skyline/app/src/main/cpp/skyline/common/signal.cpp

// SPDX-License-Identifier: MPL-2.0
// Copyright © 2020 Skyline Team and Contributors (https://github.com/skyline-emu/)

#include <unistd.h>
#include <dlfcn.h>
#include <unwind.h>
#include "signal.h"

namespace skyline::signal {
    thread_local std::exception_ptr SignalExceptionPtr;

    void ExceptionThrow() {
        std::rethrow_exception(SignalExceptionPtr);
    }

    std::terminate_handler terminateHandler{};

    void TerminateHandler() {
        auto exception{std::current_exception()};
        if (terminateHandler && exception && exception == SignalExceptionPtr) {
            struct StackFrame {
                StackFrame *next;
                void *lr;
            } *frame;

            asm("MOV %0, FP" : "=r"(frame));
            frame = frame->next->next;

            if (_Unwind_FindEnclosingFunction(frame->lr) == &ExceptionThrow) // We're in a loop, skip past a frame
                frame = frame->next->next;
            else if (_Unwind_FindEnclosingFunction(frame->next->next->next->next->next->lr) == &ExceptionThrow) // We're in a deeper loop, just terminate
                frame = frame->next->next->next->next->next->next->next;

            asm("MOV SP, %x0\n\t" // Stack frame is the first item on a function's stack, it's used to calculate calling function's stack pointer
                "MOV LR, %x1\n\t"
                "MOV FP, %x2\n\t" // The stack frame of the calling function should be set
                "BR %x3"
            : : "r"(frame + 1), "r"(frame->lr), "r"(frame->next), "r"(&ExceptionThrow));

            __builtin_unreachable();
        } else {
            terminateHandler();
        }
    }

    void ExceptionalSignalHandler(int signal, siginfo *info, ucontext *context) {
        SignalException signalException;
        signalException.signal = signal;
        signalException.pc = context->uc_mcontext.pc;
        if (signal == SIGSEGV)
            signalException.faultAddress = info->si_addr;
        SignalExceptionPtr = std::make_exception_ptr(signalException);
        context->uc_mcontext.pc = reinterpret_cast<u64>(&ExceptionThrow);

        auto handler{std::get_terminate()};
        if (handler != TerminateHandler) {
            terminateHandler = handler;
            std::set_terminate(TerminateHandler);
        }
    }

    template<typename Signature>
    Signature GetLibcFunction(const char *symbol) {
        void *libc{dlopen("libc.so", RTLD_LOCAL | RTLD_LAZY)};
        if (!libc)
            throw exception("dlopen-ing libc has failed with: {}", dlerror());
        auto function{reinterpret_cast<Signature>(dlsym(libc, symbol))};
        if (!function)
            throw exception("Cannot find '{}' in libc: {}", symbol, dlerror());
        return function;
    }

    void Sigaction(int signal, const struct sigaction *action, struct sigaction *oldAction) {
        static decltype(&sigaction) real{};
        if (!real)
            real = GetLibcFunction<decltype(&sigaction)>("sigaction");
        if (real(signal, action, oldAction) < 0)
            throw exception("sigaction has failed with {}", strerror(errno));
    }

    static void *(*TlsRestorer)(){};

    void SetTlsRestorer(void *(*function)()) {
        TlsRestorer = function;
    }

    struct DefaultSignalHandler {
        void (*function)(int, struct siginfo *, void *){};

        ~DefaultSignalHandler();
    };

    std::array<DefaultSignalHandler, NSIG> DefaultSignalHandlers;

    DefaultSignalHandler::~DefaultSignalHandler() {
        if (function) {
            int signal{static_cast<int>(this - DefaultSignalHandlers.data())};

            struct sigaction oldAction;
            Sigaction(signal, nullptr, &oldAction);

            struct sigaction action{
                .sa_sigaction = function,
                .sa_flags = oldAction.sa_flags,
            };
            Sigaction(signal, &action);
        }
    }

    thread_local std::array<SignalHandler, NSIG> ThreadSignalHandlers{};

    __attribute__((no_stack_protector)) // Stack protector stores data in TLS at the function epilog and verifies it at the prolog, we cannot allow writes to guest TLS and may switch to an alternative TLS during the signal handler and have disabled the stack protector as a result
    void ThreadSignalHandler(int signal, siginfo *info, ucontext *context) {
        void *tls{}; // The TLS value prior to being restored if it is
        if (TlsRestorer)
            tls = TlsRestorer();

        auto handler{ThreadSignalHandlers.at(signal)};
        if (handler) {
            handler(signal, info, context, &tls);
        } else {
            auto defaultHandler{DefaultSignalHandlers.at(signal).function};
            if (defaultHandler)
                defaultHandler(signal, info, context);
        }

        if (tls)
            asm volatile("MSR TPIDR_EL0, %x0"::"r"(tls));
    }

    void SetSignalHandler(std::initializer_list<int> signals, SignalHandler function) {
        static std::array<std::once_flag, NSIG> signalHandlerOnce{};

        stack_t stack;
        sigaltstack(nullptr, &stack);
        struct sigaction action{
            .sa_sigaction = reinterpret_cast<void (*)(int, siginfo *, void *)>(ThreadSignalHandler),
            .sa_flags = SA_RESTART | SA_SIGINFO | (stack.ss_sp && stack.ss_size ? SA_ONSTACK : 0),
        };

        for (int signal : signals) {
            std::call_once(signalHandlerOnce[signal], [signal, action]() {
                struct sigaction oldAction;
                Sigaction(signal, &action, &oldAction);
                if (oldAction.sa_flags && oldAction.sa_flags != action.sa_flags)
                    throw exception("Old sigaction flags aren't equivalent to the replaced signal: {:#b} | {:#b}", oldAction.sa_flags, action.sa_flags);

                DefaultSignalHandlers.at(signal).function = (oldAction.sa_flags & SA_SIGINFO) ? oldAction.sa_sigaction : reinterpret_cast<void (*)(int, struct siginfo *, void *)>(oldAction.sa_handler);
            });
            ThreadSignalHandlers.at(signal) = function;
        }
    }

    void Sigprocmask(int how, const sigset_t &set, sigset_t *oldSet) {
        static decltype(&pthread_sigmask) real{};
        if (!real)
            real = GetLibcFunction<decltype(&sigprocmask)>("sigprocmask");
        if (real(how, &set, oldSet) < 0)
            throw exception("sigprocmask has failed with {}", strerror(errno));
    }
}