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

    > Eric W. Biederman [ebiederm@xmission.com] wrote:
    > | > Could you clarify ? How is the call to alloc_pidmap() from clone3() different
    > | > from the call from clone() itself ?
    > |
    > | I think it is totally inappropriate to assign pids in a pid namespace
    > | where there are user space processes already running.
    >
    > Honestly, I don't understand why it is inappropriate or how this differs
    > from normal clone() - which also assigns pids in own and ancestor pid
    > namespaces.

    The fact we can specify which pids we want. I won't claim it is as
    exploitable as NULL pointer deferences have been but it has that kind
    of feel to it.

    > | > | How we handle a clone extension depends critically on if we want to
    > | > | create a processes for restart in user space or kernel space.
    > | > |
    > | > | Could some one give me or point me at a strong case for creating the
    > | > | processes for restart in user space?
    > | >
    > | > There has been a lot of discussion on this with reference to the
    > | > Checkpoint/Restart patchset. See http://lkml.org/lkml/2009/4/13/401
    > | > for instance.
    > |
    > | Just read it. Thank you.
    >
    > Sorry. I should have mentioned the reason here. (Like you mention below),
    > flexibility is the main reason.
    >
    > | Now I am certain clone_with_pids() is not useful functionality to be
    > | exporting to userspace.
    > |
    > | The only real argument in favor of doing this in user space is greater
    > | flexibility. I can see checkpointing/restoring a single thread process
    > | without a pid namespace. Anything more and you are just asking for
    > | trouble.
    > |
    > | A design that weakens security. Increases maintenance costs. All for
    > | an unreliable result seems like a bad one to me.
    > |
    > | > | The pid assignment code is currently ugly. I asked that we just pass
    > | > | in the min max pid pids that already exist into the core pid
    > | > | assignment function and a constrained min/max that only admits a
    > | > | single pid when we are allocating a struct pid for restart. That was
    > | > | not done and now we have a weird abortion with unnecessary special cases.
    > | >
    > | > I did post a version of the patch attemptint to implement that. As
    > | > pointed out in:
    > | >
    > | > http://lkml.org/lkml/2009/8/17/445
    > | >
    > | > we would need more checks in alloc_pidmap() to cover cases like min or max
    > | > being invalid or min being greater than max or max being greater than pid_max
    > | > etc. Those checks also made the code ugly (imo).
    > |
    > | If you need more checks you are doing it wrong. The code already has min
    > | and max values, and even a start value. I was just strongly suggesting
    > | we generalize where we get the values from, and then we have not special
    > | cases.
    >
    > Well, if alloc_pidmap(pid_ns, min, max) does not have to check the
    > parameters passed in (ie assumes that callers pass it in correctly)
    > it might be simple. But when user specifies the pid, the
    >
    > min == max == user's target pid
    >
    > so we will need to check the values either here or in callers.

    Agreed. When you are talking about the target pid. That code path
    needs the extra check.

    > Yes the code already has values and a start value. But these are
    > controlled by alloc_pidmap() and not passed in from the user space.

    I was only thinking passed in from someplace else in kernel/pid.c

    > alloc_pidmap() needs to assign the next available pid or a specific
    > target pid. Generalizing it to alloc a pid in a range seemed be a
    > bit of an over kill for currently known usages.

    alloc_pidmap in assigning the next available pid is allocating a pid
    in a range.

    > I will post a version of the patch outside this patchset with min
    > and max parameters and we can see if it can be optimized/beautified.

    Thanks,
    Eric



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