Handle host accesses for NCE Memory Trapping API

We cannot ignore accesses from the host to a region protected by the NCE Memory Trapping API, there's often access to regions which have overlap with a protected region unintentionally and those accesses need to be handled correctly rather than leading to a crash. This is done by implementing an additional signal handler `NCE::HostSignalHandler` to lookup any potential traps on a `SIGSEGV` and handle them correctly or when there isn't a corresponding trap raise a `SIGTRAP` when debugger is connected or delegate to `signal::ExceptionalSignalHandler` when it isn't.

app/src/main/cpp/skyline/nce.cpp

@@ -107,7 +107,17 @@ namespace skyline::nce {
 
             *tls = nullptr;
         } else { // If TLS wasn't restored then this occurred in host code
+            HostSignalHandler(signal, info, ctx);
+        }
+    }
+
+    static NCE* staticNce{nullptr}; //!< A static instance of NCE for use in the signal handler
+
+    void NCE::HostSignalHandler(int signal, siginfo *info, ucontext *ctx) {
         if (signal == SIGSEGV) {
+            if (staticNce && staticNce->TrapHandler(reinterpret_cast<u8 *>(info->si_addr), true))
+                return;
+
             bool runningUnderDebugger{[]() {
                 static std::ifstream status("/proc/self/status");
                 status.seekg(0);
@@ -134,8 +144,7 @@ namespace skyline::nce {
                 raise(SIGTRAP); // Notify the debugger if we've got a SIGSEGV as the debugger doesn't catch them by default as they might be hooked
         }
 
-            signal::ExceptionalSignalHandler(signal, info, ctx); //!< Delegate throwing a host exception to the exceptional signal handler
+        signal::ExceptionalSignalHandler(signal, info, ctx); // Delegate throwing a host exception to the exceptional signal handler
-        }
     }
 
     void *NceTlsRestorer() {
@@ -149,6 +158,7 @@ namespace skyline::nce {
 
     NCE::NCE(const DeviceState &state) : state(state) {
         signal::SetTlsRestorer(&NceTlsRestorer);
+        staticNce = this;
     }
 
     constexpr u8 MainSvcTrampolineSize{17}; // Size of the main SVC trampoline function in u32 units

app/src/main/cpp/skyline/nce.h

@@ -64,6 +64,15 @@ namespace skyline::nce {
          */
         static void SignalHandler(int signal, siginfo *info, ucontext *ctx, void **tls);
 
+        /**
+         * @brief Handles signals for any host threads which may access NCE trapped memory
+         * @note Any untrapped SIGSEGVs will emit SIGTRAP when a debugger is attached rather than throwing an exception
+         */
+        static void HostSignalHandler(int signal, siginfo *info, ucontext *ctx);
+
+        /**
+         * @note There should only be one instance of NCE concurrently
+         */
         NCE(const DeviceState &state);
 
         struct PatchData {

app/src/main/cpp/skyline/soc/gm20b/gpfifo.cpp

@@ -330,7 +330,8 @@ namespace skyline::soc::gm20b {
     void ChannelGpfifo::Run() {
         pthread_setname_np(pthread_self(), "GPFIFO");
         try {
-            signal::SetSignalHandler({SIGINT, SIGILL, SIGTRAP, SIGBUS, SIGFPE, SIGSEGV}, signal::ExceptionalSignalHandler);
+            signal::SetSignalHandler({SIGINT, SIGILL, SIGTRAP, SIGBUS, SIGFPE}, signal::ExceptionalSignalHandler);
+            signal::SetSignalHandler({SIGSEGV}, nce::NCE::HostSignalHandler); // We may access NCE trapped memory
 
             gpEntries.Process([this](GpEntry gpEntry) {
                 Logger::Debug("Processing pushbuffer: 0x{:X}, Size: 0x{:X}", gpEntry.Address(), +gpEntry.size);

app/src/main/cpp/skyline/soc/host1x/command_fifo.cpp

@@ -2,6 +2,7 @@
 // Copyright © 2021 Skyline Team and Contributors (https://github.com/skyline-emu/)
 
 #include <common/signal.h>
+#include <nce.h>
 #include <loader/loader.h>
 #include <kernel/types/KProcess.h>
 #include <soc.h>
@@ -115,7 +116,8 @@ namespace skyline::soc::host1x {
     void ChannelCommandFifo::Run() {
         pthread_setname_np(pthread_self(), "ChannelCommandFifo");
         try {
-            signal::SetSignalHandler({SIGINT, SIGILL, SIGTRAP, SIGBUS, SIGFPE, SIGSEGV}, signal::ExceptionalSignalHandler);
+            signal::SetSignalHandler({SIGINT, SIGILL, SIGTRAP, SIGBUS, SIGFPE}, signal::ExceptionalSignalHandler);
+            signal::SetSignalHandler({SIGSEGV}, nce::NCE::HostSignalHandler); // We may access NCE trapped memory
 
             gatherQueue.Process([this](span<u32> gather) {
                 Logger::Debug("Processing pushbuffer: 0x{:X}, size: 0x{:X}", gather.data(), gather.size());