Re: [PATCH v9 1/2] fork: extend clone3() to support setting a PID

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Nov 14, 2019 at 08:07:08AM +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@redhat.com>

I have no quarrels with the core patch anymore.
Note, once Oleg has said he's fine with this patch too I will likely
reword the kernel-doc and the comment in alloc_pid() and the commit
message a little before applying; but really just minor things that are
not worth resending for.

Thanks!
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>