coverage-analysis

SKILL.md

Coverage Analysis

Coverage analysis is essential for understanding which parts of your code are exercised during fuzzing. It helps identify fuzzing blockers like magic value checks and tracks the effectiveness of harness improvements over time.

Overview

Code coverage during fuzzing serves two critical purposes:

Assessing harness effectiveness: Understand which parts of your application are actually executed by your fuzzing harnesses

Tracking fuzzing progress: Monitor how coverage changes when updating harnesses, fuzzers, or the system under test (SUT)

Coverage is a proxy for fuzzer capability and performance. While coverage is not ideal for measuring fuzzer performance in absolute terms, it reliably indicates whether your harness works effectively in a given setup.

Key Concepts

Concept
Description

Coverage instrumentation
Compiler flags that track which code paths are executed

Corpus coverage
Coverage achieved by running all test cases in a fuzzing corpus

Magic value checks
Hard-to-discover conditional checks that block fuzzer progress

Coverage-guided fuzzing
Fuzzing strategy that prioritizes inputs that discover new code paths

Coverage report
Visual or textual representation of executed vs. unexecuted code

When to Apply

Apply this technique when:

Starting a new fuzzing campaign to establish a baseline

Fuzzer appears to plateau without finding new paths

After harness modifications to verify improvements

When migrating between different fuzzers

Identifying areas requiring dictionary entries or seed inputs

Debugging why certain code paths aren't reached

Skip this technique when:

Fuzzing campaign is actively finding crashes

Coverage infrastructure isn't set up yet

Working with extremely large codebases where full coverage reports are impractical

Fuzzer's internal coverage metrics are sufficient for your needs

Quick Reference

Task
Command/Pattern

LLVM coverage instrumentation (C/C++)
-fprofile-instr-generate -fcoverage-mapping

GCC coverage instrumentation
-ftest-coverage -fprofile-arcs

cargo-fuzz coverage (Rust)
cargo +nightly fuzz coverage <target>

Generate LLVM profile data
llvm-profdata merge -sparse file.profraw -o file.profdata

LLVM coverage report
llvm-cov report ./binary -instr-profile=file.profdata

LLVM HTML report
llvm-cov show ./binary -instr-profile=file.profdata -format=html -output-dir html/

gcovr HTML report
gcovr --html-details -o coverage.html

Ideal Coverage Workflow

The following workflow represents best practices for integrating coverage analysis into your fuzzing campaigns:

[Fuzzing Campaign]
       |
       v
[Generate Corpus]
       |
       v
[Coverage Analysis]
       |
       +---> Coverage Increased? --> Continue fuzzing with larger corpus
       |
       +---> Coverage Decreased? --> Fix harness or investigate SUT changes
       |
       +---> Coverage Plateaued? --> Add dictionary entries or seed inputs

Key principle: Use the corpus generated after each fuzzing campaign to calculate coverage, rather than real-time fuzzer statistics. This approach provides reproducible, comparable measurements across different fuzzing tools.

Step-by-Step

Step 1: Build with Coverage Instrumentation

Choose your instrumentation method based on toolchain:

LLVM/Clang (C/C++):

clang++ -fprofile-instr-generate -fcoverage-mapping \
  -O2 -DNO_MAIN \
  main.cc harness.cc execute-rt.cc -o fuzz_exec

GCC (C/C++):

g++ -ftest-coverage -fprofile-arcs \
  -O2 -DNO_MAIN \
  main.cc harness.cc execute-rt.cc -o fuzz_exec_gcov

Rust:

rustup toolchain install nightly --component llvm-tools-preview
cargo +nightly fuzz coverage fuzz_target_1

Step 2: Create Execution Runtime (C/C++ only)

For C/C++ projects, create a runtime that executes your corpus:

// execute-rt.cc
#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <stdint.h>

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);

void load_file_and_test(const char *filename) {
    FILE *file = fopen(filename, "rb");
    if (file == NULL) {
        printf("Failed to open file: %s\n", filename);
        return;
    }

    fseek(file, 0, SEEK_END);
    long filesize = ftell(file);
    rewind(file);

    uint8_t *buffer = (uint8_t*) malloc(filesize);
    if (buffer == NULL) {
        printf("Failed to allocate memory for file: %s\n", filename);
        fclose(file);
        return;
    }

    long read_size = (long) fread(buffer, 1, filesize, file);
    if (read_size != filesize) {
        printf("Failed to read file: %s\n", filename);
        free(buffer);
        fclose(file);
        return;
    }

    LLVMFuzzerTestOneInput(buffer, filesize);

    free(buffer);
    fclose(file);
}

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: %s <directory>\n", argv[0]);
        return 1;
    }

    DIR *dir = opendir(argv[1]);
    if (dir == NULL) {
        printf("Failed to open directory: %s\n", argv[1]);
        return 1;
    }

    struct dirent *entry;
    while ((entry = readdir(dir)) != NULL) {
        if (entry->d_type == DT_REG) {
            char filepath[1024];
            snprintf(filepath, sizeof(filepath), "%s/%s", argv[1], entry->d_name);
            load_file_and_test(filepath);
        }
    }

    closedir(dir);
    return 0;
}

Step 3: Execute on Corpus

LLVM (C/C++):

LLVM_PROFILE_FILE=fuzz.profraw ./fuzz_exec corpus/

GCC (C/C++):

./fuzz_exec_gcov corpus/

Rust:
Coverage data is automatically generated when running cargo fuzz coverage.

Step 4: Process Coverage Data

LLVM:

# Merge raw profile data
llvm-profdata merge -sparse fuzz.profraw -o fuzz.profdata

# Generate text report
llvm-cov report ./fuzz_exec \
  -instr-profile=fuzz.profdata \
  -ignore-filename-regex='harness.cc|execute-rt.cc'

# Generate HTML report
llvm-cov show ./fuzz_exec \
  -instr-profile=fuzz.profdata \
  -ignore-filename-regex='harness.cc|execute-rt.cc' \
  -format=html -output-dir fuzz_html/

GCC with gcovr:

# Install gcovr (via pip for latest version)
python3 -m venv venv
source venv/bin/activate
pip3 install gcovr

# Generate report
gcovr --gcov-executable "llvm-cov gcov" \
  --exclude harness.cc --exclude execute-rt.cc \
  --root . --html-details -o coverage.html

Rust:

# Install required tools
cargo install cargo-binutils rustfilt

# Create HTML generation script
cat <<'EOF' > ./generate_html
#!/bin/sh
if [ $# -lt 1 ]; then
    echo "Error: Name of fuzz target is required."
    echo "Usage: $0 fuzz_target [sources...]"
    exit 1
fi
FUZZ_TARGET="$1"
shift
SRC_FILTER="$@"
TARGET=$(rustc -vV | sed -n 's|host: ||p')
cargo +nightly cov -- show -Xdemangler=rustfilt \
  "target/$TARGET/coverage/$TARGET/release/$FUZZ_TARGET" \
  -instr-profile="fuzz/coverage/$FUZZ_TARGET/coverage.profdata" \
  -show-line-counts-or-regions -show-instantiations \
  -format=html -o fuzz_html/ $SRC_FILTER
EOF
chmod +x ./generate_html

# Generate HTML report
./generate_html fuzz_target_1 src/lib.rs

Step 5: Analyze Results

Review the coverage report to identify:

Uncovered code blocks: Areas that may need better seed inputs or dictionary entries

Magic value checks: Conditional statements with hardcoded values that block progress

Dead code: Functions that may not be reachable through your harness

Coverage changes: Compare against baseline to track improvements or regressions

Common Patterns

Pattern: Identifying Magic Values

Problem: Fuzzer cannot discover paths guarded by magic value checks.

Coverage reveals:

// Coverage shows this block is never executed
if (buf == 0x7F454C46) {  // ELF magic number
    // start parsing buf
}

Solution: Add magic values to dictionary file:

# magic.dict
"\x7F\x45\x4C\x46"

Pattern: Handling Crashing Inputs

Problem: Coverage generation fails when corpus contains crashing inputs.

Before:

./fuzz_exec corpus/  # Crashes on bad input, no coverage generated

After:

// Fork before executing to isolate crashes
int main(int argc, char **argv) {
    // ... directory opening code ...

    while ((entry = readdir(dir)) != NULL) {
        if (entry->d_type == DT_REG) {
            pid_t pid = fork();
            if (pid == 0) {
                // Child process - crash won't affect parent
                char filepath[1024];
                snprintf(filepath, sizeof(filepath), "%s/%s", argv[1], entry->d_name);
                load_file_and_test(filepath);
                exit(0);
            } else {
                // Parent waits for child
                waitpid(pid, NULL, 0);
            }
        }
    }
}

Pattern: CMake Integration

Use Case: Adding coverage builds to CMake projects.

project(FuzzingProject)
cmake_minimum_required(VERSION 3.0)

# Main binary
add_executable(program main.cc)

# Fuzzing binary
add_executable(fuzz main.cc harness.cc)
target_compile_definitions(fuzz PRIVATE NO_MAIN=1)
target_compile_options(fuzz PRIVATE -g -O2 -fsanitize=fuzzer)
target_link_libraries(fuzz -fsanitize=fuzzer)

# Coverage execution binary
add_executable(fuzz_exec main.cc harness.cc execute-rt.cc)
target_compile_definitions(fuzz_exec PRIVATE NO_MAIN)
target_compile_options(fuzz_exec PRIVATE -O2 -fprofile-instr-generate -fcoverage-mapping)
target_link_libraries(fuzz_exec -fprofile-instr-generate)

Build:

cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ .
cmake --build . --target fuzz_exec

Advanced Usage

Tips and Tricks

Tip
Why It Helps

Use LLVM 18+ with -show-directory-coverage
Organizes large reports by directory structure instead of flat file list

Export to lcov format for better HTML
llvm-cov export -format=lcov + genhtml provides cleaner per-file reports

Compare coverage across campaigns
Store .profdata files with timestamps to track progress over time

Filter harness code from reports
Use -ignore-filename-regex to focus on SUT coverage only

Automate coverage in CI/CD
Generate coverage reports automatically after scheduled fuzzing runs

Use gcovr 5.1+ for Clang 14+
Older gcovr versions have compatibility issues with recent LLVM

Incremental Coverage Updates

GCC's gcov instrumentation incrementally updates .gcda files across multiple runs. This is useful for tracking coverage as you add test cases:

# First run
./fuzz_exec_gcov corpus_batch_1/
gcovr --html coverage_v1.html

# Second run (adds to existing coverage)
./fuzz_exec_gcov corpus_batch_2/
gcovr --html coverage_v2.html

# Start fresh
gcovr --delete  # Remove .gcda files
./fuzz_exec_gcov corpus/

Handling Large Codebases

For projects with hundreds of source files:

Filter by prefix: Only generate reports for relevant directories

llvm-cov show ./fuzz_exec -instr-profile=fuzz.profdata /path/to/src/

Use directory coverage: Group by directory to reduce clutter (LLVM 18+)

llvm-cov show -show-directory-coverage -format=html -output-dir html/

Generate JSON for programmatic analysis:

llvm-cov export -format=lcov > coverage.json

Differential Coverage

Compare coverage between two fuzzing campaigns:

# Campaign 1
LLVM_PROFILE_FILE=campaign1.profraw ./fuzz_exec corpus1/
llvm-profdata merge -sparse campaign1.profraw -o campaign1.profdata

# Campaign 2
LLVM_PROFILE_FILE=campaign2.profraw ./fuzz_exec corpus2/
llvm-profdata merge -sparse campaign2.profraw -o campaign2.profdata

# Compare
llvm-cov show ./fuzz_exec \
  -instr-profile=campaign2.profdata \
  -instr-profile=campaign1.profdata \
  -show-line-counts-or-regions

Anti-Patterns

Anti-Pattern
Problem
Correct Approach

Using fuzzer-reported coverage for comparisons
Different fuzzers calculate coverage differently, making cross-tool comparison meaningless
Use dedicated coverage tools (llvm-cov, gcovr) for reproducible measurements

Generating coverage with optimizations
-O3 optimizations can eliminate code, making coverage misleading
Use -O2 or -O0 for coverage builds

Not filtering harness code
Harness coverage inflates numbers and obscures SUT coverage
Use -ignore-filename-regex or --exclude to filter harness files

Mixing LLVM and GCC instrumentation
Incompatible formats cause parsing failures
Stick to one toolchain for coverage builds

Ignoring crashing inputs
Crashes prevent coverage generation, hiding real coverage data
Fix crashes first, or use process forking to isolate them

Not tracking coverage over time
One-time coverage checks miss regressions and improvements
Store coverage data with timestamps and track trends

Tool-Specific Guidance

libFuzzer

libFuzzer uses LLVM's SanitizerCoverage by default for guiding fuzzing, but you need separate instrumentation for generating reports.

Build for coverage:

clang++ -fprofile-instr-generate -fcoverage-mapping \
  -O2 -DNO_MAIN \
  main.cc harness.cc execute-rt.cc -o fuzz_exec

Execute corpus and generate report:

LLVM_PROFILE_FILE=fuzz.profraw ./fuzz_exec corpus/
llvm-profdata merge -sparse fuzz.profraw -o fuzz.profdata
llvm-cov show ./fuzz_exec -instr-profile=fuzz.profdata -format=html -output-dir html/

Integration tips:

Don't use -fsanitize=fuzzer for coverage builds (it conflicts with profile instrumentation)

Reuse the same harness function (LLVMFuzzerTestOneInput) with a different main function

Use the -ignore-filename-regex flag to exclude harness code from coverage reports

Consider using llvm-cov's -show-instantiation flag for template-heavy C++ code

AFL++

AFL++ provides its own coverage feedback mechanism, but for detailed reports use standard LLVM/GCC tools.

Build for coverage with LLVM:

clang++ -fprofile-instr-generate -fcoverage-mapping \
  -O2 main.cc harness.cc execute-rt.cc -o fuzz_exec

Build for coverage with GCC:

AFL_USE_ASAN=0 afl-gcc -ftest-coverage -fprofile-arcs \
  main.cc harness.cc execute-rt.cc -o fuzz_exec_gcov

Execute and generate report:

# LLVM approach
LLVM_PROFILE_FILE=fuzz.profraw ./fuzz_exec afl_output/queue/
llvm-profdata merge -sparse fuzz.profraw -o fuzz.profdata
llvm-cov report ./fuzz_exec -instr-profile=fuzz.profdata

# GCC approach
./fuzz_exec_gcov afl_output/queue/
gcovr --html-details -o coverage.html

Integration tips:

Don't use AFL++'s instrumentation (afl-clang-fast) for coverage builds

Use standard compilers with coverage flags instead

AFL++'s queue/ directory contains your corpus

AFL++'s built-in coverage statistics are useful for real-time monitoring but not for detailed analysis

cargo-fuzz (Rust)

cargo-fuzz provides built-in coverage generation using LLVM tools.

Install prerequisites:

rustup toolchain install nightly --component llvm-tools-preview
cargo install cargo-binutils rustfilt

Generate coverage data:

cargo +nightly fuzz coverage fuzz_target_1

Create HTML report script:

cat <<'EOF' > ./generate_html
#!/bin/sh
FUZZ_TARGET="$1"
shift
SRC_FILTER="$@"
TARGET=$(rustc -vV | sed -n 's|host: ||p')
cargo +nightly cov -- show -Xdemangler=rustfilt \
  "target/$TARGET/coverage/$TARGET/release/$FUZZ_TARGET" \
  -instr-profile="fuzz/coverage/$FUZZ_TARGET/coverage.profdata" \
  -show-line-counts-or-regions -show-instantiations \
  -format=html -o fuzz_html/ $SRC_FILTER
EOF
chmod +x ./generate_html

Generate report:

./generate_html fuzz_target_1 src/lib.rs

Integration tips:

Always use the nightly toolchain for coverage

The -Xdemangler=rustfilt flag makes function names readable

Filter by source files (e.g., src/lib.rs) to focus on crate code

Use -show-line-counts-or-regions and -show-instantiations for better Rust-specific output

Corpus is located in fuzz/corpus/<target>/

honggfuzz

honggfuzz works with standard LLVM/GCC coverage instrumentation.

Build for coverage:

# Use standard compiler, not honggfuzz compiler
clang -fprofile-instr-generate -fcoverage-mapping \
  -O2 harness.c execute-rt.c -o fuzz_exec

Execute corpus:

LLVM_PROFILE_FILE=fuzz.profraw ./fuzz_exec honggfuzz_workspace/

Integration tips:

Don't use hfuzz-clang for coverage builds

honggfuzz corpus is typically in a workspace directory

Use the same LLVM workflow as libFuzzer

Troubleshooting

Issue
Cause
Solution

error: no profile data available
Profile wasn't generated or wrong path
Verify LLVM_PROFILE_FILE was set and .profraw file exists

Failed to load coverage
Mismatch between binary and profile data
Rebuild binary with same flags used during execution

Coverage reports show 0%
Wrong binary used for report generation
Use the instrumented binary, not the fuzzing binary

no_working_dir_found error (gcovr)
.gcda files in unexpected location
Add --gcov-ignore-errors=no_working_dir_found flag

Crashes prevent coverage generation
Corpus contains crashing inputs
Filter crashes or use forking approach to isolate failures

Coverage decreases after harness change
Harness now skips certain code paths
Review harness logic; may need to support more input formats

HTML report is flat file list
Using older LLVM version
Upgrade to LLVM 18+ and use -show-directory-coverage

incompatible instrumentation
Mixing LLVM and GCC coverage
Rebuild everything with same toolchain

Related Skills

Tools That Use This Technique

Skill
How It Applies

libfuzzer
Uses SanitizerCoverage for feedback; coverage analysis evaluates harness effectiveness

aflpp
Uses edge coverage for feedback; detailed analysis requires separate instrumentation

cargo-fuzz
Built-in cargo fuzz coverage command for Rust projects

honggfuzz
Uses edge coverage; analyze with standard LLVM/GCC tools

Related Techniques

Skill
Relationship

fuzz-harness-writing
Coverage reveals which code paths harness reaches; guides harness improvements

fuzzing-dictionaries
Coverage identifies magic value checks that need dictionary entries

corpus-management
Coverage analysis helps curate corpora by identifying redundant test cases

sanitizers
Coverage helps verify sanitizer-instrumented code is actually executed

Resources

Key External Resources

LLVM Source-Based Code Coverage
Comprehensive guide to LLVM's profile instrumentation, including advanced features like branch coverage, region coverage, and integration with existing build systems. Covers compiler flags, runtime behavior, and profile data formats.

llvm-cov Command Guide
Detailed CLI reference for llvm-cov commands including show, report, and export. Documents all filtering options, output formats, and integration with llvm-profdata.

gcovr Documentation
Complete guide to gcovr tool for generating coverage reports from gcov data. Covers HTML themes, filtering options, multi-directory projects, and CI/CD integration patterns.

SanitizerCoverage Documentation
Low-level documentation for LLVM's SanitizerCoverage instrumentation. Explains inline 8-bit counters, PC tables, and how fuzzers use coverage feedback for guidance.

On the Evaluation of Fuzzer Performance
Research paper examining limitations of coverage as a fuzzing performance metric. Argues for more nuanced evaluation methods beyond simple code coverage percentages.

Video Resources

Not applicable - coverage analysis is primarily a tooling and workflow topic best learned through documentation and hands-on practice.