LibC: Implement Itanium C++ ABI for static variable guards

This is __cxa_guard_acquire, __cxa_guard_release, and __cxa_guard_abort.

We put these symbols in a 'fake' libstdc++ to trick gcc into thinking it
has libstdc++. These symbols are necessary for C++ programs and not C
programs, so, seems file. There's no way to tell gcc that, for example,
the standard lib it should use is libc++ or libc. So, this is what we
have for now.

When threaded code enters a block that is trying to call the constructor
for a block-scope static, the compiler will emit calls to these methods
to handle the "call_once" nature of block-scope statics.

The compiler creates a 64-bit guard variable, which it checks the first
byte of to determine if the variable should be intialized or not.

If the compiler-generated code reads that byte as a 0, it will call
__cxa_guard_acquire to try and be the thread to call the constructor for
the static variable. If the first byte is 1, it will assume that the
variable's constructor was called, and go on to access it.

__cxa_guard_acquire uses one of the 7 implementation defined bytes of
the guard variable as an atomic 8 bit variable. To control a state
machine that lets each entering thread know if they gained
'initialization rights', someone is working on the varaible, someone is
working on the varaible and there's at least one thread waiting for it
to be intialized, or if the variable was initialized and it's time to
access it. We only store a 1 to the byte the compiler looks at in
__cxa_guard_release, and use a futex to handle waiting.

5 files changed, 158 insertions, 5 deletions

diff --git a/Libraries/CMakeLists.txt b/Libraries/CMakeLists.txt
index 3954f99c43..9bce4e6bbd 100644
--- a/Libraries/CMakeLists.txt
+++ b/Libraries/CMakeLists.txt
@@ -2,6 +2,7 @@ add_subdirectory(LibAudio)
 add_subdirectory(LibC)
 add_subdirectory(LibCore)
 add_subdirectory(LibCrypto)
+add_subdirectory(LibCxx)
 add_subdirectory(LibDebug)
 add_subdirectory(LibDesktop)
 add_subdirectory(LibGemini)
diff --git a/Libraries/LibC/CMakeLists.txt b/Libraries/LibC/CMakeLists.txt
index d27a5b4719..d5e94d1b96 100644
--- a/Libraries/LibC/CMakeLists.txt
+++ b/Libraries/LibC/CMakeLists.txt
@@ -3,7 +3,7 @@ set(LIBC_SOURCES
     assert.cpp
     crt0.cpp
     ctype.cpp
-    cxxabi.cpp
+    cxaatexit.cpp
     dirent.cpp
     dlfcn.cpp
     fcntl.cpp
@@ -51,8 +51,6 @@ file(GLOB AK_SOURCES "../../AK/*.cpp")
 file(GLOB ELF_SOURCES "../LibELF/*.cpp")
 set(ELF_SOURCES ${ELF_SOURCES} ../LibELF/Arch/i386/plt_trampoline.S)
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DSERENITY_LIBC_BUILD")
-
 add_library(crt0 STATIC crt0.cpp)
 add_custom_command(
     TARGET crt0
@@ -62,4 +60,4 @@ add_custom_command(
 set(SOURCES ${LIBC_SOURCES} ${AK_SOURCES} ${ELF_SOURCES})
 serenity_libc(LibC c)
 target_link_libraries(LibC crt0)
-add_dependencies(LibC LibM)
+add_dependencies(LibC LibM LibCxx)
diff --git a/Libraries/LibC/cxxabi.cpp b/Libraries/LibC/cxaatexit.cpp
index 68c9e33f3e..1b59e1d32c 100644
--- a/Libraries/LibC/cxxabi.cpp
+++ b/Libraries/LibC/cxaatexit.cpp
@@ -25,7 +25,6 @@
  */
 
 #include <AK/Types.h>
-#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 
diff --git a/Libraries/LibCxx/CMakeLists.txt b/Libraries/LibCxx/CMakeLists.txt
new file mode 100644
index 0000000000..f9cec683bb
--- /dev/null
+++ b/Libraries/LibCxx/CMakeLists.txt
@@ -0,0 +1,7 @@
+set(SOURCES
+    CxaGuard.cpp
+)
+
+# Shhh don't worry gcc, we are totally libstdc++
+serenity_libc(LibCxx stdc++)
+target_link_libraries(LibCxx LibC)
diff --git a/Libraries/LibCxx/CxaGuard.cpp b/Libraries/LibCxx/CxaGuard.cpp
new file mode 100644
index 0000000000..dc8b011a43
--- /dev/null
+++ b/Libraries/LibCxx/CxaGuard.cpp
@@ -0,0 +1,148 @@
+/*
+ * Copyright (c) 2020, Andrew Kaster <andrewdkaster@gmail.com>
+ * 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 <serenity.h>
+#include <stdint.h>
+#include <sys/types.h>
+
+// Static variable guard logic, using futexes
+// Written while looking at libc++ from a distance ;) This is super tricky...
+// From itanium spec: Get passed a pointer to a 64 bit guard "object"
+// [ 1 guard byte (checked by compiler), 7 bytes to work with  ]
+class CxaGuard {
+public:
+    enum GuardState : uint8_t {
+        NotInitialized = 0,
+        DoneInit = 1,
+        InitInProgress = 2,
+        WaitingOnInit = 4
+    };
+
+    CxaGuard(uint64_t* guard_pointer)
+        : m_full_guard_var(guard_pointer)
+        , m_compiler_guard_byte((uint8_t*)guard_pointer)
+        , m_local_state_byte(&((uint8_t*)guard_pointer)[1])
+    {
+    }
+
+    int acquire()
+    {
+        // Someone else already initailzied this by storing to the 'really done' slot
+        if (GuardState::NotInitialized != m_compiler_guard_byte.load(AK::memory_order_acquire))
+            return 0;
+
+        // Time to begin the waiting game..
+        while (true) {
+            // Try to claim initialization for this thread from the initial state
+            uint8_t last_state = GuardState::NotInitialized;
+            if (m_local_state_byte.compare_exchange_strong(last_state, GuardState::InitInProgress, AK::memory_order_acq_rel))
+                return 1; // Tell compiler-generated code to init the variable
+
+            // Someone else set the state to done, we're done here
+            if (last_state == DoneInit)
+                return 0;
+
+            // Someone else set InitInProgress, time to wait (maybe)
+            if (last_state & GuardState::InitInProgress) {
+                // Try to set the in progress + done bits
+                if ((last_state & GuardState::WaitingOnInit) == 0) {
+                    if (!m_local_state_byte.compare_exchange_strong(last_state, (GuardState::InitInProgress | GuardState::WaitingOnInit), AK::memory_order_acq_rel)) {
+                        if (last_state == GuardState::DoneInit)
+                            return 0;
+                        if (last_state == GuardState::NotInitialized)
+                            continue; // start over, and try to initialize in this thread
+                        // else wait, someone else set the wait bit before us
+                    }
+                }
+                // State is InitInProgress | WaitingOnInit, time to sleep
+                wait_on_futex();
+            }
+        }
+    }
+
+    void release()
+    {
+        // Store the value the compiler will look at, we're done! yay
+        m_compiler_guard_byte.store(GuardState::DoneInit, AK::memory_order_release);
+
+        // Set the value other threads chilling in our loop will look at to done
+        uint8_t old_local_state = m_local_state_byte.exchange(GuardState::DoneInit, AK::memory_order_acq_rel);
+
+        // Wake up sleepy heads, the variable is initialized!
+        if (old_local_state & GuardState::WaitingOnInit)
+            wake_futex();
+    }
+
+    void abort()
+    {
+        uint8_t old_local_state = m_local_state_byte.exchange(GuardState::NotInitialized, AK::memory_order_acq_rel);
+
+        // Wake up sleepy heads, someone else gets to try...
+        if (old_local_state & GuardState::WaitingOnInit)
+            wake_futex();
+    }
+
+private:
+    void wait_on_futex()
+    {
+        int futex_value = (GuardState::InitInProgress | GuardState::WaitingOnInit) << 8;
+        futex((int32_t*)m_full_guard_var, FUTEX_WAIT, futex_value, nullptr);
+    }
+
+    void wake_futex()
+    {
+        futex((int32_t*)m_full_guard_var, FUTEX_WAKE, INT32_MAX, nullptr);
+    }
+
+    void* m_full_guard_var;
+    AtomicRef<uint8_t> m_compiler_guard_byte;
+    AtomicRef<uint8_t> m_local_state_byte; // we'll load and set this atomically
+
+    // Note that we're not using 6/8 bytes of the guard object...
+};
+
+extern "C" {
+
+int __cxa_guard_acquire(uint64_t* guard_object)
+{
+    CxaGuard g(guard_object);
+    return g.acquire();
+}
+
+void __cxa_guard_release(uint64_t* guard_object)
+{
+    CxaGuard g(guard_object);
+    g.release();
+}
+
+void __cxa_guard_abort(uint64_t* guard_object)
+{
+    CxaGuard g(guard_object);
+    g.abort();
+}
+
+} // extern "C"