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

[Andrei Vagin]

On Mon, Aug 12, 2019 at 1:10 PM Adrian Reber <areber@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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?...

> +               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
>