LibPthread: Add magic bytes to the start of sem_t structures

This helps ensure random pointers are not passed in as semaphores, but
more importantly once named semaphores are implemented, this will
ensure that random files are not used as semaphores.

Userland/Libraries/LibC/semaphore.cpp

@@ -12,6 +12,8 @@
 #include <semaphore.h>
 #include <serenity.h>
 
+static constexpr u32 SEM_MAGIC = 0x78951230;
+
 // Whether sem_wait() or sem_post() is responsible for waking any sleeping
 // threads.
 static constexpr u32 POST_WAKES = 1 << 31;
@@ -50,19 +52,31 @@ int sem_init(sem_t* sem, int shared, unsigned int value)
         return -1;
     }
 
+    sem->magic = SEM_MAGIC;
     sem->value = value;
     return 0;
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_destroy.html
-int sem_destroy(sem_t*)
+int sem_destroy(sem_t* sem)
 {
+    if (sem->magic != SEM_MAGIC) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    sem->magic = 0;
     return 0;
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_getvalue.html
 int sem_getvalue(sem_t* sem, int* sval)
 {
+    if (sem->magic != SEM_MAGIC) {
+        errno = EINVAL;
+        return -1;
+    }
+
     u32 value = AK::atomic_load(&sem->value, AK::memory_order_relaxed);
     *sval = value & ~POST_WAKES;
     return 0;
@@ -71,6 +85,11 @@ int sem_getvalue(sem_t* sem, int* sval)
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_post.html
 int sem_post(sem_t* sem)
 {
+    if (sem->magic != SEM_MAGIC) {
+        errno = EINVAL;
+        return -1;
+    }
+
     u32 value = AK::atomic_fetch_add(&sem->value, 1u, AK::memory_order_release);
     // Fast path: no need to wake.
     if (!(value & POST_WAKES)) [[likely]]
@@ -91,6 +110,11 @@ int sem_post(sem_t* sem)
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_trywait.html
 int sem_trywait(sem_t* sem)
 {
+    if (sem->magic != SEM_MAGIC) {
+        errno = EINVAL;
+        return -1;
+    }
+
     u32 value = AK::atomic_load(&sem->value, AK::memory_order_relaxed);
     u32 count = value & ~POST_WAKES;
     if (count == 0) {
@@ -111,12 +135,22 @@ int sem_trywait(sem_t* sem)
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_wait.html
 int sem_wait(sem_t* sem)
 {
+    if (sem->magic != SEM_MAGIC) {
+        errno = EINVAL;
+        return -1;
+    }
+
     return sem_timedwait(sem, nullptr);
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_timedwait.html
 int sem_timedwait(sem_t* sem, const struct timespec* abstime)
 {
+    if (sem->magic != SEM_MAGIC) {
+        errno = EINVAL;
+        return -1;
+    }
+
     u32 value = AK::atomic_load(&sem->value, AK::memory_order_relaxed);
     bool responsible_for_waking = false;
 

Userland/Libraries/LibC/semaphore.h

@@ -14,6 +14,7 @@
 __BEGIN_DECLS
 
 typedef struct {
+    uint32_t magic;
     uint32_t value;
 } sem_t;