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    SubjectRe: [PATCH v5 1/2] fork: extend clone3() to support CLONE_SET_TID
    On 08/11, Adrian Reber wrote:
    >
    > include/linux/pid.h | 2 +-
    > include/linux/sched/task.h | 1 +
    > include/uapi/linux/sched.h | 1 +
    > kernel/fork.c | 22 ++++++++++++++++++++--
    > kernel/pid.c | 36 +++++++++++++++++++++++++++++-------
    > 5 files changed, 52 insertions(+), 10 deletions(-)

    Looks good to me...

    A couple of nits below, but I won't insist, feel free to ignore.

    > +/*
    > + * Different sizes of struct clone_args
    > + */
    > +#define CLONE3_ARGS_SIZE_V0 64

    I don't really understand why do we want the "size < CLONE3_ARGS_SIZE_V0"
    check in copy_clone_args_from_user(), but I won't argue.

    > +/* V1 includes set_tid */
    > +#define CLONE3_ARGS_SIZE_V1 72

    unused?

    > @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
    > stackleak_task_init(p);
    >
    > if (pid != &init_struct_pid) {
    > - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
    > + if (args->set_tid && !ns_capable(
    > + p->nsproxy->pid_ns_for_children->user_ns,
    > + CAP_SYS_ADMIN)) {
    > + retval = -EPERM;
    > + goto bad_fork_cleanup_thread;
    > + }
    > + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);

    copy_process() is already huge and complex, why not move this check into
    alloc_pid() ? Again, note that is accepts the same ->pid_ns_for_children.

    > @@ -166,6 +166,9 @@ struct pid *alloc_pid(struct pid_namespace *ns)
    > struct upid *upid;
    > int retval = -ENOMEM;
    >
    > + if (set_tid < 0 || set_tid >= pid_max)
    > + return ERR_PTR(-EINVAL);
    > +
    > pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
    > if (!pid)
    > return ERR_PTR(retval);
    > @@ -186,12 +189,31 @@ struct pid *alloc_pid(struct pid_namespace *ns)
    > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
    > pid_min = RESERVED_PIDS;
    >
    > - /*
    > - * Store a null pointer so find_pid_ns does not find
    > - * a partially initialized PID (see below).
    > - */
    > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
    > - pid_max, GFP_ATOMIC);
    > + if (set_tid) {
    > + /*
    > + * Also fail if a PID != 1 is requested
    > + * and no PID 1 exists.
    > + */
    > + nr = -EINVAL;
    > + if (set_tid == 1 || !idr_is_empty(&tmp->idr))

    On the second thought, I think we should check ns->child_reaper != NULL
    rather than !idr_is_empty(), this looks more robust and clean.

    And this way alloc_pid() can do everything lockless at the start,

    if (set_tid) {
    if (set_tid < 0 || set_tid >= pid_max)
    return ERR_PTR(-EINVAL);
    if (set_tid != 1 && !ns->child_reaper)
    return ERR_PTR(-EINVAL);
    if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
    return ERR_PTR(-EPERM);
    }


    and just for record... this is off-topic and I need to recheck, but
    today "ns->pid_allocated = 0" in free_pid() doesn't look right to me...
    This logic predates unshare(CLONE_PIDNS).

    Oleg.

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