Kernel+LibC: Implement sigtimedwait()

This includes a new Thread::Blocker called SignalBlocker which blocks
until a signal of a matching type is pending. The current Blocker
implementation in the Kernel is very complicated, but cleaning it up is
a different yak for a different day.

Author: IdanHo

Kernel/API/Syscall.h

@@ -176,6 +176,7 @@ enum class NeedsBigProcessLock {
     S(sigpending, NeedsBigProcessLock::Yes)                 \
     S(sigprocmask, NeedsBigProcessLock::Yes)                \
     S(sigreturn, NeedsBigProcessLock::Yes)                  \
+    S(sigtimedwait, NeedsBigProcessLock::Yes)               \
     S(socket, NeedsBigProcessLock::Yes)                     \
     S(socketpair, NeedsBigProcessLock::Yes)                 \
     S(stat, NeedsBigProcessLock::Yes)                       \

Kernel/Process.h

@@ -338,6 +338,7 @@ class Process final
     ErrorOr<FlatPtr> sys$sigaltstack(Userspace<const stack_t*> ss, Userspace<stack_t*> old_ss);
     ErrorOr<FlatPtr> sys$sigprocmask(int how, Userspace<const sigset_t*> set, Userspace<sigset_t*> old_set);
     ErrorOr<FlatPtr> sys$sigpending(Userspace<sigset_t*>);
+    ErrorOr<FlatPtr> sys$sigtimedwait(Userspace<sigset_t const*>, Userspace<siginfo_t*>, Userspace<const timespec*>);
     ErrorOr<FlatPtr> sys$getgroups(size_t, Userspace<gid_t*>);
     ErrorOr<FlatPtr> sys$setgroups(size_t, Userspace<const gid_t*>);
     ErrorOr<FlatPtr> sys$pipe(int pipefd[2], int flags);

Kernel/Syscalls/sigaction.cpp

@@ -307,4 +307,35 @@ ErrorOr<FlatPtr> Process::sys$sigaltstack(Userspace<const stack_t*> ss, Userspac
     return 0;
 }
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigtimedwait.html
+ErrorOr<FlatPtr> Process::sys$sigtimedwait(Userspace<const sigset_t*> set, Userspace<siginfo_t*> info, Userspace<const timespec*> timeout)
+{
+    VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this)
+    REQUIRE_PROMISE(sigaction);
+
+    sigset_t set_value;
+    TRY(copy_from_user(&set_value, set));
+
+    Thread::BlockTimeout block_timeout = {};
+    if (timeout) {
+        auto timeout_time = TRY(copy_time_from_user(timeout));
+        block_timeout = Thread::BlockTimeout(false, &timeout_time);
+    }
+
+    siginfo_t info_value = {};
+    auto block_result = Thread::current()->block<Thread::SignalBlocker>(block_timeout, set_value, info_value);
+    if (block_result.was_interrupted())
+        return EINTR;
+    // We check for an unset signal instead of directly checking for a timeout interruption
+    // in order to allow polling the pending signals by setting the timeout to 0.
+    if (info_value.si_signo == SIGINVAL) {
+        VERIFY(block_result == Thread::BlockResult::InterruptedByTimeout);
+        return EAGAIN;
+    }
+
+    if (info)
+        TRY(copy_to_user(info, &info_value));
+    return info_value.si_signo;
+}
+
 }

Kernel/Thread.cpp

@@ -637,6 +637,7 @@ void Thread::send_signal(u8 signal, [[maybe_unused]] Process* sender)
 
     m_pending_signals |= 1 << (signal - 1);
     m_have_any_unmasked_pending_signals.store((pending_signals_for_state() & ~m_signal_mask) != 0, AK::memory_order_release);
+    m_signal_blocker_set.unblock_all_blockers_whose_conditions_are_met();
 
     if (!has_unmasked_pending_signals())
         return;

Kernel/Thread.h

@@ -292,6 +292,7 @@ class Thread
             Queue,
             Routing,
             Sleep,
+            Signal,
             Wait
         };
         virtual ~Blocker();
@@ -698,6 +699,41 @@ class Thread
         bool m_did_unblock { false };
     };
 
+    class SignalBlocker final : public Blocker {
+    public:
+        explicit SignalBlocker(sigset_t pending_set, siginfo_t& result);
+        virtual StringView state_string() const override { return "Pending Signal"sv; }
+        virtual Type blocker_type() const override { return Type::Signal; }
+        void will_unblock_immediately_without_blocking(UnblockImmediatelyReason) override;
+        virtual bool setup_blocker() override;
+        bool check_pending_signals(bool from_add_blocker);
+
+    private:
+        sigset_t m_pending_set { 0 };
+        siginfo_t& m_result;
+        bool m_did_unblock { false };
+    };
+
+    class SignalBlockerSet final : public BlockerSet {
+    public:
+        void unblock_all_blockers_whose_conditions_are_met()
+        {
+            BlockerSet::unblock_all_blockers_whose_conditions_are_met([&](auto& b, void*, bool&) {
+                VERIFY(b.blocker_type() == Blocker::Type::Signal);
+                auto& blocker = static_cast<Thread::SignalBlocker&>(b);
+                return blocker.check_pending_signals(false);
+            });
+        }
+
+    private:
+        bool should_add_blocker(Blocker& b, void*) override
+        {
+            VERIFY(b.blocker_type() == Blocker::Type::Signal);
+            auto& blocker = static_cast<Thread::SignalBlocker&>(b);
+            return !blocker.check_pending_signals(true);
+        }
+    };
+
     class WaitBlocker final : public Blocker {
     public:
         enum class UnblockFlags {
@@ -1302,6 +1338,7 @@ class Thread
     u32 m_signal_mask { 0 };
     FlatPtr m_alternative_signal_stack { 0 };
     FlatPtr m_alternative_signal_stack_size { 0 };
+    SignalBlockerSet m_signal_blocker_set;
     FlatPtr m_kernel_stack_base { 0 };
     FlatPtr m_kernel_stack_top { 0 };
     OwnPtr<Memory::Region> m_kernel_stack_region;

Kernel/ThreadBlockers.cpp

@@ -446,6 +446,48 @@ void Thread::SelectBlocker::was_unblocked(bool did_timeout)
     }
 }
 
+Thread::SignalBlocker::SignalBlocker(sigset_t pending_set, siginfo_t& result)
+    : m_pending_set(pending_set)
+    , m_result(result)
+{
+}
+
+void Thread::SignalBlocker::will_unblock_immediately_without_blocking(UnblockImmediatelyReason unblock_immediately_reason)
+{
+    if (unblock_immediately_reason != UnblockImmediatelyReason::TimeoutInThePast)
+        return;
+    // If the specified timeout is 0 the caller is simply trying to poll once for pending signals,
+    // so simply calling check_pending_signals should populate the requested information.
+    check_pending_signals(false);
+}
+
+bool Thread::SignalBlocker::setup_blocker()
+{
+    return add_to_blocker_set(thread().m_signal_blocker_set);
+}
+
+bool Thread::SignalBlocker::check_pending_signals(bool from_add_blocker)
+{
+    {
+        SpinlockLocker lock(m_lock);
+        if (m_did_unblock)
+            return false;
+
+        auto matching_pending_signal = __builtin_ffsl(thread().pending_signals() & m_pending_set);
+        if (matching_pending_signal == 0)
+            return false;
+
+        m_did_unblock = true;
+        m_result = {};
+        m_result.si_signo = matching_pending_signal;
+        m_result.si_code = 0; // FIXME: How can we determine this?
+    }
+
+    if (!from_add_blocker)
+        unblock_from_blocker();
+    return true;
+}
+
 Thread::WaitBlockerSet::ProcessBlockInfo::ProcessBlockInfo(NonnullRefPtr<Process>&& process, WaitBlocker::UnblockFlags flags, u8 signal)
     : process(move(process))
     , flags(flags)

Userland/Libraries/LibC/signal.cpp

@@ -163,6 +163,12 @@ int sigsuspend(const sigset_t* set)
     return pselect(0, nullptr, nullptr, nullptr, nullptr, set);
 }
 
+int sigtimedwait(sigset_t const* set, siginfo_t* info, struct timespec const* timeout)
+{
+    int rc = syscall(Syscall::SC_sigtimedwait, set, info, timeout);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 const char* sys_signame[] = {
     "INVAL",
     "HUP",

Userland/Libraries/LibC/signal.h

@@ -27,6 +27,7 @@ int sigismember(const sigset_t*, int sig);
 int sigprocmask(int how, const sigset_t* set, sigset_t* old_set);
 int sigpending(sigset_t*);
 int sigsuspend(const sigset_t*);
+int sigtimedwait(sigset_t const*, siginfo_t*, struct timespec const*);
 int raise(int sig);
 int getsignalbyname(const char*);
 const char* getsignalname(int);