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Subject[RFC][v7][PATCH 9/9]: Document clone2() syscall


Subject: [RFC][v7][PATCH 9/9]: Document clone2() syscall

This gives a brief overview of the clone2() system call. We should
eventually describe more details in existing clone(2) man page or in
a new man page.

Changelog[v7]:
- Rename clone_with_pids() to clone2()
- Changes to reflect new prototype of clone2() (using clone_struct).

Signed-off-by: Sukadev Bhattiprolu <sukadev@vnet.linux.ibm.com>
---
Documentation/clone2 | 85 +++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 85 insertions(+)
Index: linux-2.6/Documentation/clone2
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/Documentation/clone2 2009-09-18 18:48:00.000000000 -0700
@@ -0,0 +1,85 @@
+
+struct clone_struct {
+ u64 flags;
+ u64 child_stack;
+ u32 nr_pids;
+ u32 parent_tid;
+ u32 child_tid;
+ u32 reserved1;
+ u64 reserved2;
+};
+
+clone2(struct clone_struct * __user clone_args, pid_t * __user pids)
+
+ In addition to doing everything that clone() system call does,
+ the clone2() system call:
+
+ - allows additional clone flags (all 32 bits in the flags
+ parameter to clone() are in use)
+
+ - allows user to specify a pid for the child process in its
+ active and ancestor pid name spaces.
+
+ This system call is meant to be used when restarting an application
+ from a checkpoint. Such restart requires that the processes in the
+ application have the same pids they had when the application was
+ checkpointed. When containers are nested, the processes within the
+ containers exist in multiple pid namespaces and hence have multiple
+ pids to specify during restart.
+
+ The @pids defines the set of pids that should be assigned to the child
+ process in its active and ancestor pid name spaces. The descendant pid
+ namespaces do not matter since a process does not have a pid in
+ descendant namespaces, unless the process is in a new pid namespace
+ in which case the process is a container-init (and must have the pid 1
+ in that namespace).
+
+ See CLONE_NEWPID section of clone(2) man page for details about pid
+ namespaces.
+
+ The order pids in @pids corresponds to the nesting order of pid-
+ namespaces, with @pids[0] corresponding to the init_pid_ns.
+
+ If a pid in the @pids list is 0, the kernel will assign the next
+ available pid in the pid namespace, for the process.
+
+ If a pid in the @pids list is non-zero, the kernel tries to assign
+ the specified pid in that namespace. If that pid is already in use
+ by another process, the system call fails with -EBUSY.
+
+ On success, the system call returns the pid of the child process in
+ the parent's active pid namespace.
+
+ On failure, clone2() returns -1 and sets 'errno' to one of following
+ values (the child process is not created).
+
+ EPERM Caller does not have the SYS_ADMIN privilege needed to excute
+ this call.
+
+ EINVAL The number of pids specified in 'clone_args.nr_pids' exceeds
+ the current nesting level of parent process
+
+ EBUSY A requested pid is in use by another process in that name space.
+
+Example:
+
+ pid_t pids[] = { 77, 99 };
+ struct clone_struct cs;
+
+ cs.flags = (u64) SIGCHLD;
+ cs.child_stack = (u64) setup_child_stack();
+ cs.nr_pids = 2;
+ cs.parent_tid = 0;
+ cs.child_tid = 0;
+
+ rc = syscall(__NR_clone2, &cs, pids);
+
+ if (rc < 0) {
+ perror("clone2()");
+ exit(1);
+ } else if (rc) {
+ /* Parent */
+ } else {
+ /* Child */
+ }
+

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