Re: [PATCH v6 1/2] fork: extend clone3() to support setting a PID

Adrian Reber
2019-08-12T21:03:01Z

On Mon, Aug 12, 2019 at 01:43:53PM -0700, Andrei Vagin wrote:
> On Mon, Aug 12, 2019 at 1:10 PM Adrian Reber <areber@redhat.com> wrote:
> >
> > 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.
> >
> > Signed-off-by: Adrian Reber <areber@redhat.com>
> > ---
> > v2:
> > - Removed (size < sizeof(struct clone_args)) as discussed with
> > Christian and Dmitry
> > - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
> > - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
> >
> > v3:
> > - Return EEXIST if PID is already in use (Christian)
> > - Drop CLONE_SET_TID (Christian and Oleg)
> > - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> > - Handle different `struct clone_args` sizes (Dmitry)
> >
> > v4:
> > - Rework struct size check with defines (Christian)
> > - Reduce number of set_tid checks (Oleg)
> > - Less parentheses and more robust code (Oleg)
> > - Do ns_capable() on correct user_ns (Oleg, Christian)
> >
> > v5:
> > - make set_tid checks earlier in alloc_pid() (Christian)
> > - remove unnecessary comment and struct size check (Christian)
> >
> > v6:
> > - remove CLONE_SET_TID from description (Christian)
> > - add clone3() tests for different clone_args sizes (Christian)
> > - move more set_tid checks to alloc_pid() (Oleg)
> > - make all set_tid checks lockless (Oleg)
> > ---
> > include/linux/pid.h | 2 +-
> > include/linux/sched/task.h | 1 +
> > include/uapi/linux/sched.h | 1 +
> > kernel/fork.c | 14 ++++++++++++--
> > kernel/pid.c | 37 ++++++++++++++++++++++++++++++-------
> > 5 files changed, 45 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > index 2a83e434db9d..052000db0ced 100644
> > --- a/include/linux/pid.h
> > +++ b/include/linux/pid.h
> > @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
> > extern struct pid *find_get_pid(int nr);
> > extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
> >
> > -extern struct pid *alloc_pid(struct pid_namespace *ns);
> > +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
> > extern void free_pid(struct pid *pid);
> > extern void disable_pid_allocation(struct pid_namespace *ns);
> >
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index 0497091e40c1..4f2a80564332 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -26,6 +26,7 @@ struct kernel_clone_args {
> > unsigned long stack;
> > unsigned long stack_size;
> > unsigned long tls;
> > + pid_t set_tid;
> > };
> >
> > /*
> > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > index b3105ac1381a..e1ce103a2c47 100644
> > --- a/include/uapi/linux/sched.h
> > +++ b/include/uapi/linux/sched.h
> > @@ -45,6 +45,7 @@ struct clone_args {
> > __aligned_u64 stack;
> > __aligned_u64 stack_size;
> > __aligned_u64 tls;
> > + __aligned_u64 set_tid;
> > };
> >
> > /*
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 2852d0e76ea3..8317d408a8d6 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -117,6 +117,11 @@
> > */
> > #define MAX_THREADS FUTEX_TID_MASK
> >
> > +/*
> > + * For different sizes of struct clone_args
> > + */
> > +#define CLONE3_ARGS_SIZE_V0 64
> > +
> > /*
> > * Protected counters by write_lock_irq(&tasklist_lock)
> > */
> > @@ -2031,7 +2036,7 @@ static __latent_entropy struct task_struct *copy_process(
> > 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);
> > if (IS_ERR(pid)) {
> > retval = PTR_ERR(pid);
> > goto bad_fork_cleanup_thread;
> > @@ -2535,9 +2540,13 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > if (unlikely(size > PAGE_SIZE))
> > return -E2BIG;
> >
> > - if (unlikely(size < sizeof(struct clone_args)))
> > + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> > return -EINVAL;
> >
> > + if (size < sizeof(struct clone_args))
> > + memset((void *)&args + size, 0,
> > + sizeof(struct clone_args) - size);
> > +
> > if (unlikely(!access_ok(uargs, size)))
> > return -EFAULT;
> >
> > @@ -2571,6 +2580,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > .stack = args.stack,
> > .stack_size = args.stack_size,
> > .tls = args.tls,
> > + .set_tid = args.set_tid,
> > };
> >
> > return 0;
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 0a9f2e437217..5cdab73b9094 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
> > call_rcu(&pid->rcu, delayed_put_pid);
> > }
> >
> > -struct pid *alloc_pid(struct pid_namespace *ns)
> > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> > {
> > struct pid *pid;
> > enum pid_type type;
> > @@ -166,6 +166,16 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > struct upid *upid;
> > int retval = -ENOMEM;
> >
> > + if (set_tid) {
> > + if (set_tid < 0 || set_tid >= pid_max)
> > + return ERR_PTR(-EINVAL);
> > + /* Also fail if a PID != 1 is requested and no PID 1 exists */
> > + if (set_tid != 1 && !ns->child_reaper)
> > + return ERR_PTR(-EINVAL);
>
> What will happen if I will create a new pid namespace
> (unsahre(CLONE_NEWPID)) and then call clone3 with tid which isn't
> equal to 1?
>
> Will I get the init process with the 5 pid?...

No, you will get -EINVAL. In the selftest exactly this is tested. A PID
1 is still required even with this patch.

> > + if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
> > + return ERR_PTR(-EPERM);
> > + }
> > +
> > pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
> > if (!pid)
> > return ERR_PTR(retval);
> > @@ -186,12 +196,25 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> > pid_min = RESERVED_PIDS;
> >
> > - /*
> > - * Store a null pointer so find_pid_ns does not find
> > - * a partially initialized PID (see below).
> > - */
> > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > - pid_max, GFP_ATOMIC);
> > + if (set_tid) {
> > + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> > + set_tid + 1, GFP_ATOMIC);
> > + /*
> > + * If ENOSPC is returned it means that the PID is
> > + * alreay in use. Return EEXIST in that case.
> > + */
> > + if (nr == -ENOSPC)
> > + nr = -EEXIST;
> > + /* Only use set_tid for one PID namespace. */
> > + set_tid = 0;
> > + } else {
> > + /*
> > + * Store a null pointer so find_pid_ns does not find
> > + * a partially initialized PID (see below).
> > + */
> > + nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > + pid_max, GFP_ATOMIC);
> > + }
> > spin_unlock_irq(&pidmap_lock);
> > idr_preload_end();
> >
> > --
> > 2.21.0
> >