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ICO: Add header validation in strict mode #3033

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ICO: Add header validation in strict mode #3033

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ICO: Add header validation in strict mode
RunDevelopment Jun 11, 2026
42069b7
Merge branch 'main' into ico-header-valid
RunDevelopment Jun 12, 2026
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23 changes: 20 additions & 3 deletions src/codecs/ico/decoder.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use byteorder_lite::ReadBytesExt;
use byteorder_lite::{LittleEndian, ReadBytesExt};
use std::io::{BufRead, Read, Seek};
use std::{error, fmt};

Expand Down Expand Up @@ -175,8 +175,25 @@ impl<R: BufRead + Seek> IcoDecoder<R> {
fn read_entries<R: Read>(r: &mut R, spec: SpecCompliance) -> ImageResult<Vec<DirEntry>> {
let mut header = [0u8; 6];
r.read_exact(&mut header)?;
// header[0..2] = reserved, header[2..4] = type, header[4..6] = count
let count = u16::from_le_bytes(header[4..6].try_into().unwrap());
let mut header = header.as_slice();

let reserved = header.read_u16::<LittleEndian>()?;

@197g 197g Jun 13, 2026

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I'd use split_first_chunk here to instead of an IO-facing library. Alternatively, but only since this is uniformly u16, you might turn header into [[u8; 2]; 3] and read it with as_flattened? Lots of alternatives that do not involve an IO error for a statically sized array and constant condition.

@RunDevelopment RunDevelopment Jun 13, 2026

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TBH, I just copied what TGA is doing. Should we not do that there too?

I also considered the u16 approach, but didn't like it because it makes it really easy to mess up endian. E.g.

let mut header = [0_u16; 3];
reader.read_exact(bytemuck::cast_slice_mut(&mut header))?;
let [reserved, id_type, entries] = header; // only correct on LE systems

But I agree with you that the IO interface is a little overkill. Maybe we could just have a 2 or 3 util methods like this:

// Panics when OOB
fn read_u16<Endian>(slice: &[u8], offset: usize) -> u16;

I saw this pattern in Chrome, so that might be an idea.

let id_type = header.read_u16::<LittleEndian>()?;
let count = header.read_u16::<LittleEndian>()?;

if spec == SpecCompliance::Strict && reserved != 0 {
return Err(ImageError::Decoding(DecodingError::new(
ImageFormat::Ico.into(),
format!("Reserved field must be 0, but found 0x{reserved:X}"),
)));
}
if spec == SpecCompliance::Strict && !matches!(id_type, 1 | 2) {
return Err(ImageError::Decoding(DecodingError::new(
ImageFormat::Ico.into(),
format!("Invalid header type. Expected 1 (ICO) or 2 (CUR), but found {id_type}"),
)));
}

(0..count).map(|_| read_entry(r, spec)).collect()
}

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