[RFC PATCH] proc, pidns: Add highpid

[Andy Lutomirski]

Pid reuse is common, which means that it's difficult or impossible
to read information about a pid from /proc without races.

This introduces a second number associated with each (task, pidns)
pair called highpid.  Highpid is a 64-bit number, and, barring
extremely unlikely circumstances or outright error, a (highpid, pid)
will never be reused.

With just this change, a program can open /proc/PID/status, read the
"Highpid" field, and confirm that it has the expected value.  If the
pid has been reused, then highpid will be different.

The initial implementation is straightforward: highpid is simply a
64-bit counter. If a high-end system can fork every 3 ns (which
would be amazing, given that just allocating a pid requires at
atomic operation), it would take well over 1000 years for highpid to
wrap.

For CRIU's benefit, the next highpid can be set by a privileged
user.

NB: The sysctl stuff only works on 64-bit systems.  If the approach
looks good, I'll fix that somehow.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---

If this goes in, there's plenty of room to add new interfaces to
make this more useful.  For example, we could add a fancier tgkill
that adds and validates hightgid and highpid, and we might want to
add a syscall to read one's own hightgid and highpid.  These would
be quite useful for pidfiles.

David, would this be useful for kdbus?

CRIU people: will this be unduly difficult to support in CRIU?

If you all think this is a good idea, I'll fix the sysctl stuff and
document it.  It might also be worth adding "Hightgid" to status.

 fs/proc/array.c               |  5 ++++-
 include/linux/pid.h           |  2 ++
 include/linux/pid_namespace.h |  1 +
 kernel/pid.c                  | 19 +++++++++++++++----
 kernel/pid_namespace.c        | 22 ++++++++++++++++++++++
 5 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index cd3653e4f35c..f1e0e69d19f9 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -159,6 +159,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 	int g;
 	struct fdtable *fdt = NULL;
 	const struct cred *cred;
+	const struct upid *upid;
 	pid_t ppid, tpid;
 
 	rcu_read_lock();
@@ -170,12 +171,14 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		if (tracer)
 			tpid = task_pid_nr_ns(tracer, ns);
 	}
+	upid = pid_upid_ns(pid, ns);
 	cred = get_task_cred(p);
 	seq_printf(m,
 		"State:\t%s\n"
 		"Tgid:\t%d\n"
 		"Ngid:\t%d\n"
 		"Pid:\t%d\n"
+		"Highpid:\t%llu\n"
 		"PPid:\t%d\n"
 		"TracerPid:\t%d\n"
 		"Uid:\t%d\t%d\t%d\t%d\n"
@@ -183,7 +186,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		get_task_state(p),
 		task_tgid_nr_ns(p, ns),
 		task_numa_group_id(p),
-		pid_nr_ns(pid, ns),
+		upid ? upid->nr : 0, upid ? upid->highnr : 0,
 		ppid, tpid,
 		from_kuid_munged(user_ns, cred->uid),
 		from_kuid_munged(user_ns, cred->euid),
diff --git a/include/linux/pid.h b/include/linux/pid.h
index 23705a53abba..ece70b64d04c 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -51,6 +51,7 @@ struct upid {
 	/* Try to keep pid_chain in the same cacheline as nr for find_vpid */
 	int nr;
 	struct pid_namespace *ns;
+	u64 highnr;
 	struct hlist_node pid_chain;
 };
 
@@ -170,6 +171,7 @@ static inline pid_t pid_nr(struct pid *pid)
 }
 
 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
+const struct upid *pid_upid_ns(struct pid *pid, struct pid_namespace *ns);
 pid_t pid_vnr(struct pid *pid);
 
 #define do_each_pid_task(pid, type, task)				\
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 1997ffc295a7..1f9f0455d7ef 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -26,6 +26,7 @@ struct pid_namespace {
 	struct rcu_head rcu;
 	int last_pid;
 	unsigned int nr_hashed;
+	atomic64_t next_highpid;
 	struct task_struct *child_reaper;
 	struct kmem_cache *pid_cachep;
 	unsigned int level;
diff --git a/kernel/pid.c b/kernel/pid.c
index 9b9a26698144..863d11a9bfbf 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -312,6 +312,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 
 		pid->numbers[i].nr = nr;
 		pid->numbers[i].ns = tmp;
+		pid->numbers[i].highnr =
+			atomic64_inc_return(&tmp->next_highpid) - 1;
 		tmp = tmp->parent;
 	}
 
@@ -492,17 +494,26 @@ struct pid *find_get_pid(pid_t nr)
 }
 EXPORT_SYMBOL_GPL(find_get_pid);
 
-pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
+const struct upid *pid_upid_ns(struct pid *pid, struct pid_namespace *ns)
 {
 	struct upid *upid;
-	pid_t nr = 0;
 
 	if (pid && ns->level <= pid->level) {
 		upid = &pid->numbers[ns->level];
 		if (upid->ns == ns)
-			nr = upid->nr;
+			return upid;
 	}
-	return nr;
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(pid_upid_ns);
+
+pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
+{
+	const struct upid *upid = pid_upid_ns(pid, ns);
+
+	if (!upid)
+		return 0;
+	return upid->nr;
 }
 EXPORT_SYMBOL_GPL(pid_nr_ns);
 
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index db95d8eb761b..e246802b4361 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -268,6 +268,22 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
 	return proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
 }
 
+static int pid_ns_next_highpid_handler(struct ctl_table *table, int write,
+		void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct pid_namespace *pid_ns = task_active_pid_ns(current);
+	struct ctl_table tmp = *table;
+
+	if (write && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
+		return -EPERM;
+
+	/* This needs to be fixed. */
+	BUILD_BUG_ON(sizeof(u64) != sizeof(unsigned long));
+
+	tmp.data = &pid_ns->next_highpid;
+	return proc_dointvec(&tmp, write, buffer, lenp, ppos);
+}
+
 extern int pid_max;
 static int zero = 0;
 static struct ctl_table pid_ns_ctl_table[] = {
@@ -279,6 +295,12 @@ static struct ctl_table pid_ns_ctl_table[] = {
 		.extra1 = &zero,
 		.extra2 = &pid_max,
 	},
+	{
+		.procname = "ns_next_highpid",
+		.maxlen = sizeof(u64),
+		.mode = 0666, /* permissions are checked in the handler */
+		.proc_handler = pid_ns_next_highpid_handler,
+	},
 	{ }
 };
 static struct ctl_path kern_path[] = { { .procname = "kernel", }, { } };
-- 
1.9.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/