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SubjectRe: [PATCH v21 011/100] eclone (11/11): Document sys_eclone
On Sat,  1 May 2010 10:14:53 -0400 Oren Laadan wrote:

> From: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
>
> This gives a brief overview of the eclone() system call. We should
> eventually describe more details in existing clone(2) man page or in
> a new man page.
>
> Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
> Acked-by: Serge E. Hallyn <serue@us.ibm.com>
> Acked-by: Oren Laadan <orenl@cs.columbia.edu>
> ---
> Documentation/eclone | 348 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 348 insertions(+), 0 deletions(-)
> create mode 100644 Documentation/eclone
>
> diff --git a/Documentation/eclone b/Documentation/eclone
> new file mode 100644
> index 0000000..c2f1b4b
> --- /dev/null
> +++ b/Documentation/eclone
> @@ -0,0 +1,348 @@
> +
> +struct clone_args {
> + u64 clone_flags_high;
> + u64 child_stack;
> + u64 child_stack_size;
> + u64 parent_tid_ptr;
> + u64 child_tid_ptr;
> + u32 nr_pids;
> + u32 reserved0;
> +};
> +
> +
> +sys_eclone(u32 flags_low, struct clone_args * __user cargs, int cargs_size,
> + pid_t * __user pids)
> +
> + In addition to doing everything that clone() system call does, the

that the clone()

> + eclone() 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 namespaces.
> +
> + 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.

in the existing

> +
> + The fields in 'struct clone_args' are meant to be used as follows:
> +
> + u64 clone_flags_high:
> +
> + When eclone() supports more than 32 flags, the additional 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;
> + u64 child_stack_size;
> +
> + These two fields correspond to the 'child_stack' fields in
> + clone() and clone2() (on IA64) system calls. The usage of
> + these two fields depends on the processor architecture.
> +
> + Most architectures use ->child_stack to pass-in a stack-pointer

to pass in

> + itself and don't need the ->child_stack_size field. On these
> + architectures the ->child_stack_size field must be 0.
> +
> + Some architectures, eg IA64, use ->child_stack to pass-in the

e.g. to pass in

> + base of the region allocated for stack. These architectures
> + must pass in the size of the stack-region in ->child_stack_size.

stack region

Seems unfortunate that different architectures use the fields differently.

> +
> + 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

system call.

> +
> + u32 nr_pids;
> +
> + nr_pids specifies the number of pids in the @pids array
> + parameter to eclone() (see below). nr_pids should not exceed
> + the current nesting level of the calling process (i.e if the

i.e.

> + 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 reserved0;
> + u64 reserved1;
> +
> + These fields are intended to extend the functionality of the
> + eclone() in the future, while preserving backward compatibility.
> + They must be set to 0 for now.

The struct does not have a reserved1 field AFAICT.

> + The @cargs_size parameter 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.
> +
> + The @pids parameter defines the set of pids that should be assigned to
> + the child process in its active and ancestor pid namespaces. 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

of the clone(2)

> + namespaces.
> +
> + If a pid in the @pids list is 0, the kernel will assign the next
> + available pid in the pid namespace.
> +
> + 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).
> +
> + The order of pids in @pids is oldest in pids[0] to youngest pid
> + namespace in pids[nr_pids-1]. If the number of pids specified in the
> + @pids list is fewer than the nesting level of the process, the pids
> + are applied from youngest namespace. i.e if the process is nested in

the youngest namespace. I.e.

> + a level-6 pid namespace and @pids only specifies 3 pids, the 3 pids
> + are applied to levels 6, 5 and 4. Levels 0 through 3 are assumed to
> + have a pid of '0' (the kernel will assign a pid in those namespaces).
> +
> + On success, the system call returns the pid of the child process in
> + the parent's active pid namespace.
> +
> + On failure, eclone() returns -1 and sets 'errno' to one of following
> + values (the child process is not created).
> +
> + EPERM Caller does not have the CAP_SYS_ADMIN privilege needed to
> + specify the pids in this call (if pids are not specifed
> + CAP_SYS_ADMIN is not required).
> +
> + EINVAL The number of pids specified in 'clone_args.nr_pids' exceeds
> + the current nesting level of parent process

process.

> +
> + EINVAL Not all specified clone-flags are valid.
> +
> + EINVAL The reserved fields in the clone_args argument are not 0.
> +
> + EINVAL The child_stack_size field is not 0 (on architectures that
> + pass in a stack pointer in ->child_stack field)

field).

> +
> + EBUSY A requested pid is in use by another process in that namespace.
> +
> +---


Is this example program meant to build only on i386?

On x86_64 I get:

eclone-syscall-test.c: In function 'do_clone':
eclone-syscall-test.c:166: warning: assignment makes pointer from integer without a cast
/tmp/cc0OrhU3.o: In function `do_clone':
eclone-syscall-test.c:(.text+0x173): undefined reference to `setup_stack'
eclone-syscall-test.c:(.text+0x1e2): undefined reference to `eclone'


> +/*
> + * Example eclone() usage - Create a child process with pid CHILD_TID1 in
> + * the current pid namespace. The child gets the usual "random" pid in any
> + * ancestor pid namespaces.
> + */
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <signal.h>
> +#include <errno.h>
> +#include <unistd.h>
> +#include <wait.h>
> +#include <sys/syscall.h>
> +
> +#define __NR_eclone 337
> +#define CLONE_NEWPID 0x20000000
> +#define CLONE_CHILD_SETTID 0x01000000
> +#define CLONE_PARENT_SETTID 0x00100000
> +#define CLONE_UNUSED 0x00001000
> +
> +#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;
> + u64 child_stack_size;
> +
> + u64 parent_tid_ptr;
> + u64 child_tid_ptr;
> +
> + u32 nr_pids;
> +
> + u32 reserved0;
> +};
> +
> +#define exit _exit
> +
> +/*
> + * Following eclone() is based on code posted by Oren Laadan at:
> + * https://lists.linux-foundation.org/pipermail/containers/2009-June/018463.html
> + */
> +#if defined(__i386__) && defined(__NR_eclone)
> +
> +int eclone(u32 flags_low, struct clone_args *clone_args, int args_size,
> + int *pids)
> +{
> + long retval;
> +
> + __asm__ __volatile__(
> + "movl %3, %%ebx\n\t" /* flags_low -> 1st (ebx) */
> + "movl %4, %%ecx\n\t" /* clone_args -> 2nd (ecx)*/
> + "movl %5, %%edx\n\t" /* args_size -> 3rd (edx) */
> + "movl %6, %%edi\n\t" /* pids -> 4th (edi)*/
> +
> + "pushl %%ebp\n\t" /* save value of ebp */
> + "int $0x80\n\t" /* Linux/i386 system call */
> + "testl %0,%0\n\t" /* check return value */
> + "jne 1f\n\t" /* jump if parent */
> +
> + "popl %%esi\n\t" /* get subthread function */
> + "call *%%esi\n\t" /* start subthread function */
> + "movl %2,%0\n\t"
> + "int $0x80\n" /* exit system call: exit subthread */
> + "1:\n\t"
> + "popl %%ebp\t" /* restore parent's ebp */
> +
> + :"=a" (retval)
> +
> + :"0" (__NR_eclone),
> + "i" (__NR_exit),
> + "m" (flags_low),
> + "m" (clone_args),
> + "m" (args_size),
> + "m" (pids)
> + );
> +
> + if (retval < 0) {
> + errno = -retval;
> + retval = -1;
> + }
> + return retval;
> +}
> +
> +/*
> + * Allocate a stack for the clone-child and arrange to have the child
> + * execute @child_fn with @child_arg as the argument.
> + */
> +void *setup_stack(int (*child_fn)(void *), void *child_arg, int size)
> +{
> + void *stack_base;
> + void **stack_top;
> +
> + stack_base = malloc(size + size);
> + if (!stack_base) {
> + perror("malloc()");
> + exit(1);
> + }
> +
> + stack_top = (void **)((char *)stack_base + (size - 4));
> + *--stack_top = child_arg;
> + *--stack_top = child_fn;
> +
> + return stack_top;
> +}
> +#endif
> +
> +/* gettid() is a bit more useful than getpid() when messing with clone() */
> +int gettid()
> +{
> + int rc;
> +
> + rc = syscall(__NR_gettid, 0, 0, 0);
> + if (rc < 0) {
> + printf("rc %d, errno %d\n", rc, errno);
> + exit(1);
> + }
> + return rc;
> +}
> +
> +#define CHILD_TID1 377
> +#define CHILD_TID2 1177
> +#define CHILD_TID3 2799
> +
> +struct clone_args clone_args;
> +void *child_arg = &clone_args;
> +int child_tid;
> +
> +int do_child(void *arg)
> +{
> + struct clone_args *cs = (struct clone_args *)arg;
> + int ctid;
> +
> + /* Verify we pushed the arguments correctly on the stack... */
> + if (arg != child_arg) {
> + printf("Child: Incorrect child arg pointer, expected %p,"
> + "actual %p\n", child_arg, arg);
> + exit(1);
> + }
> +
> + /* ... and that we got the thread-id we expected */
> + ctid = *((int *)(unsigned long)cs->child_tid_ptr);
> + if (ctid != CHILD_TID1) {
> + printf("Child: Incorrect child tid, expected %d, actual %d\n",
> + CHILD_TID1, ctid);
> + exit(1);
> + } else {
> + printf("Child got the expected tid, %d\n", gettid());
> + }
> + sleep(2);
> +
> + printf("[%d, %d]: Child exiting\n", getpid(), ctid);
> + exit(0);
> +}
> +
> +static int do_clone(int (*child_fn)(void *), void *child_arg,
> + unsigned int flags_low, int nr_pids, pid_t *pids_list)
> +{
> + int rc;
> + void *stack;
> + struct clone_args *ca = &clone_args;
> + int args_size;
> +
> + stack = setup_stack(child_fn, child_arg, STACKSIZE);
> +
> + memset(ca, 0, sizeof(*ca));
> +
> + ca->child_stack = (u64)(unsigned long)stack;
> + ca->child_stack_size = (u64)0;
> + ca->child_tid_ptr = (u64)(unsigned long)&child_tid;
> + ca->nr_pids = nr_pids;
> +
> + args_size = sizeof(struct clone_args);
> + rc = eclone(flags_low, ca, args_size, pids_list);
> +
> + printf("[%d, %d]: eclone() returned %d, error %d\n", getpid(), gettid(),
> + rc, errno);
> + return rc;
> +}
> +
> +/*
> + * Multiple pid_t pid_t values in pids_list[] here are just for illustration.
> + * The test case creates a child in the current pid namespace and uses only
> + * the first value, CHILD_TID1.
> + */
> +pid_t pids_list[] = { CHILD_TID1, CHILD_TID2, CHILD_TID3 };
> +int main()
> +{
> + int rc, pid, status;
> + unsigned long flags;
> + int nr_pids = 1;
> +
> + flags = SIGCHLD|CLONE_CHILD_SETTID;
> +
> + pid = do_clone(do_child, &clone_args, flags, nr_pids, pids_list);
> +
> + printf("[%d, %d]: Parent waiting for %d\n", getpid(), gettid(), pid);
> +
> + rc = waitpid(pid, &status, __WALL);
> + if (rc < 0) {
> + printf("waitpid(): rc %d, error %d\n", rc, errno);
> + } else {
> + printf("[%d, %d]: child %d:\n\t wait-status 0x%x\n", getpid(),
> + gettid(), rc, status);
> +
> + if (WIFEXITED(status)) {
> + printf("\t EXITED, %d\n", WEXITSTATUS(status));
> + } else if (WIFSIGNALED(status)) {
> + printf("\t SIGNALED, %d\n", WTERMSIG(status));
> + }
> + }
> + return 0;
> +}
> --


---
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***


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