[Devel] [PATCH rh7]: ve: Provide interface for current tty inheritance

[Cyrill Gorcunov]

When fork() is called the current controlling terminal is inherited
by a child process. But in criu we fork all process first and then
restore their files, thus if terminal is opened in some children
its reference get lost. We refuse to checkpoint such configurations
at the moment in criu itself.

So to be able to restore this kind of container we need a way to
propagate controlling terminal to children processes, and here
is an interface "ve.ctty" entry on toplevel ve cgroup. One have
to pass pid of the donor task in first position followed by a series
of recipient pids.

https://jira.sw.ru/browse/PSBM-76490

CC: Andrey Vagin <avagin at virtuozzo.com>
CC: Andrey Ryabinin <aryabinin at virtuozzo.com>
CC: Konstantin Khorenko <khorenko at virtuozzo.com>
CC: "Denis V. Lunev" <den at virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov at virtuozzo.com>
---
Guys, take a look please. Note the patch on its own is useless
because CRIU support needed as well. I already have a working
version of patches for CRIU, but they are ~300 lines of code
so I'm trying to shrink them because it's too much.

 kernel/ve/ve.c |   96 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)

Index: linux-pcs7.git/kernel/ve/ve.c
===================================================================
--- linux-pcs7.git.orig/kernel/ve/ve.c
+++ linux-pcs7.git/kernel/ve/ve.c
@@ -37,6 +37,7 @@
 #include <linux/fs_struct.h>
 #include <linux/task_work.h>
 #include <linux/ctype.h>
+#include <linux/tty.h>
 
 #include <uapi/linux/vzcalluser.h>
 #include <linux/vziptable_defs.h>
@@ -1214,6 +1215,7 @@ enum {
 	VE_CF_NETNS_NR,
 	VE_CF_NETIF_MAX_NR,
 	VE_CF_NETIF_NR,
+	VE_CF_CTTY,
 };
 
 static int ve_ts_read(struct cgroup *cg, struct cftype *cft, struct seq_file *m)
@@ -1393,6 +1395,94 @@ static int ve_write_running_u64(struct c
 	return _ve_write_u64(cg, cft, value, 1);
 }
 
+static int ve_write_ctty(struct cgroup *cg, struct cftype *cft, const char *buffer)
+{
+	struct task_struct *tsk_from, *tsk_to;
+	struct tty_struct *tty_from, *tty_to;
+	pid_t pid_from, pid_to;
+	unsigned long flags;
+	char *pids;
+	int ret;
+
+	/*
+	 * Buffer format is the following
+	 *
+	 * 	pid_from pid_to pid_to ...
+	 *
+	 * where pid_to are pids to propagate
+	 * current terminal into.
+	 */
+
+	pids = skip_spaces(buffer);
+	if (sscanf(pids, "%d", &pid_from) != 1)
+		return -EINVAL;
+	pids = strchr(pids, ' ');
+	if (!pids)
+		return -EINVAL;
+	pids = skip_spaces(pids);
+
+	rcu_read_lock();
+	tsk_from = find_task_by_vpid(pid_from);
+	if (tsk_from)
+		get_task_struct(tsk_from);
+	rcu_read_unlock();
+
+	if (!tsk_from)
+		return -ESRCH;
+
+	spin_lock_irqsave(&tsk_from->sighand->siglock, flags);
+	tty_from = tty_kref_get(tsk_from->signal->tty);
+	spin_unlock_irqrestore(&tsk_from->sighand->siglock, flags);
+
+	if (!tty_from) {
+		ret = -ENOTTY;
+		goto out;
+	}
+
+	ret = 0;
+	while (pids && *pids) {
+		if (sscanf(pids, "%d", &pid_to) != 1) {
+			ret = -EINVAL;
+			goto out;
+		}
+		pids = strchr(pids, ' ');
+		if (pids)
+			pids = skip_spaces(pids);
+
+		rcu_read_lock();
+		tsk_to = find_task_by_vpid(pid_to);
+		if (tsk_to)
+			get_task_struct(tsk_to);
+		rcu_read_unlock();
+
+		if (!tsk_to) {
+			ret = -ESRCH;
+			goto out;
+		}
+
+		if (tsk_from->task_ve == tsk_to->task_ve) {
+			spin_lock_irqsave(&tsk_to->sighand->siglock, flags);
+			tty_to = tsk_to->signal->tty;
+			if (!tty_to)
+				tsk_to->signal->tty = tty_kref_get(tty_from);
+			else
+				ret = -EBUSY;
+			spin_unlock_irqrestore(&tsk_to->sighand->siglock, flags);
+		} else
+			ret = -EINVAL;
+
+		put_task_struct(tsk_to);
+
+		if (ret)
+			goto out;
+	}
+
+out:
+	tty_kref_put(tty_from);
+	put_task_struct(tsk_from);
+	return ret;
+}
+
 static struct cftype ve_cftypes[] = {
 	{
 		.name			= "state",
@@ -1496,6 +1586,12 @@ static struct cftype ve_cftypes[] = {
 		.read_u64		= ve_read_u64,
 		.private		= VE_CF_NETIF_NR,
 	},
+	{
+		.name			= "ctty",
+		.flags			= CFTYPE_ONLY_ON_ROOT,
+		.write_string		= ve_write_ctty,
+		.private		= VE_CF_CTTY,
+	},
 	{ }
 };