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.

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Install skill "coverage-analysis" with this command: npx skills add trailofbits/skills/trailofbits-skills-coverage-analysis

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.

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