fork: extend clone3() to support setting a PID
The main motivation to add set_tid to clone3() is CRIU.
To restore a process with the same PID/TID CRIU currently uses
/proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
ns_last_pid and then (quickly) does a clone(). This works most of the
time, but it is racy. It is also slow as it requires multiple syscalls.
Extending clone3() to support *set_tid makes it possible restore a
process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
race free (as long as the desired PID/TID is available).
This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
on clone3() with *set_tid as they are currently in place for ns_last_pid.
The original version of this change was using a single value for
set_tid. At the 2019 LPC, after presenting set_tid, it was, however,
decided to change set_tid to an array to enable setting the PID of a
process in multiple PID namespaces at the same time. If a process is
created in a PID namespace it is possible to influence the PID inside
and outside of the PID namespace. Details also in the corresponding
selftest.
To create a process with the following PIDs:
PID NS level Requested PID
0 (host) 31496
1 42
2 1
For that example the two newly introduced parameters to struct
clone_args (set_tid and set_tid_size) would need to be:
set_tid[0] = 1;
set_tid[1] = 42;
set_tid[2] = 31496;
set_tid_size = 3;
If only the PIDs of the two innermost nested PID namespaces should be
defined it would look like this:
set_tid[0] = 1;
set_tid[1] = 42;
set_tid_size = 2;
The PID of the newly created process would then be the next available
free PID in the PID namespace level 0 (host) and 42 in the PID namespace
at level 1 and the PID of the process in the innermost PID namespace
would be 1.
The set_tid array is used to specify the PID of a process starting
from the innermost nested PID namespaces up to set_tid_size PID namespaces.
set_tid_size cannot be larger then the current PID namespace level.
Signed-off-by: Adrian Reber <areber@redhat.com>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Acked-by: Andrei Vagin <avagin@gmail.com>
Link: https://lore.kernel.org/r/20191115123621.142252-1-areber@redhat.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
diff --git a/kernel/fork.c b/kernel/fork.c
index 954e875..4175702 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2087,7 +2087,8 @@
stackleak_task_init(p);
if (pid != &init_struct_pid) {
- pid = alloc_pid(p->nsproxy->pid_ns_for_children);
+ pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid,
+ args->set_tid_size);
if (IS_ERR(pid)) {
retval = PTR_ERR(pid);
goto bad_fork_cleanup_thread;
@@ -2590,6 +2591,7 @@
{
int err;
struct clone_args args;
+ pid_t *kset_tid = kargs->set_tid;
if (unlikely(usize > PAGE_SIZE))
return -E2BIG;
@@ -2600,6 +2602,15 @@
if (err)
return err;
+ if (unlikely(args.set_tid_size > MAX_PID_NS_LEVEL))
+ return -EINVAL;
+
+ if (unlikely(!args.set_tid && args.set_tid_size > 0))
+ return -EINVAL;
+
+ if (unlikely(args.set_tid && args.set_tid_size == 0))
+ return -EINVAL;
+
/*
* Verify that higher 32bits of exit_signal are unset and that
* it is a valid signal
@@ -2617,8 +2628,16 @@
.stack = args.stack,
.stack_size = args.stack_size,
.tls = args.tls,
+ .set_tid_size = args.set_tid_size,
};
+ if (args.set_tid &&
+ copy_from_user(kset_tid, u64_to_user_ptr(args.set_tid),
+ (kargs->set_tid_size * sizeof(pid_t))))
+ return -EFAULT;
+
+ kargs->set_tid = kset_tid;
+
return 0;
}
@@ -2662,6 +2681,9 @@
int err;
struct kernel_clone_args kargs;
+ pid_t set_tid[MAX_PID_NS_LEVEL];
+
+ kargs.set_tid = set_tid;
err = copy_clone_args_from_user(&kargs, uargs, size);
if (err)