rebuild-harness

name	rebuild-harness
description	How to rebuild the harness with source modifications using libCRS apply-patch-build

Rebuild Harness

Rebuild the harness after modifying source code (e.g., adding debug logs, instrumentation, or testing a hypothesis). Uses the builder sidecar to compile inside the target environment.

When to Use

• Adding printf/fprintf(stderr, ...) to trace execution paths
• Adding assertions to test hypotheses about vulnerable code
• Modifying the harness to reach different code paths
• Instrumenting code to understand input parsing

Workflow

# 1. Edit source files in {source_dir}
#    (the source is a git repo — use git diff to generate patches)

# 2. Generate a patch
cd {source_dir}
git add -A
git diff --cached > /tmp/debug.diff

# 3. Build with the patch applied
libCRS apply-patch-build /tmp/debug.diff /tmp/build_001

# 4. Check build result
cat /tmp/build_001/retcode
# 0 = success, non-zero = build failed

# If build failed, inspect logs:
cat /tmp/build_001/stderr.log
cat /tmp/build_001/stdout.log

# 5. Get the rebuild ID (only exists if build succeeded)
cat /tmp/build_001/rebuild_id

# 6. Run POV against the new build
libCRS run-pov /tmp/candidate.bin /tmp/run_debug \
  --harness {harness} --rebuild-id $(cat /tmp/build_001/rebuild_id)

# 7. Check output (your debug prints will appear here)
cat /tmp/run_debug/stdout.log
cat /tmp/run_debug/stderr.log

Example: Adding Debug Logging

# Add a debug print to trace which branch is taken
cd {source_dir}
# Edit the file...

git add -A
git diff --cached > /tmp/debug_trace.diff

# Build and test
libCRS apply-patch-build /tmp/debug_trace.diff /tmp/build_debug
REBUILD_ID=$(cat /tmp/build_debug/rebuild_id)

libCRS run-pov /tmp/candidate.bin /tmp/run_debug \
  --harness {harness} --rebuild-id $REBUILD_ID

# See your debug output
cat /tmp/run_debug/stderr.log

Notes

• Rebuild IDs are content-addressed: same patch → same rebuild ID (cached).
• Failed builds are NOT cached — you can fix and retry.
• Always reset source after debugging: git checkout -- .
• For final POV verification, omit --rebuild-id (runs against the original vulnerable build), not your debug build.
• Builds can be slow (recompiles the full project). Review your diff before building.