[PATCH 1/1] sched: implement adopt() system call

[Vito Caputo]

	This implements a proposed facility for adopting a process
	by another.  The immediate use case is programs like GNU
	screen which lose their parent-child relationship when
	detached.  Using this sytem call on reattach, the relation
	can be restored, which is particularly of use to those
	taking advantage of the /proc/$pid/task/$pid/children list
	provided by CONFIG_CHECKPOINT_RESTORE.

	This implementation applies permission checks similar to
	that of kill(), in addition to preventing the adoption of
	an ancestor process.

	I have tested it and use the change myself to complement
	process subtree monitoring, without which I lose all
	visibility of the descendants of my screen sessions on
	reattach.

Signed-off-by: Vito Caputo <vcaputo@xxxxxxxxxxxxxxxxx>
---
 arch/x86/syscalls/syscall_32.tbl |    1 +
 arch/x86/syscalls/syscall_64.tbl |    1 +
 include/linux/syscalls.h         |    2 ++
 kernel/exit.c                    |   59 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 63 insertions(+)

diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
index aabfb83..d219781 100644
--- a/arch/x86/syscalls/syscall_32.tbl
+++ b/arch/x86/syscalls/syscall_32.tbl
@@ -357,3 +357,4 @@
 348	i386	process_vm_writev	sys_process_vm_writev		compat_sys_process_vm_writev
 349	i386	kcmp			sys_kcmp
 350	i386	finit_module		sys_finit_module
+351	i386	adopt			sys_adopt
diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
index 38ae65d..6345ebf 100644
--- a/arch/x86/syscalls/syscall_64.tbl
+++ b/arch/x86/syscalls/syscall_64.tbl
@@ -320,6 +320,7 @@
 311	64	process_vm_writev	sys_process_vm_writev
 312	common	kcmp			sys_kcmp
 313	common	finit_module		sys_finit_module
+314	common	adopt			sys_adopt
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 4147d70..3997cde 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -841,4 +841,6 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
 asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
 			 unsigned long idx1, unsigned long idx2);
 asmlinkage long sys_finit_module(int fd, const char __user *uargs, int flags);
+
+asmlinkage long sys_adopt(pid_t upid);
 #endif
diff --git a/kernel/exit.c b/kernel/exit.c
index af2eb3c..5b554c8 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1681,3 +1681,62 @@ SYSCALL_DEFINE3(waitpid, pid_t, pid, int __user *, stat_addr, int, options)
 }
 
 #endif
+
+/*
+ * sys_adopt() adopts the process specified as upid by the current
+ * process if permitted.  This is linux-specific and provided so
+ * programs such as GNU screen may restore the parent-child
+ * relationship lost in detaching when reattaching.
+ */
+SYSCALL_DEFINE1(adopt, pid_t, upid)
+{
+	long ret = -ENOENT;
+	struct pid *pid;
+
+	if ((pid = find_get_pid(upid))) {
+		struct task_struct	*p;
+
+		rcu_read_lock();
+		write_lock_irq(&tasklist_lock);
+		p = pid_task(pid, PIDTYPE_PID);
+		if (p) {
+			struct task_struct *t;
+			const struct cred *cred = current_cred();
+			const struct cred *tcred = __task_cred(p);
+
+			if (!uid_eq(cred->euid, tcred->suid) &&
+			    !uid_eq(cred->euid, tcred->uid)  &&
+			    !uid_eq(cred->uid,  tcred->suid) &&
+			    !uid_eq(cred->uid,  tcred->uid) &&
+			    !ns_capable(cred->user_ns, CAP_KILL)) {
+				ret = -EPERM;
+				goto out_unlock;
+			}
+
+
+			/* upid cannot be current nor an ancestor of current */
+			for (t = current;; t = t->real_parent) {
+				if (t == p) {
+					ret = -EINVAL;
+					goto out_unlock;
+				}
+				if (is_global_init(t))
+					break;
+			}
+
+			t = p;
+			do {
+				t->real_parent = current;
+			} while_each_thread(p, t);
+
+			list_move_tail(&p->sibling, &p->real_parent->children);
+			ret = 0;
+		} /* else { ret = -ENOENT } */
+out_unlock:
+		write_unlock_irq(&tasklist_lock);
+		rcu_read_unlock();
+		put_pid(pid);
+	} /* else { ret = -ENOENT } */
+
+	return ret;
+}
-- 
1.7.10.4


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