hakmem/docs/status/P0_BUG_STATUS.md

P0 SEGV Bug - Current Status & Next Steps

Last Update: 2025-11-12

🐛 Bug Summary

Symptom: SEGV crash at iterations 28,440 and 38,985 (deterministic with seed 42) Pattern: Corrupted address 0x7fff00008000 in TLS SLL chain Root Cause: STALE NEXT POINTERS in carved chains

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🎁 Box Theory Implementation (完了済み)

✅ Box 3 (Pointer Conversion Box)

• File: core/box/ptr_conversion_box.h (267 lines)
• 役割: BASE ↔ USER pointer conversion
• API:
  • ptr_base_to_user(base, class_idx) - C0-C6: base+1, C7: base
  • ptr_user_to_base(user, class_idx) - C0-C6: user-1, C7: user
• Status: ✅ Committed (1713 lines added total)

✅ Box E (Expansion Box)

• File: core/box/superslab_expansion_box.h/c
• 役割: SuperSlab expansion with TLS state guarantee
• 機能: expansion_expand_with_tls_guarantee() - Expand後に slab 0 を即座にバインド
• Status: ✅ Committed

✅ Box I (Integrity Box) - 703 lines!

• File: core/box/integrity_box.h (267行) + integrity_box.c (436行)
• 役割: Comprehensive integrity verification system
• Priority ALPHA: 5つの Slab Metadata 不変条件チェック
  1. carved <= capacity
  2. used <= carved
  3. used <= capacity
  4. free_count == (carved - used)
  5. capacity <= 512
• 機能:
  • integrity_validate_slab_metadata() - メタデータ検証
  • validate_ptr_range() - ポインタ範囲検証（null-page, kernel-space, 0xa2/0xcc/0xdd/0xfe パターン）
• Status: ✅ Committed

✅ Box TLS-SLL (今回の修正対象)

• File: core/box/tls_sll_box.h
• 役割: TLS Single-Linked List management (C7-safe)
• API:
  • tls_sll_push() - Push to SLL (C7 rejected)
  • tls_sll_pop() - Pop from SLL (returns base pointer)
  • tls_sll_splice() - Batch push
• 今回の発見:
  • Fix #1: tls_sll_pop で next をクリア（C0-C6 は base+1 で）
  • But: carved chain の tail が NULL 終端されていない（Fix #2 必要）
• Status: ⚠️ Fix #1 適用済み、Fix #2 未適用

✅ その他のBox (既存)

• Front Gate Box: core/box/front_gate_box.h/c + front_gate_classifier.c
• Free Local/Remote/Publish Box: core/box/free_local_box.c, free_remote_box.c, free_publish_box.c
• Mailbox Box: core/box/mailbox_box.h/c

Commit Info:

• Commit: "Add Box I (Integrity), Box E (Expansion)..."
• Files: 23 files changed, 1713 insertions(+), 56 deletions(-)
• Date: Recent (before P0 debug session)

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🔍 Investigation History

✅ Completed Investigations

1. Valgrind (O0 build): 0 errors, 29K iterations passed

   • Conclusion: Bug is optimization-dependent (-O3 triggers it)

2. Task Agent GDB Analysis:

   • Found crash location: tls_sll_pop line 169
   • Hypothesis: use-after-allocate (next pointer at base+1 is user memory)

3. Box I, E, 3 Implementation: 703 lines of integrity checks

   • All checks passed before crash
   • Validation didn't catch the bug

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🛠️ Fixes Applied (Partial Success)

Fix #1: Clear next pointer in tls_sll_pop ✅ (INCOMPLETE)

File: core/box/tls_sll_box.h:254-262

Change:

// OLD (WRONG): Only cleared for C7
if (__builtin_expect(class_idx == 7, 0)) {
    *(void**)base = NULL;
}

// NEW: Clear for C0-C6 too
#if HAKMEM_TINY_HEADER_CLASSIDX
    if (class_idx == 7) {
        *(void**)base = NULL;  // C7: clear at base (offset 0)
    } else {
        *(void**)((uint8_t*)base + 1) = NULL;  // C0-C6: clear at base+1 (offset 1)
    }
#else
    *(void**)base = NULL;
#endif

Result:

• ✅ Passed 29K iterations (previous crash point)
• ❌ Still crashes at 38,985 iterations

---

🚨 NEW DISCOVERY: Root Cause Found!

Fix #2: NULL-terminate carved chain tail (NOT YET APPLIED)

File: core/tiny_refill_opt.h:229-234

BUG: Tail block's next pointer is NOT NULL-terminated!

// Current code (BUGGY):
for (uint32_t i = 1; i < batch; i++) {
    uint8_t* next = cursor + stride;
    *(void**)(cursor + next_offset) = (void*)next;  // Links blocks 0→1, 1→2, ...
    cursor = next;
}
void* tail = (void*)cursor;  // tail = last block
// ❌ BUG: tail's next pointer is NEVER set to NULL!
// It contains GARBAGE from previous allocation!

IMPACT:

1. Chain is carved: head → block1 → block2 → ... → tail → [GARBAGE]
2. Chain spliced to TLS SLL
3. Later, tls_sll_pop traverses the chain
4. Reads garbage next pointer → SEGV at 0x7fff00008000

FIX (add after line 233):

for (uint32_t i = 1; i < batch; i++) {
    uint8_t* next = cursor + stride;
    *(void**)(cursor + next_offset) = (void*)next;
    cursor = next;
}
void* tail = (void*)cursor;

// ✅ FIX: NULL-terminate the tail
*(void**)((uint8_t*)tail + next_offset) = NULL;

---

🚨 CURRENT STATUS (2025-11-12 UPDATED)

Fixes Applied:

1. ✅ Fix #1: Clear next pointer in tls_sll_pop (C0-C6 at base+1)
2. ✅ Fix #2: NULL-terminate tail in trc_linear_carve()
3. ✅ Fix #3: Clean rebuild with HEADER_CLASSIDX=1
4. ✅ Fix #4: Increase canary check frequency (1000 → 100 ops)
5. ✅ Fix #5: Add bounds check to tls_sll_push()

Test Results:

• ❌ Still crashes at iteration 28,410 (call 14269)
• Canaries: NOT corrupted (corruption is immediate)
• Bounds check: NOT triggered (class_idx is valid)
• Task agent finding: External corruption of g_tls_sll_head[0]

Analysis:

• Fix #1 and Fix #2 ARE working correctly (Task agent verified)
• Corruption happens IMMEDIATELY before crash (canaries at 100-op interval miss it)
• class_idx is valid [0-7] when corruption happens (bounds check doesn't trigger)
• Crash is deterministic at call 14269

📋 Next Steps (NEEDS USER INPUT)

Option A: Deep GDB Investigation (SLOW)

• Set hardware watchpoint on g_tls_sll_head[0]
• Run to call 14250, then watch for corruption
• Time: 1-2 hours, may not work with optimization

Option B: Disable Optimizations (DIAGNOSTIC)

• Rebuild with -O0 to see if bug disappears
• If so, likely compiler optimization bug or UB
• Time: 10 minutes

Option C: Simplified Stress Test (QUICK)

• Disable P0 batch optimization temporarily
• Disable SFC temporarily
• Test with simpler code path
• Time: 20 minutes

After Fix Verified

4. Commit P0 fix:

   • Fix #1: Clear next in tls_sll_pop
   • Fix #2: NULL-terminate in trc_linear_carve
   • Box I/E/3 validation infrastructure
   • Double-free detection

5. Update CLAUDE.md with findings

6. Performance benchmark (release build)

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🎯 Expected Outcome

After applying Fix #2, the allocator should:

• ✅ Pass 100K iterations without crash
• ✅ Pass 1M iterations without crash
• ✅ Maintain performance (~2.7M ops/s for 256B)

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📝 Lessons Learned

1. Stale pointers are dangerous: Always NULL-terminate linked lists
2. Optimization exposes bugs: -O3 can hide initialization in debug builds
3. Multiple fixes needed: Fix #1 alone was insufficient
4. Chain integrity: Carved chains MUST be properly terminated

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🔧 Build Flags (CRITICAL)

MUST use these flags:

HEADER_CLASSIDX=1
AGGRESSIVE_INLINE=1
PREWARM_TLS=1

Why: HAKMEM_TINY_HEADER_CLASSIDX=1 is required for Fix #1 to execute!

Use build.sh to ensure correct flags:

./build.sh bench_random_mixed_hakmem