sanitizers

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Purpose

Guide agents through choosing, enabling, and interpreting compiler runtime sanitizers for finding memory errors, undefined behaviour, data races, and memory leaks.

Triggers

"My program has a memory error — which sanitizer do I use?"
"How do I enable ASan?"
"How do I interpret an ASan/UBSan/TSan report?"
"ASan says heap-buffer-overflow — what does that mean?"
"How do I suppress false positives in sanitizers?"
"Can I use sanitizers in CI?"

Workflow

1. Decision tree: which sanitizer?

bash

Bug class?
├── Memory OOB, use-after-free, double-free → AddressSanitizer (ASan)
├── Stack OOB, global OOB → ASan (all three covered)
├── Uninitialised reads → MemorySanitizer (MSan, Clang only, requires all-clang build)
├── Undefined behaviour (int overflow, null deref, bad cast) → UBSan
├── Data races (multi-thread) → ThreadSanitizer (TSan)
├── Memory leaks only → LeakSanitizer (LSan, standalone or via ASan)
└── Multiple classes → ASan + UBSan (common combo); cannot combine with TSan or MSan

2. AddressSanitizer (ASan)

bash

# GCC or Clang
gcc -fsanitize=address -fno-omit-frame-pointer -g -O1 -o prog main.c
# Or
clang -fsanitize=address -fno-omit-frame-pointer -g -O1 -o prog main.c

Runtime options (via ASAN_OPTIONS):

bash

ASAN_OPTIONS=detect_leaks=1:abort_on_error=1:log_path=/tmp/asan.log ./prog

`ASAN_OPTIONS` key	Effect
`detect_leaks=0/1`	Enable LeakSanitizer (default 1 on Linux)
`abort_on_error=1`	Call `abort()` instead of `_exit()` (for core dumps)
`log_path=path`	Write report to file
`symbolize=1`	Symbolize addresses (needs `llvm-symbolizer` in PATH)
`fast_unwind_on_malloc=0`	More accurate stacks (slower)
`quarantine_size_mb=256`	Delay reuse of freed memory

Interpreting ASan output:

text

==12345==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000000050
READ of size 4 at 0x602000000050 thread T0
    #0 0x401234 in foo /home/user/src/main.c:15
    #1 0x401567 in main /home/user/src/main.c:42

0x602000000050 is located 0 bytes after a 40-byte region
[0x602000000028, 0x602000000050) allocated at:
    #0 0x7f12345 in malloc ...
    #1 0x401234 in main /home/user/src/main.c:10

Reading: the top frame in WRITE/READ is the access site; the allocated at stack shows the allocation. The region is 40 bytes at [start, end) and the access is at end = one byte past the end (classic off-by-one).

3. UndefinedBehaviorSanitizer (UBSan)

bash

gcc -fsanitize=undefined -g -O1 -o prog main.c
# More complete: add specific checks
gcc -fsanitize=undefined,integer -g -O1 -o prog main.c

Common UBSan checks:

signed-integer-overflow
unsigned-integer-overflow (not in undefined by default)
null — null pointer dereference
bounds — array index OOB (compile-time knowable bounds)
alignment — misaligned pointer access
float-cast-overflow — float-to-int conversion overflow
vptr — C++ vtable type mismatch
shift-exponent — shift >= bit width

bash

# Enable everything including integer overflow
gcc -fsanitize=undefined \
    -fsanitize=signed-integer-overflow,unsigned-integer-overflow,float-cast-overflow \
    -fno-sanitize-recover=all \   # abort instead of continue
    -g -O1 -o prog main.c

-fno-sanitize-recover=all: makes UBSan abort on first error (important for CI).

Interpreting UBSan output:

text

src/main.c:15:12: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

4. ThreadSanitizer (TSan)

bash

# Clang or GCC (GCC ≥ 4.8)
clang -fsanitize=thread -g -O1 -o prog main.c

# TSan is incompatible with ASan and MSan

Interpreting TSan output:

text

WARNING: ThreadSanitizer: data race (pid=12345)
  Write of size 4 at 0x7f... by thread T2:
    #0 increment /home/user/src/counter.c:8
  Previous read of size 4 at 0x7f... by thread T1:
    #0 read_counter /home/user/src/counter.c:3

5. MemorySanitizer (MSan)

MSan detects reads of uninitialised memory. Clang only. Requires all-instrumented build (no mixing of MSan and non-MSan objects).

bash

clang -fsanitize=memory -fno-omit-frame-pointer -g -O1 -o prog main.c
# With origin tracking (slower but shows where uninit value came from)
clang -fsanitize=memory -fsanitize-memory-track-origins=2 -g -O1 -o prog main.c

System libraries must be rebuilt with MSan or substituted with MSan-instrumented wrappers. Use msan-libs toolchain from LLVM.

6. ASan + UBSan combined

bash

gcc -fsanitize=address,undefined -fno-sanitize-recover=all \
    -fno-omit-frame-pointer -g -O1 -o prog main.c

Do not combine with TSan or MSan.

7. Suppressions

bash

# ASan suppression file
cat > asan.supp << 'EOF'
# Suppress leaks from OpenSSL init
leak:CRYPTO_malloc
EOF

LSAN_OPTIONS=suppressions=asan.supp ./prog

# UBSan suppression
cat > ubsan.supp << 'EOF'
signed-integer-overflow:third_party/fast_math.c
EOF
UBSAN_OPTIONS=suppressions=ubsan.supp:print_stacktrace=1 ./prog

8. CMake integration

cmake

option(SANITIZE "Enable sanitizers" OFF)
if(SANITIZE)
    set(san_flags -fsanitize=address,undefined -fno-sanitize-recover=all
                  -fno-omit-frame-pointer -g -O1)
    add_compile_options(${san_flags})
    add_link_options(${san_flags})
endif()

9. CI integration

yaml

# GitHub Actions example
- name: Build with ASan+UBSan
  run: |
    cmake -S . -B build -DSANITIZE=ON
    cmake --build build -j$(nproc)

- name: Run tests under sanitizers
  run: |
    ASAN_OPTIONS=abort_on_error=1:detect_leaks=1 \
    UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 \
    ctest --test-dir build -j$(nproc) --output-on-failure

10. HWASan (Hardware-Assisted AddressSanitizer)

Lower overhead than ASan on supported ARM64 hardware with TBI (Top Byte Ignore) or MTE.

bash

# Clang/LLVM HWASan (userspace)
clang -fsanitize=hwaddress -g -O1 -o app app.c

# Requires ARM64 with TBI (most Android/arm64 servers) or HWASan tagging support
# Cannot combine with ASan on same build

Sanitizer	Overhead	Platform
ASan	~2x	x86, arm64
HWASan	~1.2–1.5x	arm64 with TBI/MTE
MSan	~3x	LLVM only

11. MemTagSanitizer (ARM MTE)

Uses ARM Memory Tagging Extension hardware tags for heap/stack/memory safety.

bash

# Experimental — LLVM with MTE-capable hardware (arm64)
clang -fsanitize=memtag -g -O1 -o app app.c

# Kernel MTE (separate from userspace MemTagSanitizer)
# CONFIG_ARM64_MTE=y — hardware tagging in kernel allocator

12. GWP-ASan (production sampling)

Sampled guard-page ASan suitable for production (used in Android; upstream glibc integration is ongoing).

bash

# LLVM GWP-ASan (link-time, sampled allocations)
clang -fsanitize=gwp-asan -O2 -o app app.c

# Android: enabled in some system components for sampled crash detection
# glibc: experimental GWP-ASan allocator integration (check distro release notes)

Catches heap OOB/UAF probabilistically with near-zero steady-state overhead.

13. KASAN for kernel modules

bash

# Build test kernel with KASAN
# CONFIG_KASAN=y CONFIG_KASAN_INLINE=y or CONFIG_KASAN_OUTLINE=y

# Boot KASAN kernel in QEMU for module development
qemu-system-x86_64 -kernel bzImage -append "kasan=on" ...

# Load module — KASAN reports appear in dmesg
sudo insmod mymod.ko
dmesg | tail -30

Pair with skills/kernel/kernel-testing for KUnit tests under KASAN. See skills/security/kernel-security for KASAN report triage.

For a quick flag reference, see references/flags.md. For report interpretation examples, see references/reports.md.

Related skills

Use skills/profilers/valgrind for Memcheck when ASan is unavailable
Use skills/runtimes/fuzzing to auto-generate inputs that trigger sanitizer errors
Use skills/compilers/gcc or skills/compilers/clang for build flag context

sanitizers

Sanitizers

Purpose

Triggers

Workflow

1. Decision tree: which sanitizer?

2. AddressSanitizer (ASan)

3. UndefinedBehaviorSanitizer (UBSan)

4. ThreadSanitizer (TSan)

5. MemorySanitizer (MSan)

6. ASan + UBSan combined

7. Suppressions

8. CMake integration

9. CI integration

10. HWASan (Hardware-Assisted AddressSanitizer)

11. MemTagSanitizer (ARM MTE)

12. GWP-ASan (production sampling)

13. KASAN for kernel modules

Related skills

関連する Skills

clang

static-analysis

linux-perf

llvm