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 08:57:59AM +0100, Christian Brauner wrote:
> 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>

And thanks for the --base=<commit> information you've added to this
version. :)

	Christian