Kernel: Add safe atomic functions

This allows us to perform atomic operations on potentially unsafe
user space pointers.

Kernel/Arch/i386/SafeMem.h

@@ -0,0 +1,120 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Atomic.h>
+#include <AK/Optional.h>
+#include <AK/Types.h>
+
+#pragma once
+
+namespace Kernel {
+
+struct RegisterState;
+
+[[nodiscard]] bool safe_memcpy(void* dest_ptr, const void* src_ptr, size_t n, void*& fault_at);
+[[nodiscard]] ssize_t safe_strnlen(const char* str, size_t max_n, void*& fault_at);
+[[nodiscard]] bool safe_memset(void* dest_ptr, int c, size_t n, void*& fault_at);
+[[nodiscard]] Optional<u32> safe_atomic_fetch_add_relaxed(volatile u32* var, u32 val);
+[[nodiscard]] Optional<u32> safe_atomic_exchange_relaxed(volatile u32* var, u32 val);
+[[nodiscard]] Optional<u32> safe_atomic_load_relaxed(volatile u32* var);
+[[nodiscard]] bool safe_atomic_store_relaxed(volatile u32* var, u32 val);
+[[nodiscard]] Optional<bool> safe_atomic_compare_exchange_relaxed(volatile u32* var, u32& expected, u32 val);
+
+[[nodiscard]] ALWAYS_INLINE Optional<u32> safe_atomic_fetch_and_relaxed(volatile u32* var, u32 val)
+{
+    auto expected_value = safe_atomic_load_relaxed(var);
+    if (!expected_value.has_value())
+        return {}; // fault
+    u32& expected = expected_value.value();
+    for (;;) {
+        auto result = safe_atomic_compare_exchange_relaxed(var, expected, expected & val);
+        if (!result.has_value())
+            return {}; // fault
+        if (result.value())
+            return expected; // exchanged
+
+        // This is only so that we don't saturate the bus...
+        AK::atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);
+    }
+}
+
+[[nodiscard]] ALWAYS_INLINE Optional<u32> safe_atomic_fetch_and_not_relaxed(volatile u32* var, u32 val)
+{
+    auto expected_value = safe_atomic_load_relaxed(var);
+    if (!expected_value.has_value())
+        return {}; // fault
+    u32& expected = expected_value.value();
+    for (;;) {
+        auto result = safe_atomic_compare_exchange_relaxed(var, expected, expected & ~val);
+        if (!result.has_value())
+            return {}; // fault
+        if (result.value())
+            return expected; // exchanged
+
+        // This is only so that we don't saturate the bus...
+        AK::atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);
+    }
+}
+
+[[nodiscard]] ALWAYS_INLINE Optional<u32> safe_atomic_fetch_or_relaxed(volatile u32* var, u32 val)
+{
+    auto expected_value = safe_atomic_load_relaxed(var);
+    if (!expected_value.has_value())
+        return {}; // fault
+    u32& expected = expected_value.value();
+    for (;;) {
+        auto result = safe_atomic_compare_exchange_relaxed(var, expected, expected | val);
+        if (!result.has_value())
+            return {}; // fault
+        if (result.value())
+            return expected; // exchanged
+
+        // This is only so that we don't saturate the bus...
+        AK::atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);
+    }
+}
+
+[[nodiscard]] ALWAYS_INLINE Optional<u32> safe_atomic_fetch_xor_relaxed(volatile u32* var, u32 val)
+{
+    auto expected_value = safe_atomic_load_relaxed(var);
+    if (!expected_value.has_value())
+        return {}; // fault
+    u32& expected = expected_value.value();
+    for (;;) {
+        auto result = safe_atomic_compare_exchange_relaxed(var, expected, expected ^ val);
+        if (!result.has_value())
+            return {}; // fault
+        if (result.value())
+            return expected; // exchanged
+
+        // This is only so that we don't saturate the bus...
+        AK::atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);
+    }
+}
+
+bool handle_safe_access_fault(RegisterState& regs, u32 fault_address);
+
+}