From 2d9f1781c6cdec8d74f44f98624af2eacbd21810 Mon Sep 17 00:00:00 2001
From: Vito Caputo <vcaputo@gnugeneration.com>
Date: Mon, 10 Jun 2013 17:28:49 -0700
Subject: [PATCH 1/1] sched: implement adopt() system call
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@gnugeneration.com>
---
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