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    Subject[PATCH 3/3] pids: Make it possible to clone tasks with given pids
    When restoring a task (or a set of tasks) we need to recreate them
    with exactly the same pid(s) as they had before. Thus we need the
    ability to create a task with specified pid. The proposal is to reuse
    the already free CLONE_STOPPED clone flag, introduce the new one
    called CLONE_CHILD_USEPIDS and point to the desired pids with the
    child_tidptr.

    The child_tidptr points to an array of pids for current namespace and
    its ancestors. When 0 is met in this array the pid number for the
    corresponding namespace is generated, rather than set.

    For security reasons after a regular clone/fork is done in a namespace
    further cloning with predefined pid is not allowed.

    Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

    ---
    include/linux/pid.h | 2 +-
    include/linux/sched.h | 3 +--
    kernel/fork.c | 4 +++-
    kernel/pid.c | 39 +++++++++++++++++++++++++++++++++++++--
    4 files changed, 42 insertions(+), 6 deletions(-)

    diff --git a/include/linux/pid.h b/include/linux/pid.h
    index b152d44..95aa618 100644
    --- a/include/linux/pid.h
    +++ b/include/linux/pid.h
    @@ -119,7 +119,7 @@ extern struct pid *find_get_pid(int nr);
    extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
    int next_pidmap(struct pid_namespace *pid_ns, unsigned int last);

    -extern struct pid *alloc_pid(struct pid_namespace *ns);
    +extern struct pid *alloc_pid(struct pid_namespace *ns, int __user *want_pids);
    extern void free_pid(struct pid *pid);

    /*
    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 41d0237..5472c4e 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -21,8 +21,7 @@
    #define CLONE_DETACHED 0x00400000 /* Unused, ignored */
    #define CLONE_UNTRACED 0x00800000 /* set if the tracing process can't force CLONE_PTRACE on this clone */
    #define CLONE_CHILD_SETTID 0x01000000 /* set the TID in the child */
    -/* 0x02000000 was previously the unused CLONE_STOPPED (Start in stopped state)
    - and is now available for re-use. */
    +#define CLONE_CHILD_USEPIDS 0x02000000 /* use the pids given by user */
    #define CLONE_NEWUTS 0x04000000 /* New utsname group? */
    #define CLONE_NEWIPC 0x08000000 /* New ipcs */
    #define CLONE_NEWUSER 0x10000000 /* New user namespace */
    diff --git a/kernel/fork.c b/kernel/fork.c
    index 45a5f54..26c67ff 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -1253,7 +1253,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
    goto bad_fork_cleanup_io;

    if (pid != &init_struct_pid) {
    - pid = alloc_pid(p->nsproxy->pid_ns);
    + pid = alloc_pid(p->nsproxy->pid_ns,
    + (clone_flags & CLONE_CHILD_USEPIDS) ?
    + child_tidptr : NULL);
    if (IS_ERR(pid)) {
    retval = PTR_ERR(pid);
    goto bad_fork_cleanup_io;
    diff --git a/kernel/pid.c b/kernel/pid.c
    index 700dda3..fb6f74f 100644
    --- a/kernel/pid.c
    +++ b/kernel/pid.c
    @@ -224,6 +224,27 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
    return -ENOMEM;
    }

    +static int set_pidmap(struct pid_namespace *pid_ns, int pid)
    +{
    + int offset;
    + struct pidmap *map;
    +
    + offset = pid & BITS_PER_PAGE_MASK;
    + map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
    +
    + if (unlikely(!map->page))
    + if (alloc_pidmap_page(map))
    + return -ENOMEM;
    +
    + if (pid_ns->last_pid != 0)
    + return -EPERM;
    +
    + if (test_and_set_bit(offset, map->page))
    + return -EBUSY;
    +
    + return pid;
    +}
    +
    int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
    {
    int offset;
    @@ -284,7 +305,7 @@ void free_pid(struct pid *pid)
    call_rcu(&pid->rcu, delayed_put_pid);
    }

    -struct pid *alloc_pid(struct pid_namespace *ns)
    +struct pid *alloc_pid(struct pid_namespace *ns, int __user *want_pids)
    {
    struct pid *pid;
    enum pid_type type;
    @@ -298,7 +319,21 @@ struct pid *alloc_pid(struct pid_namespace *ns)

    tmp = ns;
    for (i = ns->level; i >= 0; i--) {
    - nr = alloc_pidmap(tmp);
    + nr = 0;
    + if (unlikely(want_pids != NULL)) {
    + if (get_user(nr, want_pids)) {
    + nr = -EFAULT;
    + goto out_free;
    + }
    +
    + if (nr != 0) {
    + want_pids++;
    + nr = set_pidmap(tmp, nr);
    + } else
    + want_pids = NULL; /* optimize above */
    + }
    + if (nr == 0)
    + nr = alloc_pidmap(tmp);
    if (nr < 0)
    goto out_free;

    --
    1.5.5.6

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