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Using LLVM's libfuzzer with Swift

Background

Of the many tools available for fuzzing, I've found that LLVM's libfuzzer is the easiest to use for Swift on macOS. Other tools seem to have a list of requirements taht are difficult to meet. libfuzzer on the other hand is built into LLVM and is available on macOS in the custom Swift toolchains: https://www.swift.org/download/

In this document I'll describe how to use libfuzzer with Swift and Swift Packages.

I used this setup to fuzz an SVG Renderer package that I am building. I was able to find and fix a number of bugs in my SVG parsing code using libfuzzer in basically no time at all.

mayoff

import SwiftUI

extension UnitPoint {
    /// - returns: The point on the perimeter of the unit square that is at angle `angle` relative to the center of the unit square.
    init(_ angle: Angle) {
        // Inspired by https://math.stackexchange.com/a/4041510/399217
        // Also see https://www.desmos.com/calculator/k13553cbgk

        let s = sin(angle.radians)
        let c = cos(angle.radians)

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// https://adventofcode.com/2022/day/23

import AOC
import Preamble

let input = readInput()
let xinput = """
....#..
..###.#
#...#.#

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# https://twitter.com/hillelogram/status/1461103115260805123/photo/1
# https://pikchr.org/home/pikchrshow

linerad = 5px

B1: box "1"
B8: box "8"
B9: box "9"
B1_2: [ box; "1" color blue at last box ]
B2_2: [ box; "2" color blue at last box ]

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import SwiftUI

fileprivate struct WidthPreferenceKey: PreferenceKey {
    static var defaultValue: [AnyHashable: CGFloat] { [:] }

    static func reduce(value: inout [AnyHashable : CGFloat], nextValue: () -> [AnyHashable : CGFloat]) {
        value.merge(nextValue(), uniquingKeysWith: { max($0, $1) })
    }
}

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.dqd.stop-simulator-spotlight</string>
    <key>ProcessType</key>
    <string>Background</string>
    <key>ProgramArguments</key>
    <array>

mayoff

You probably have a WidgetBundle type in your widget extension like this:

@main
struct MyWidgets: WidgetBundle {
    var body: some Widget {
        SmallWidget()
        MediumWidget()
        LargeWidget()
    }

mayoff

import SwiftUI

fileprivate struct IconWidthKey: PreferenceKey {
    static var defaultValue: CGFloat? { nil }

    static func reduce(value: inout CGFloat?, nextValue: () -> CGFloat?) {
        switch (value, nextValue()) {
        case (nil, let next): value = next
        case (_, nil): break
        case (.some(let current), .some(let next)): value = max(current, next)

mayoff

@testable import SwiftUICaseStudies
import XCTest

class RefreshableTests: XCTestCase {

  func testVanilla() async {
    var k: CheckedContinuation<Void, Never>? = nil

    let viewModel = PullToRefreshViewModel(
      fetch: {

mayoff

#if canImport(Combine)

import Combine
import CasePaths

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
public typealias Effect<Output> = AnyPublisher<Output, Never>

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
extension Effect {