::from(vec![0; max_frame_len + 1]), &mut dst);
+
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err().kind(), io::ErrorKind::InvalidInput);
+        assert_eq!(dst, original);
+    }
+}
+
 #[test]
 fn neg_adjusted_frame_does_not_fit() {
     let codec = LengthDelimitedCodec::builder()