Subject: [v9][PATCH 9/9] Document clone3() syscall
This gives a brief overview of the clone3() system call. We should
eventually describe more details in existing clone(2) man page or in
a new man page.
Changelog[v9]:
- [Pavel Machek]: Fix an inconsistency and rename new file to
Documentation/clone3.
- [Roland McGrath, H. Peter Anvin] Updates to description and
example to reflect new prototype of clone3() and the updated/
renamed 'struct clone_args'.
Changelog[v8]:
- clone2() is already in use in IA64. Rename syscall to clone3()
- Add notes to say that we return -EINVAL if invalid clone flags
are specified or if the reserved fields are not 0.
Changelog[v7]:
- Rename clone_with_pids() to clone2()
- Changes to reflect new prototype of clone2() (using clone_struct).
Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
---
Documentation/clone3 | 191 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 191 insertions(+), 0 deletions(-)
create mode 100644 Documentation/clone3
diff --git a/Documentation/clone3 b/Documentation/clone3
new file mode 100644
index 0000000..466fac2
--- /dev/null
+++ b/Documentation/clone3
@@ -0,0 +1,191 @@
+
+struct clone_args {
+ u64 clone_flags_high;
+ u64 child_stack_base;
+ u64 child_stack_size;
+ u64 parent_tid_ptr;
+ u64 child_tid_ptr;
+ u32 nr_pids;
+ u32 clone_args_size;
+ u64 reserved1;
+};
+
+
+clone3(u32 flags_low, struct clone_args * __user cargs, pid_t * __user pids)
+
+ In addition to doing everything that clone() system call does,
+ the clone3() system call:
+
+ - allows additional clone flags (31 of 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 @flags_low parameter is identical to the 'clone_flags' parameter
+ in existing clone() system call.
+
+ The fields in 'struct clone_args' are meant to be used as follows:
+
+ u64 clone_flags_high:
+
+ When clone3() supports more than 32 clone flags, the higher
+ bits in the clone_flags should be specified in this field.
+ This field is currently unused and must be set to 0.
+
+ u64 child_stack_base;
+ u64 child_stack_size;
+
+ These two fields correspond to the 'child_stack' fields
+ in clone() and clone2() system calls (on IA64).
+
+ u64 parent_tid_ptr;
+ u64 child_tid_ptr;
+
+ These two fields correspond to the 'parent_tid_ptr' and
+ 'child_tid_ptr' fields in the clone() system call
+
+ u32 nr_pids;
+
+ nr_pids specifies the number of pids in the @pids array
+ parameter to clone3() (see below). nr_pids should not exceed
+ the current nesting level of the calling process (i.e if the
+ process is in init_pid_ns, nr_pids must be 1, if process is
+ in a pid namespace that is a child of init-pid-ns, nr_pids
+ cannot exceed 2, and so on).
+
+ u32 clone_args_size;
+
+ clone_args_size specifes the sizeof(struct clone_args) and is
+ intended to enable extending this structure in the future,
+ while preserving backward compatibility. For now, this field
+ must be set to the sizeof(struct clone_args) and this size must
+ match the kernel's view of the structure.
+
+ u64 reserved1;
+
+ reserved1 is intended to enable extending the functionality
+ of the clone3() system call in the future, while preserving
+ backward compatibility. It must currently be set to 0.
+
+
+ The @pids parameter 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 (see EBUSY below).
+
+ On success, the system call returns the pid of the child process in
+ the parent's active pid namespace.
+
+ On failure, clone3() 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
+
+ EINVAL Not all specified clone-flags are valid.
+
+ EINVAL The reserved fields in the clone_args argument are not 0.
+
+ EBUSY A requested pid is in use by another process in that name space.
+
+---
+/* Example usage of clone3() on i386 */
+
+#include <stdio.h>
+#include <signal.h>
+#include <errno.h>
+
+#define __NR_clone3 337
+#define TEST_PID 399
+#define STACKSIZE 8192
+
+typedef unsigned long long u64;
+typedef unsigned int u32;
+typedef int pid_t;
+
+struct clone_args {
+ u64 clone_flags_high;
+ u64 child_stack_base;
+ u64 child_stack_size;
+ u64 parent_tid_ptr;
+ u64 child_tid_ptr;
+ u32 nr_pids;
+ u32 clone_args_size;
+ u64 reserved1;
+};
+
+int do_child(void *arg)
+{
+ printf("Child, pid %d, arg %s\n", getpid(), arg);
+
+ if (getpid() != TEST_PID)
+ printf("Expected pid %d, actual %d\n", TEST_PID, getpid());
+
+ _Exit(0);
+}
+
+main()
+{
+ int rc;
+ void **stack;
+ struct clone_args cargs;
+
+ u32 flags_low = SIGCHLD;
+ char *arg_str = "Args for child: abcdefg";
+ pid_t pids[] = { 377, TEST_PID };
+
+ stack = (void **)(malloc(STACKSIZE) + STACKSIZE - 1);
+
+ /* Set up stack for child */
+ *--stack = arg_str;
+ *--stack = NULL;
+ *--stack = do_child;
+
+ cargs.clone_flags_high = (u64)0;
+ cargs.child_stack_base = (u64)stack;
+ cargs.child_stack_size = (u64)0;
+
+ cargs.nr_pids = 2; /* assumes we are in a child pid ns */
+ cargs.parent_tid_ptr = (u64)0;
+ cargs.child_tid_ptr = (u64)0;
+
+ cargs.clone_args_size = sizeof(cargs);
+ cargs.reserved1 = (u64)0;
+
+ rc = syscall(__NR_clone3, flags_low, &cargs, &pids);
+
+ if (rc != TEST_PID) {
+ printf("Parent: expected rc %d, actual %d, errno %d\n",
+ TEST_PID, rc, errno);
+ } else {
+ printf("Parent: clone3() returns %d, errno %d\n", rc, errno);
+ }
+
+ waitpid(-1, NULL, 0);
+}
--
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