Fix handling of fdAT chunks shorter than 4 bytes.

This has accidentally regressed in a recent commit and was caught during
PR review.

src/decoder/stream.rs

@@ -217,6 +217,8 @@ pub(crate) enum FormatErrorInner {
     NoMoreImageData,
     /// Bad text encoding
     BadTextEncoding(TextDecodingError),
+    /// fdAT shorter than 4 bytes
+    FdatShorterThanFourBytes,
 }
 
 impl error::Error for DecodingError {
@@ -337,6 +339,7 @@ impl fmt::Display for FormatError {
                     }
                 }
             }
+            FdatShorterThanFourBytes => write!(fmt, "fdAT chunk shorter than 4 bytes"),
         }
     }
 }
@@ -651,7 +654,13 @@ impl StreamingDecoder {
                     ApngSequenceNumber => {
                         debug_assert_eq!(self.current_chunk.type_, chunk::fdAT);
                         let next_seq_no = val;
-                        self.current_chunk.remaining -= 4;
+                        if self.current_chunk.remaining < 4 {
+                            return Err(DecodingError::Format(
+                                FormatErrorInner::FdatShorterThanFourBytes.into(),
+                            ));
+                        } else {
+                            self.current_chunk.remaining -= 4;
+                        }
 
                         if let Some(seq_no) = self.current_seq_no {
                             if next_seq_no != seq_no + 1 {
@@ -1363,7 +1372,11 @@ impl Default for ChunkState {
 mod tests {
     use super::ScaledFloat;
     use super::SourceChromaticities;
+    use crate::test_utils::*;
+    use crate::{Decoder, DecodingError};
+    use byteorder::WriteBytesExt;
     use std::fs::File;
+    use std::io::Write;
 
     #[test]
     fn image_gamma() -> Result<(), ()> {
@@ -1611,4 +1624,100 @@ mod tests {
         // content (`b"test iccp contents"`) which would have a different CRC (797351983).
         assert_eq!(4070462061, crc32fast::hash(&icc_profile));
     }
+
+    /// Writes an acTL chunk.
+    /// See https://wiki.mozilla.org/APNG_Specification#.60acTL.60:_The_Animation_Control_Chunk
+    fn write_actl(w: &mut impl Write, animation: &crate::AnimationControl) {
+        let mut data = Vec::new();
+        data.write_u32::<byteorder::BigEndian>(animation.num_frames)
+            .unwrap();
+        data.write_u32::<byteorder::BigEndian>(animation.num_plays)
+            .unwrap();
+        write_chunk(w, b"acTL", &data);
+    }
+
+    /// Writes an fcTL chunk.
+    /// See https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk
+    fn write_fctl(w: &mut impl Write, frame: &crate::FrameControl) {
+        let mut data = Vec::new();
+        data.write_u32::<byteorder::BigEndian>(frame.sequence_number)
+            .unwrap();
+        data.write_u32::<byteorder::BigEndian>(frame.width).unwrap();
+        data.write_u32::<byteorder::BigEndian>(frame.height)
+            .unwrap();
+        data.write_u32::<byteorder::BigEndian>(frame.x_offset)
+            .unwrap();
+        data.write_u32::<byteorder::BigEndian>(frame.y_offset)
+            .unwrap();
+        data.write_u16::<byteorder::BigEndian>(frame.delay_num)
+            .unwrap();
+        data.write_u16::<byteorder::BigEndian>(frame.delay_den)
+            .unwrap();
+        data.write_u8(frame.dispose_op as u8).unwrap();
+        data.write_u8(frame.blend_op as u8).unwrap();
+        write_chunk(w, b"fcTL", &data);
+    }
+
+    /// Writes PNG signature and chunks that can precede an fdAT chunk that is expected
+    /// to have
+    /// - `sequence_number` set to 0
+    /// - image data with rgba8 pixels in a `width` by `width` image
+    fn write_fdat_prefix(w: &mut impl Write, num_frames: u32, width: u32) {
+        write_png_sig(w);
+        write_rgba8_ihdr_with_width(w, width);
+        write_actl(
+            w,
+            &crate::AnimationControl {
+                num_frames,
+                num_plays: 0,
+            },
+        );
+
+        let mut fctl = crate::FrameControl {
+            width,
+            height: width,
+            ..Default::default()
+        };
+        write_fctl(w, &fctl);
+        write_rgba8_idat_with_width(w, width);
+
+        fctl.sequence_number += 1;
+        write_fctl(w, &fctl);
+    }
+
+    #[test]
+    fn test_fdat_chunk_payload_length_0() {
+        let mut png = Vec::new();
+        write_fdat_prefix(&mut png, 2, 8);
+        write_chunk(&mut png, b"fdAT", &[]);
+
+        let decoder = Decoder::new(png.as_slice());
+        let mut reader = decoder.read_info().unwrap();
+        let mut buf = vec![0; reader.output_buffer_size()];
+        reader.next_frame(&mut buf).unwrap();
+
+        // 0-length fdAT should result in an error.
+        let err = reader.next_frame(&mut buf).unwrap_err();
+        assert!(matches!(&err, DecodingError::Format(_)));
+        let msg = format!("{err}");
+        assert_eq!("fdAT chunk shorter than 4 bytes", msg);
+    }
+
+    #[test]
+    fn test_fdat_chunk_payload_length_3() {
+        let mut png = Vec::new();
+        write_fdat_prefix(&mut png, 2, 8);
+        write_chunk(&mut png, b"fdAT", &[1, 0, 0]);
+
+        let decoder = Decoder::new(png.as_slice());
+        let mut reader = decoder.read_info().unwrap();
+        let mut buf = vec![0; reader.output_buffer_size()];
+        reader.next_frame(&mut buf).unwrap();
+
+        // 3-bytes-long fdAT should result in an error.
+        let err = reader.next_frame(&mut buf).unwrap_err();
+        assert!(matches!(&err, DecodingError::Format(_)));
+        let msg = format!("{err}");
+        assert_eq!("fdAT chunk shorter than 4 bytes", msg);
+    }
 }

src/lib.rs

@@ -81,5 +81,8 @@ pub use crate::decoder::{
 pub use crate::encoder::{Encoder, EncodingError, StreamWriter, Writer};
 pub use crate::filter::{AdaptiveFilterType, FilterType};
 
+#[cfg(test)]
+pub(crate) mod test_utils;
+
 #[cfg(feature = "benchmarks")]
 pub mod benchable_apis;

src/test_utils.rs

@@ -1,4 +1,10 @@
 //! A set of test utilities.
+//!
+//! There is some overlap between this module and `src/encoder.rs` module, but:
+//!
+//! * This module (unlike `src/encoder.rs`) performs no validation of the data being written - this
+//!   allows building testcases that use arbitrary, potentially invalid PNGs as input.
+//! * This module can be reused from `benches/decoder.rs` (a separate crate).
 
 use byteorder::WriteBytesExt;
 use std::io::Write;
@@ -20,19 +26,24 @@ use std::io::Write;
 /// [this
 /// discussion](https://github.com/image-rs/image-png/discussions/416#discussioncomment-7436871)
 /// for more details).
+#[allow(dead_code)] // Used from `benches/decoder.rs`
 pub fn write_noncompressed_png(w: &mut impl Write, width: u32) {
     write_png_sig(w);
-    write_ihdr(w, width);
-    write_noncompressed_idat(w, width);
+    write_rgba8_ihdr_with_width(w, width);
+    write_rgba8_idat_with_width(w, width);
     write_iend(w);
 }
 
-fn write_png_sig(w: &mut impl Write) {
+/// Writes PNG signature.
+/// See http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html#PNG-file-signature
+pub fn write_png_sig(w: &mut impl Write) {
     const SIG: [u8; 8] = [137, 80, 78, 71, 13, 10, 26, 10];
     w.write_all(&SIG).unwrap();
 }
 
-fn write_chunk(w: &mut impl Write, chunk_type: &[u8], data: &[u8]) {
+/// Writes an arbitrary PNG chunk.
+pub fn write_chunk(w: &mut impl Write, chunk_type: &[u8], data: &[u8]) {
+    assert_eq!(chunk_type.len(), 4);
     let crc = {
         let input = chunk_type
             .iter()
@@ -48,7 +59,9 @@ fn write_chunk(w: &mut impl Write, chunk_type: &[u8], data: &[u8]) {
     w.write_u32::<byteorder::BigEndian>(crc).unwrap();
 }
 
-fn write_ihdr(w: &mut impl Write, width: u32) {
+/// Writes an IHDR chunk that indicates a non-interlaced RGBA8 that uses the same height and
+/// `width`.  See http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.IHDR
+pub fn write_rgba8_ihdr_with_width(w: &mut impl Write, width: u32) {
     let mut data = Vec::new();
     data.write_u32::<byteorder::BigEndian>(width).unwrap();
     data.write_u32::<byteorder::BigEndian>(width).unwrap(); // height
@@ -60,7 +73,8 @@ fn write_ihdr(w: &mut impl Write, width: u32) {
     write_chunk(w, b"IHDR", &data);
 }
 
-fn write_noncompressed_idat(w: &mut impl Write, width: u32) {
+/// Writes an IDAT chunk.
+pub fn write_rgba8_idat_with_width(w: &mut impl Write, width: u32) {
     // Generate arbitrary test pixels.
     let image_pixels = {
         let mut row = Vec::new();