Kernel+LibC: Add sys$disown() for disowning child processes

This syscall allows a parent process to disown a child process, setting
its parent PID to 0.

Unparented processes are automatically reaped by the kernel upon exit,
and no sys$waitid() is required. This will make it much nicer to do
spawn-and-forget which is common in the GUI environment.

Base/usr/share/man/man2/disown.md

@@ -0,0 +1,25 @@
+## Name
+
+disown - disown a child process
+
+## Synopsis
+
+```**c++
+#include <serenity.h>
+
+int disown(pid_t pid);
+```
+
+## Description
+
+`disown()` unparents a child process of the calling process. The child's parent PID is set to zero, which allows the kernel to automatically reap it upon death.
+
+## Pledge
+
+In pledged programs, the `proc` promise is required for this system call.
+
+## Errors
+
+* `ESRCH`: No process with PID `pid` was found.
+* `ECHILD`: The target process is not a child of the calling process.
+

Kernel/API/Syscall.h

@@ -194,7 +194,8 @@ namespace Kernel {
     S(sendfd)                 \
     S(recvfd)                 \
     S(sysconf)                \
-    S(set_process_name)
+    S(set_process_name)       \
+    S(disown)
 
 namespace Syscall {
 

Kernel/CMakeLists.txt

@@ -96,6 +96,7 @@ set(KERNEL_SOURCES
     Syscalls/chroot.cpp
     Syscalls/clock.cpp
     Syscalls/debug.cpp
+    Syscalls/disown.cpp
     Syscalls/dup.cpp
     Syscalls/execve.cpp
     Syscalls/exit.cpp

Kernel/Process.h

@@ -333,6 +333,7 @@ public:
     int sys$sendfd(int sockfd, int fd);
     int sys$recvfd(int sockfd);
     long sys$sysconf(int name);
+    int sys$disown(pid_t);
 
     template<bool sockname, typename Params>
     int get_sock_or_peer_name(const Params&);

Kernel/Syscalls/disown.cpp

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 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$disown(pid_t pid)
+{
+    REQUIRE_PROMISE(proc);
+    auto process = Process::from_pid(pid);
+    if (!process)
+        return -ESRCH;
+    if (process->ppid() != this->pid())
+        return -ECHILD;
+    process->m_ppid = 0;
+    return 0;
+}
+}

Libraries/LibC/serenity.cpp

@@ -30,6 +30,12 @@
 
 extern "C" {
 
+int disown(pid_t pid)
+{
+    int rc = syscall(SC_disown, pid);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 int module_load(const char* path, size_t path_length)
 {
     int rc = syscall(SC_module_load, path, path_length);

Libraries/LibC/serenity.h

@@ -31,6 +31,8 @@
 
 __BEGIN_DECLS
 
+int disown(pid_t);
+
 int shbuf_create(int, void** buffer);
 int shbuf_allow_pid(int, pid_t peer_pid);
 int shbuf_allow_all(int);