Call Graph Reachability Analysis

Call Graph Reachability Analysis

Provides highest-confidence vulnerability assessment by proving whether vulnerable functions can actually be reached during program execution.

When to Use This Skill

Use this skill when:

• You need definitive proof that a vulnerable function is or isn't reachable
• Medium/high confidence analysis shows possible vulnerability but needs confirmation
• Generating evidence for security compliance or audit requirements
• govulncheck is unavailable or didn't find the CVE
• You need visual proof of execution paths for stakeholders

Prerequisites

Required Tools

• callgraph: go install golang.org/x/tools/cmd/callgraph@latest
• digraph: go install golang.org/x/tools/cmd/digraph@latest
• Go workspace with go.mod file

Optional Tools

• graphviz (for visualization): brew install graphviz (macOS) or sudo apt-get install graphviz (Linux)
• sfdp or dot command (part of graphviz)

Input Requirements

• CVE vulnerable function signature (e.g., <package-path>.<function-name>)
• Package path from CVE analysis
• Workspace path to analyze
• Algorithm preference (optional, default: vta) — passed via --algo from parent command
• CVE_ID — used to construct the output directory
• OUT_DIR (optional, default: .work/compliance/analyze-cve/${CVE_ID}) — where artifacts are written

Timeout and Algorithm Convention

• Use the algorithm specified by the user via --algo (default: vta).
• All callgraph invocations use timeout 300 (5 minutes) to prevent hanging on large codebases.
• If the chosen algorithm times out: fall back to the next faster algorithm (vta → rta → cha), then narrow scope to specific packages (e.g., ./cmd/..., ./pkg/...).

Implementation Steps

Step 1: Verify Tools Are Available

# Check for callgraph
which callgraph || echo "callgraph not found - install with: go install golang.org/x/tools/cmd/callgraph@latest"

# Check for digraph
which digraph || echo "digraph not found - install with: go install golang.org/x/tools/cmd/digraph@latest"

# Optional: Check for graphviz
which sfdp || echo "graphviz not found - visual graphs won't be generated (optional)"

Decision Point:

• IF callgraph OR digraph missing → Exit this skill, return to parent analysis
• IF both present → Continue

Step 2: Build Complete Call Graph

# ALGO defaults to "vta" unless user specified --algo
ALGO="${USER_ALGO:-vta}"

# Output directory — aligns with parent command report location
OUT_DIR="${OUT_DIR:-.work/compliance/analyze-cve/${CVE_ID}}"
mkdir -p "${OUT_DIR}"

# Build call graph from workspace root
# Timeout after 300s (5 minutes) to avoid hanging on large codebases
timeout 300 callgraph -algo "${ALGO}" -format=digraph . > "${OUT_DIR}/callgraph.txt"

Error Handling:

• IF build fails (compilation errors) → Note in report that call graph cannot be built
• IF command times out → Fall back to next faster algorithm (vta → rta → cha) and retry
• IF all algorithms time out → Narrow scope: timeout 300 callgraph -algo rta -format=digraph ./cmd/... ./pkg/... > "${OUT_DIR}/callgraph.txt"
• IF successful → Continue to Step 3

Output: ${OUT_DIR}/callgraph.txt containing the full program call graph

Step 3: Check if Vulnerable Function Exists in Graph

Extract the vulnerable function signature from CVE details.

# Search for exact function in the cached call graph
VULN_FUNC="<package-path>.<vulnerable-function>"
cat "${OUT_DIR}/callgraph.txt" | digraph nodes | grep "${VULN_FUNC}$"

Decision Point:

• IF function found → Continue to Step 4
• IF function NOT found → Report as LOW RISK → Recommend manual review → Exit skill

Step 4: Find Execution Paths from Entry Points

Search for paths from main entry points to the vulnerable function.

# Find path from main() to vulnerable function using cached call graph
ENTRY_POINT="command-line-arguments.main"
VULN_FUNC="<package-path>.<vulnerable-function>"

cat "${OUT_DIR}/callgraph.txt" | \
  digraph somepath "${ENTRY_POINT}" "${VULN_FUNC}"

Alternative Entry Points to Check:

• command-line-arguments.main (main program)
• Test entry points: *_test.go test functions
• Init functions: *.init
• HTTP handlers if it's a web service

Interpretation:

• IF path found → Vulnerable function IS reachable → HIGH RISK
• IF no path found → Check alternative entry points
• IF still no path → Function may be in unreachable code → MEDIUM RISK

Output: Text representation of call chain or empty result

Step 5: Generate DOT Graph for Visualization

If path exists, generate visual representation:

# Generate DOT format from cached call graph
cat "${OUT_DIR}/callgraph.txt" | \
  digraph somepath "${ENTRY_POINT}" "${VULN_FUNC}" | \
  digraph to dot > "${OUT_DIR}/callgraph.dot"

# Convert to SVG (if graphviz available)
if which sfdp > /dev/null; then
  sfdp -Tsvg -o"${OUT_DIR}/callgraph.svg" -Goverlap=scale "${OUT_DIR}/callgraph.dot"
  echo "Visual graph saved to: ${OUT_DIR}/callgraph.svg"
else
  echo "Graphviz not available - DOT file saved to: ${OUT_DIR}/callgraph.dot"
fi

Output Files:

• ${OUT_DIR}/callgraph.dot - DOT notation of call path
• ${OUT_DIR}/callgraph.svg - Visual graph (if graphviz available)

Step 6: Parse and Format Call Chain

Extract human-readable call chain from digraph output:

# Get call chain as text from cached call graph
cat "${OUT_DIR}/callgraph.txt" | \
  digraph somepath "${ENTRY_POINT}" "${VULN_FUNC}" | \
  digraph to dot | \
  grep " -> " | \
  sed 's/"//g' | \
  sed 's/;//g'

Example Output:

command-line-arguments.main -> <package-path>.Handler
<package-path>.Handler -> <package-path>.ProcessFunction
<package-path>.ProcessFunction -> <vulnerable-package>.<vulnerable-function>

Format for Report:

Execution Path Found:
main → Handler → ProcessFunction → <vulnerable-function> (VULNERABLE)

Step 7: Assess Risk Level

HIGH RISK:

• Reachable path from main() to vulnerable function
• Action: Proceed to remediation

MEDIUM RISK:

• Function in graph but no direct path from main()
• Action: Recommend manual review + remediation

LOW RISK:

• Function not found in call graph
• Action: Recommend manual review to confirm

Return Value

Return structured result to parent analysis:

{
  "method": "call-graph-reachability",
  "algorithm": "vta",
  "vulnerable_function": "<package-path>.<vulnerable-function>",
  "found_in_graph": true,
  "reachable_from_main": true,
  "call_chain": "main → Handler → ProcessFunction → <vulnerable-function>",
  "risk_level": "HIGH",
  "evidence": {
    "callgraph_file": "${OUT_DIR}/callgraph.txt",
    "dot_file": "${OUT_DIR}/callgraph.dot",
    "svg_file": "${OUT_DIR}/callgraph.svg"
  }
}

Error Handling

Build Failures

• IF project doesn't compile → Note in report, cannot perform call graph analysis
• Suggest: Fix compilation errors first

Very Large Codebases

• IF chosen algorithm times out (>5 minutes) → Fall back to next faster algorithm (vta → rta → cha)
• IF all algorithms time out → Narrow scope: timeout 300 callgraph -algo rta -format=digraph ./cmd/... ./pkg/... > "${OUT_DIR}/callgraph.txt"

Missing Entry Points

• IF command-line-arguments.main not found → Look for other entry points
• Web services: Check HTTP handler registrations
• Libraries: Note that call graph analysis may not be applicable

Tool Installation Issues

• IF tools cannot be installed → Fall back to lower confidence methods
• Document limitation in final report

Example: Generic Analysis Workflow

# Setup: set CVE_ID and output directory
$ CVE_ID="CVE-YYYY-NNNNN"
$ OUT_DIR=".work/compliance/analyze-cve/${CVE_ID}"
$ mkdir -p "${OUT_DIR}"

# Step 1: Build call graph (default: vta; user can override with --algo)
$ timeout 300 callgraph -algo vta -format=digraph . > "${OUT_DIR}/callgraph.txt"

# Step 2: Check if function is called
$ cat "${OUT_DIR}/callgraph.txt" | digraph nodes | grep "<package-path>.<vulnerable-function>$"
<package-path>.<vulnerable-function>

# Step 3: Find path from main
$ cat "${OUT_DIR}/callgraph.txt" | digraph somepath command-line-arguments.main <package-path>.<vulnerable-function>
digraph {
    "command-line-arguments.main" -> "<app-package>.Handler";
    "<app-package>.Handler" -> "<app-package>.ProcessFunction";
    "<app-package>.ProcessFunction" -> "<intermediate-package>.HelperFunction";
    "<intermediate-package>.HelperFunction" -> "<vulnerable-package>.<vulnerable-function>";
}

# Step 4: Generate visual graph
$ cat "${OUT_DIR}/callgraph.txt" | digraph somepath command-line-arguments.main <package-path>.<vulnerable-function> | digraph to dot | sfdp -Tsvg -o"${OUT_DIR}/callgraph.svg"

# Result: HIGH RISK — reachable path found
# Call chain: main → Handler → ProcessFunction → HelperFunction → <vulnerable-function>

Integration with Parent Command

This skill is called from Method 4 of the codebase-impact-analysis skill.

When to Invoke:

• After basic dependency checks show package is present
• When highest confidence assessment is needed
• When tools are available (checked in Phase 0)

Return to Parent:

• Provide risk level (HIGH/MEDIUM/LOW)
• Include evidence (call chain, graph files)
• Update report with reachability findings