|
| 1 | +# FreeBSD Segmentation Fault Analysis |
| 2 | + |
| 3 | +## Problem Description |
| 4 | +The test `MutexTest.TryLockExceptionSafety` crashes with a segmentation fault on FreeBSD 15 with GCC 13.3. |
| 5 | + |
| 6 | +### Error Message |
| 7 | +``` |
| 8 | +pid 2326 (test-ipc), jid 0, uid 0: exited on signal 11 (core dumped) |
| 9 | +core-test-ipc.pid-2326: handling rb error 14 |
| 10 | +Segmentation fault (core dumped) |
| 11 | +``` |
| 12 | + |
| 13 | +Key observations: |
| 14 | +- Signal 11 = SIGSEGV (Segmentation Fault) |
| 15 | +- `rb error 14` = likely "robust list error 14", where 14 = `EFAULT` (Bad address) |
| 16 | +- This suggests a problem with robust mutex list management in FreeBSD's libthr |
| 17 | + |
| 18 | +## Hypotheses |
| 19 | + |
| 20 | +### Hypothesis 1: Invalid Pointer in `pthread_mutex_timedlock` |
| 21 | +The `mutex_` pointer passed to `pthread_mutex_timedlock()` might be invalid or point to freed memory. |
| 22 | + |
| 23 | +**Why this could happen:** |
| 24 | +- The shared memory region might not be properly initialized |
| 25 | +- The `shm_->get()` might return an invalid pointer |
| 26 | +- There could be a race condition in `acquire_mutex()` |
| 27 | + |
| 28 | +### Hypothesis 2: Robust Mutex List Corruption |
| 29 | +FreeBSD's robust mutex implementation maintains a per-thread list of robust mutexes. The error `rb error 14 (EFAULT)` suggests that this list might be corrupted. |
| 30 | + |
| 31 | +**Why this could happen:** |
| 32 | +- After our previous fix removing `shm_->sub_ref()`, the robust list might not be properly maintained |
| 33 | +- There might be a dangling pointer in the robust list |
| 34 | +- The mutex might have been destroyed while still in the robust list |
| 35 | + |
| 36 | +### Hypothesis 3: `valid()` Check Failure |
| 37 | +The `valid()` method uses `std::memcmp()` to compare the mutex against a zero-initialized mutex. On FreeBSD, if the mutex contains internal pointers, this comparison might access invalid memory. |
| 38 | + |
| 39 | +### Hypothesis 4: Shared Memory Mapping Issue |
| 40 | +The shared memory region might not be properly mapped in all processes/threads, causing the mutex pointer to be invalid in some contexts. |
| 41 | + |
| 42 | +## Debugging Strategy |
| 43 | + |
| 44 | +### Step 1: Add Diagnostic Output |
| 45 | +Modify `try_lock()` to add detailed logging: |
| 46 | + |
| 47 | +```cpp |
| 48 | +bool try_lock() noexcept(false) { |
| 49 | + ipc::error("[DEBUG] try_lock() called\n"); |
| 50 | + ipc::error("[DEBUG] valid() = %d, shm_ = %p, ref_ = %p, mutex_ = %p\n", |
| 51 | + valid(), shm_, ref_, mutex_); |
| 52 | + |
| 53 | + if (!valid()) { |
| 54 | + ipc::error("[DEBUG] try_lock() returning false (not valid)\n"); |
| 55 | + return false; |
| 56 | + } |
| 57 | + |
| 58 | + ipc::error("[DEBUG] Calling make_timespec(0)\n"); |
| 59 | + auto ts = posix_::detail::make_timespec(0); |
| 60 | + ipc::error("[DEBUG] ts.tv_sec = %ld, ts.tv_nsec = %ld\n", ts.tv_sec, ts.tv_nsec); |
| 61 | + |
| 62 | + ipc::error("[DEBUG] Calling pthread_mutex_timedlock()\n"); |
| 63 | + int eno = ::pthread_mutex_timedlock(mutex_, &ts); |
| 64 | + ipc::error("[DEBUG] pthread_mutex_timedlock returned %d\n", eno); |
| 65 | + |
| 66 | + // ... rest of the function |
| 67 | +} |
| 68 | +``` |
| 69 | +
|
| 70 | +### Step 2: Verify Mutex Initialization |
| 71 | +Check if the mutex is properly initialized before use: |
| 72 | +
|
| 73 | +```cpp |
| 74 | +bool open(char const *name) noexcept { |
| 75 | + close(); |
| 76 | + if ((mutex_ = acquire_mutex(name)) == nullptr) { |
| 77 | + ipc::error("[DEBUG] acquire_mutex returned nullptr\n"); |
| 78 | + return false; |
| 79 | + } |
| 80 | + ipc::error("[DEBUG] acquire_mutex returned %p\n", mutex_); |
| 81 | + ipc::error("[DEBUG] shm_ = %p, ref_ = %p\n", shm_, ref_); |
| 82 | + |
| 83 | + auto self_ref = ref_->fetch_add(1, std::memory_order_relaxed); |
| 84 | + ipc::error("[DEBUG] self_ref = %d, shm_->ref() = %d\n", self_ref, shm_->ref()); |
| 85 | + |
| 86 | + if (shm_->ref() > 1 || self_ref > 0) { |
| 87 | + ipc::error("[DEBUG] Mutex already initialized, returning\n"); |
| 88 | + return valid(); |
| 89 | + } |
| 90 | + |
| 91 | + // ... rest of initialization |
| 92 | +} |
| 93 | +``` |
| 94 | + |
| 95 | +### Step 3: Check for Robust List Issues |
| 96 | +The `rb error 14` suggests that FreeBSD's robust mutex list might be corrupted. This could be caused by: |
| 97 | + |
| 98 | +1. **Improper destruction**: A robust mutex should not be destroyed while it's still locked or while it's in a thread's robust list. |
| 99 | +2. **Dangling pointers**: After destroying a mutex, its address might still be in the robust list. |
| 100 | + |
| 101 | +**Potential fix**: Ensure that mutexes are unlocked before being destroyed: |
| 102 | + |
| 103 | +```cpp |
| 104 | +~mutex() { |
| 105 | + // Ensure the mutex is unlocked before destruction |
| 106 | + if (valid()) { |
| 107 | + try { |
| 108 | + unlock(); // Try to unlock if we hold the lock |
| 109 | + } catch (...) { |
| 110 | + // Ignore errors during destruction |
| 111 | + } |
| 112 | + } |
| 113 | + close(); |
| 114 | +} |
| 115 | +``` |
| 116 | + |
| 117 | +### Step 4: Test with Non-Robust Mutex |
| 118 | +To isolate whether the problem is related to robust mutexes specifically, we could temporarily disable the `PTHREAD_MUTEX_ROBUST` attribute: |
| 119 | + |
| 120 | +```cpp |
| 121 | +// In open() method, comment out: |
| 122 | +// if ((eno = ::pthread_mutexattr_setrobust(&mutex_attr, PTHREAD_MUTEX_ROBUST)) != 0) { |
| 123 | +// ipc::error("fail pthread_mutexattr_setrobust[%d]\n", eno); |
| 124 | +// return false; |
| 125 | +// } |
| 126 | +``` |
| 127 | + |
| 128 | +If the crash disappears without robust mutexes, it confirms that the issue is in robust mutex handling. |
| 129 | + |
| 130 | +## Recommended Fix |
| 131 | + |
| 132 | +Based on the `rb error 14 (EFAULT)` message, I suspect the problem is that FreeBSD's robust mutex list contains a dangling pointer to a destroyed mutex. |
| 133 | + |
| 134 | +**Root cause**: When a mutex object is destroyed (in the destructor or `close()`), it calls `pthread_mutex_destroy()`, but if the current thread has this mutex in its robust list (even if it's not locked), FreeBSD might try to access it later and find an invalid pointer. |
| 135 | + |
| 136 | +**Proposed solution**: |
| 137 | + |
| 138 | +1. **Add explicit unlock before destruction**: |
| 139 | + ```cpp |
| 140 | + ~mutex() noexcept { |
| 141 | + // Try to unlock if we might hold the lock |
| 142 | + if (valid()) { |
| 143 | + try { |
| 144 | + ::pthread_mutex_unlock(mutex_); // Ignore errors |
| 145 | + } catch (...) {} |
| 146 | + } |
| 147 | + close(); |
| 148 | + } |
| 149 | + ``` |
| 150 | +
|
| 151 | +2. **Ensure proper cleanup order in close()**: |
| 152 | + ```cpp |
| 153 | + void close() noexcept { |
| 154 | + if ((ref_ != nullptr) && (shm_ != nullptr) && (mutex_ != nullptr)) { |
| 155 | + // Try to unlock before destroying |
| 156 | + ::pthread_mutex_unlock(mutex_); // Ignore errors |
| 157 | + |
| 158 | + if (shm_->name() != nullptr) { |
| 159 | + release_mutex(shm_->name(), [this] { |
| 160 | + auto self_ref = ref_->fetch_sub(1, std::memory_order_relaxed); |
| 161 | + if ((shm_->ref() <= 1) && (self_ref <= 1)) { |
| 162 | + int eno; |
| 163 | + if ((eno = ::pthread_mutex_destroy(mutex_)) != 0) { |
| 164 | + ipc::error("fail pthread_mutex_destroy[%d]\n", eno); |
| 165 | + } |
| 166 | + return true; |
| 167 | + } |
| 168 | + return false; |
| 169 | + }); |
| 170 | + } else shm_->release(); |
| 171 | + } |
| 172 | + shm_ = nullptr; |
| 173 | + ref_ = nullptr; |
| 174 | + mutex_ = nullptr; |
| 175 | + } |
| 176 | + ``` |
| 177 | + |
| 178 | +3. **Alternatively, track lock state explicitly**: |
| 179 | + Add a member variable to track whether we hold the lock: |
| 180 | + ```cpp |
| 181 | + std::atomic<bool> is_locked_{false}; |
| 182 | + |
| 183 | + // In lock(): |
| 184 | + if (result) is_locked_.store(true); |
| 185 | + |
| 186 | + // In unlock(): |
| 187 | + if (result) is_locked_.store(false); |
| 188 | + |
| 189 | + // In destructor/close(): |
| 190 | + if (is_locked_.load()) { |
| 191 | + ::pthread_mutex_unlock(mutex_); |
| 192 | + } |
| 193 | + ``` |
| 194 | +
|
| 195 | +## Next Steps |
| 196 | +
|
| 197 | +1. Add diagnostic output to understand where exactly the crash occurs |
| 198 | +2. Implement one of the proposed fixes |
| 199 | +3. Test on FreeBSD 15 to verify the fix |
| 200 | +4. Ensure the fix doesn't break Linux/QNX compatibility |
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