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    Subject[PATCH] implement unshare(CLONE_SIGHAND) for single-thread case
    on top of (already merged)
    unshare-use-rcu_assign_pointer-when-setting-sighand.patch
    depends on
    convert-sighand_cache-to-use-slab_destroy_by_rcu.patch

    Implement unshare(CLONE_SIGHAND) for single-thread case.

    I think it is not possible to do it for multithread case without
    complication of copy_process().

    Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

    --- MM/kernel/fork.c~ 2006-03-18 16:23:23.000000000 +0300
    +++ MM/kernel/fork.c 2006-03-18 17:00:39.000000000 +0300
    @@ -1483,18 +1483,24 @@ static int unshare_namespace(unsigned lo
    }

    /*
    - * Unsharing of sighand for tasks created with CLONE_SIGHAND is not
    - * supported yet
    + * Unsharing of sighand is only supported for single-thread applications
    */
    static int unshare_sighand(unsigned long unshare_flags, struct sighand_struct **new_sighp)
    {
    - struct sighand_struct *sigh = current->sighand;
    -
    if ((unshare_flags & CLONE_SIGHAND) &&
    - (sigh && atomic_read(&sigh->count) > 1))
    - return -EINVAL;
    - else
    - return 0;
    + atomic_read(&current->sighand->count) > 1) {
    +
    + if (!thread_group_empty(current))
    + return -EINVAL;
    +
    + *new_sighp = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
    + if (unlikely(*new_sighp == NULL))
    + return -ENOMEM;
    +
    + atomic_set(&(*new_sighp)->count, 1);
    + }
    +
    + return 0;
    }

    /*
    @@ -1579,7 +1585,18 @@ asmlinkage long sys_unshare(unsigned lon
    if ((err = unshare_semundo(unshare_flags, &new_ulist)))
    goto bad_unshare_cleanup_fd;

    - if (new_fs || new_ns || new_sigh || new_mm || new_fd || new_ulist) {
    + if (new_sigh) {
    + sigh = current->sighand;
    + write_lock_irq(&tasklist_lock);
    + spin_lock(&sigh->siglock);
    + memcpy(new_sigh->action, sigh->action, sizeof(sigh->action));
    + rcu_assign_pointer(current->sighand, new_sigh);
    + spin_unlock(&sigh->siglock);
    + write_unlock_irq(&tasklist_lock);
    + new_sigh = sigh;
    + }
    +
    + if (new_fs || new_ns || new_mm || new_fd || new_ulist) {

    task_lock(current);

    @@ -1595,12 +1612,6 @@ asmlinkage long sys_unshare(unsigned lon
    new_ns = ns;
    }

    - if (new_sigh) {
    - sigh = current->sighand;
    - rcu_assign_pointer(current->sighand, new_sigh);
    - new_sigh = sigh;
    - }
    -
    if (new_mm) {
    mm = current->mm;
    active_mm = current->active_mm;
    -
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