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    SubjectRe: [RFC][v8][PATCH 0/10] Implement clone3() system call
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    Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> writes:

    > Eric W. Biederman [ebiederm@xmission.com] wrote:
    > | > | + if (target < RESERVED_PIDS)
    > | >
    > | > Should we replace RESERVED_PIDS with 0 ? We currently allow new
    > | > containers to have pids 1..32K in the first pass and in subsequent
    > | > passes assign starting at RESERVED_PIDS.
    > |
    > | If it is a preexisting namespace pid namespace removing the RESERVED_PIDS
    > | check removes most if not all of the point of RESERVED_PIDS.
    > |
    > | In a new fresh pid namespace I have no problem with not performing
    > | the RESERVED_PIDS check.
    >
    > In that case can we do this
    >
    > if (target_pid < RESERVED_PIDS && !pid_ns->level)
    > return -EINVAL;
    >
    > instead ?
    > |
    > | So I guess that makes the check.
    > |
    > | if ((target < RESERVED_PIDS) && pid_ns->last_pid >= RESERVED_PIDS)
    > | return -EINVAL;
    >
    > I am just wondering if there is a small corner case where C/R would randomly
    > fail because of this sequence:
    >
    > - C/R code calls clone() or clone3() say about RESERVED_PIDS-1
    > times and ->last_pid == RESERVED_PIDS-1.
    >
    > - C/R code calls normal fork()/alloc_pidmap() for a short-lived
    > child - its pid == ->last_pid == RESERVED_PIDS
    >
    > - C/R code then calls clone3()/set_pidmap() to set the pid of
    > a new child to RESERVED_PID but fails (i.e it fails to restore
    > a pid even when the pid is not in use).
    >
    > We could argue that mixing alloc_pidmap() and set_pidmap() during restart
    > is bad since set_pidmap() may fail.
    >
    > The C/R developer could argue that we are forcing them to specify a pid
    > even for a short lived process that they wait()s on and thus ensure that
    > pid is not in use.
    >
    > Anyway, is RESERVED_PIDS meant for initial kernel-threads/daemons - if so
    > would it be ok enforce it only in init_pid_ns ?

    It is mean for initial user space daemons, things that start on boot.

    I don't know how much the protection matters at this date, but we have it.

    Eric


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