Panic: "remapper was overflowed" when composite glyphs reach all 65,536 glyph IDs

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Assessment

Difficulty
4/5
Estimated time
1-2 days
Newbie friendliness
56/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
backend

Research direction

Start by reading subsetter-0.2.6/src/remapper.rs around the overflowing u16 count and trace how glyf::closure remaps composite components. The reproduction in the issue demonstrates the failure when all 65,536 glyph IDs are reached; use it to check a fix. Done means subset returns an error or handles out-of-range component IDs without panicking.

Written by the indexing model from the issue text.

Description

subset panics on a TrueType font whose glyph closure reaches all 65,536 glyph IDs, in any build:

thread 'main' panicked at subsetter-0.2.6/src/remapper.rs:91:18:
remapper was overflowed

glyf::closure remaps every glyph it reaches through composite components, and Remapper::remap counts them in a u16 with .expect("remapper was overflowed"). Components may name glyph IDs past maxp's glyph count, which Table::glyph_data still reads where loca gives them an entry, so a composite glyph (or several) can reach every ID from 0 to 65535: the 65,536th glyph remapped panics. This happens with 0.2.6 and with main (b0adcc1). (#34 is the same expect reached through the Font DICT remapper of CFF fonts.)

To reproduce with subsetter 0.2.6 (default-features = false). The font is built for this report: three tables, glyf, head and loca, where .notdef is a composite of the empty glyphs 1 to 65535.

fn main() {
    // A TrueType font of three tables, built for this report: glyph 0 is a composite of
    // the empty glyphs 1 to 65535, which `loca` gives entries.
    let mut glyf = vec![0xFF, 0xFF, 0, 0, 0, 0, 0, 0, 0, 0]; // numberOfContours -1, bbox
    for glyph in 1..=u16::MAX {
        let flags: u16 = if glyph == u16::MAX { 0x0002 } else { 0x0022 }; // MORE_COMPONENTS
        glyf.extend(flags.to_be_bytes());
        glyf.extend(glyph.to_be_bytes());
        glyf.extend([0, 0]); // byte arguments
    }
    let mut loca = Vec::new();
    loca.extend(0u32.to_be_bytes());
    for _ in 0..=u16::MAX {
        loca.extend((glyf.len() as u32).to_be_bytes()); // glyphs 1.. are empty
    }
    let mut head = vec![0; 54];
    head[51] = 1; // indexToLocFormat: long
    let tables: [(&[u8; 4], Vec<u8>); 3] = [(b"glyf", glyf), (b"head", head), (b"loca", loca)];
    let mut font = vec![0, 1, 0, 0, 0, 3, 0, 32, 0, 1, 0, 16];
    let mut offset = 12 + 16 * tables.len();
    for (tag, table) in &tables {
        font.extend(*tag);
        font.extend([0; 4]); // checksum, not checked
        font.extend((offset as u32).to_be_bytes());
        font.extend((table.len() as u32).to_be_bytes());
        offset += table.len().next_multiple_of(4);
    }
    for (_, table) in &tables {
        font.extend(table);
        font.resize(font.len().next_multiple_of(4), 0);
    }
    let remapper = subsetter::GlyphRemapper::new();
    println!("{:?}", subsetter::subset(&font, 0, &remapper).map(|font| font.len()));
}

cargo run and cargo run --release both panic as above.

It was found by the independent review of a officewright PR that checks glyphs for subsetting; until it is fixed, our call to subset refuses such a subset before the subsetter runs.

A possible fix: return an error from closure (for example MalformedFont) once the remapper holds 65,536 glyphs, or skip components that are not below maxp's glyph count.

Dominant language
Rust
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