Hello all,
I'd like to propose a new system call, I know I'll probably be flamed,
but here goes.
Today there is no way to get a process which has been orphaned, and thus
become a child of init, back as a child of another process. This is
annoying me, please allow me to explain why.
The CONFIG_CHECKPOINT_RESTORE feature enables a useful little thing in
proc which I've been taking advantage of to make some interesting
omnipresent desktop monitoring features built into my own composited
window manager. That thing is the /proc/$pid/task/$pid/children file.
Using this file, I've created scoped per-window process monitors which
can efficiently limit their monitoring to only the client pid of the X
window and its decendants. These monitors are always running, but
visibility is toggled when desired by a keypress, efficiency is
important and the children file helps tremendously in this regard.
So I'm thrilled with all this and everything is fantastically efficient
and simple, until the first time I detach from my screen session and
reattach.
Initially, I'll have a scenario like:
xterm
bash
screen -S stuff
screen -S stuff
bash
bash
top
bash
vi foo.c
bash
make
sh -c cc -Wall ...
cc1 -quiet -I ...
bash
Behind these rows of text I have user/cpu graphs for each process
sliding by.
After detaching from screen, then reattaching, this is what the above
turns into, as you can probably predict:
xterm
bash
screen -dr stuff
All subsequent processes created through my interactions with this
screen are lost to the monitoring, since the "screen -S stuff" backend
is now stuck being a child of init.
With the attached adopt() syscall patch, and a 1 line change to screen's
attacher function invoking adopt(), I get the following on reattach:
xterm
bash
screen -dr stuff
screen -S stuff
bash
bash
top
bash
vi foo.c
bash
make
sh -c cc -Wall ...
cc1 -quiet -I ...
Which makes me very happy, dance around the room happy, and I'm not a
dancer.
Here is a screenshot of the results enabled by the patch:
http://pengaru.com/~swivel/vwm/screen_sys_adopt.png
Thanks for reading, and hopefuly considering this addition, I'm sure
there are other uses as well. Please CC me in any replies as I'm not
currently subscribed.
Regards,
Vito Caputo
>From 2d9f1781c6cdec8d74f44f98624af2eacbd21810 Mon Sep 17 00:00:00 2001
From: Vito Caputo <vcap...@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 <vcap...@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
