Messages in this thread | | | Date | Thu, 14 Nov 2019 08:58:00 +0100 | From | Christian Brauner <> | Subject | Re: [PATCH v9 1/2] fork: extend clone3() to support setting a PID |
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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>
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