Re: [PATCH v8 1/2] fork: extend clone3() to support setting a PID

[Christian Brauner]

On Wed, Nov 13, 2019 at 09:03:00AM +0100, Adrian Reber 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.
> 
> 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@xxxxxxxxxx>

Adrian, when you resend, can you please add --base=<commit> with the
base commit this series applies to? This makes my life easier when
applying this series.

Other from missing kernel-doc (see below) I don't have any further
complaints atm.

> ---
> 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)
> 
> v7:
>  - changed set_tid to be an array to set the PID of a process
>    in multiple nested PID namespaces at the same time as discussed
>    at LPC 2019 (container MC)
> 
> v8:
>  - skip unnecessary memset() (Rasmus)
>  - replace set_tid copy loop with a single copy (Christian)
>  - more parameter error checking (Christian)
>  - cache set_tid in alloc_pid() (Oleg)
>  - move code in "else" branch (Oleg)
> ---
>  include/linux/pid.h           |  3 +-
>  include/linux/pid_namespace.h |  2 ++
>  include/linux/sched/task.h    |  3 ++
>  include/uapi/linux/sched.h    |  2 ++
>  kernel/fork.c                 | 24 ++++++++++++-
>  kernel/pid.c                  | 64 +++++++++++++++++++++++++++--------
>  kernel/pid_namespace.c        |  2 --
>  7 files changed, 82 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 9645b1194c98..034b7df25888 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -120,7 +120,8 @@ 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,
> +			     size_t set_tid_size);
>  extern void free_pid(struct pid *pid);
>  extern void disable_pid_allocation(struct pid_namespace *ns);
>  
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index 49538b172483..2ed6af88794b 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -12,6 +12,8 @@
>  #include <linux/ns_common.h>
>  #include <linux/idr.h>
>  
> +/* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
> +#define MAX_PID_NS_LEVEL 32
>  
>  struct fs_pin;
>  
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 4b1c3b664f51..f1879884238e 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -26,6 +26,9 @@ struct kernel_clone_args {
>  	unsigned long stack;
>  	unsigned long stack_size;
>  	unsigned long tls;
> +	pid_t *set_tid;
> +	/* Number of elements in *set_tid */
> +	size_t set_tid_size;
>  };
>  
>  /*
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 25b4fa00bad1..13f74c40a629 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -72,6 +72,8 @@ struct clone_args {
>  	__aligned_u64 stack;
>  	__aligned_u64 stack_size;
>  	__aligned_u64 tls;
> +	__aligned_u64 set_tid;
> +	__aligned_u64 set_tid_size;

Please add kernel-doc comments for these two new fields to the top of
the like we did for all the other fields.

	Christian