Kernel: Move syscall implementations out of Process.cpp

This is something I've been meaning to do for a long time, and here we
finally go. This patch moves all sys$foo functions out of Process.cpp
and into files in Kernel/Syscalls/.

It's not exactly one syscall per file (although it could be, but I got
a bit tired of the repetitive work here..)

This makes hacking on individual syscalls a lot less painful since you
don't have to rebuild nearly as much code every time. I'm also hopeful
that this makes it easier to understand individual syscalls. :^)

Kernel/Syscalls/sigaction.cpp

@@ -0,0 +1,120 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * 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 <Kernel/Process.h>
+
+namespace Kernel {
+
+int Process::sys$sigprocmask(int how, const sigset_t* set, sigset_t* old_set)
+{
+    REQUIRE_PROMISE(sigaction);
+    auto current_thread = Thread::current();
+    if (old_set) {
+        if (!validate_write_typed(old_set))
+            return -EFAULT;
+        copy_to_user(old_set, &current_thread->m_signal_mask);
+    }
+    if (set) {
+        if (!validate_read_typed(set))
+            return -EFAULT;
+        sigset_t set_value;
+        copy_from_user(&set_value, set);
+        switch (how) {
+        case SIG_BLOCK:
+            current_thread->m_signal_mask &= ~set_value;
+            break;
+        case SIG_UNBLOCK:
+            current_thread->m_signal_mask |= set_value;
+            break;
+        case SIG_SETMASK:
+            current_thread->m_signal_mask = set_value;
+            break;
+        default:
+            return -EINVAL;
+        }
+    }
+    return 0;
+}
+
+int Process::sys$sigpending(sigset_t* set)
+{
+    REQUIRE_PROMISE(stdio);
+    if (!validate_write_typed(set))
+        return -EFAULT;
+    copy_to_user(set, &Thread::current()->m_pending_signals);
+    return 0;
+}
+
+int Process::sys$sigaction(int signum, const sigaction* act, sigaction* old_act)
+{
+    REQUIRE_PROMISE(sigaction);
+    if (signum < 1 || signum >= 32 || signum == SIGKILL || signum == SIGSTOP)
+        return -EINVAL;
+    if (!validate_read_typed(act))
+        return -EFAULT;
+    InterruptDisabler disabler; // FIXME: This should use a narrower lock. Maybe a way to ignore signals temporarily?
+    auto& action = Thread::current()->m_signal_action_data[signum];
+    if (old_act) {
+        if (!validate_write_typed(old_act))
+            return -EFAULT;
+        copy_to_user(&old_act->sa_flags, &action.flags);
+        copy_to_user(&old_act->sa_sigaction, &action.handler_or_sigaction, sizeof(action.handler_or_sigaction));
+    }
+    copy_from_user(&action.flags, &act->sa_flags);
+    copy_from_user(&action.handler_or_sigaction, &act->sa_sigaction, sizeof(action.handler_or_sigaction));
+    return 0;
+}
+
+int Process::sys$sigreturn(RegisterState& registers)
+{
+    REQUIRE_PROMISE(stdio);
+    SmapDisabler disabler;
+
+    //Here, we restore the state pushed by dispatch signal and asm_signal_trampoline.
+    u32* stack_ptr = (u32*)registers.userspace_esp;
+    u32 smuggled_eax = *stack_ptr;
+
+    //pop the stored eax, ebp, return address, handler and signal code
+    stack_ptr += 5;
+
+    Thread::current()->m_signal_mask = *stack_ptr;
+    stack_ptr++;
+
+    //pop edi, esi, ebp, esp, ebx, edx, ecx and eax
+    memcpy(&registers.edi, stack_ptr, 8 * sizeof(FlatPtr));
+    stack_ptr += 8;
+
+    registers.eip = *stack_ptr;
+    stack_ptr++;
+
+    registers.eflags = *stack_ptr;
+    stack_ptr++;
+
+    registers.userspace_esp = registers.esp;
+    return smuggled_eax;
+}
+
+}