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    SubjectRe: [PATCH v3 4/6] sysctl: Warn when a clamped sysctl parameter is set out of range
    On Thu,  1 Mar 2018 12:43:38 -0500 Waiman Long <longman@redhat.com> wrote:

    > Even with clamped sysctl parameters, it is still not that straight
    > forward to figure out the exact range of those parameters. One may
    > try to write extreme parameter values to see if they get clamped.
    > To make it easier, a warning with the expected range will now be
    > printed in the kernel ring buffer when a clamped sysctl parameter
    > receives an out of range value.
    >
    > ...
    >
    > + if (clamped && param->name &&
    > + !(*param->flags & CTL_FLAGS_OOR_WARNED)) {
    > + proc_ctl_warn(d, param->name,
    > + param->min ? *param->min : -INT_MAX,
    > + param->max ? *param->max : INT_MAX, val);
    > + *param->flags |= CTL_FLAGS_OOR_WARNED;
    > + }

    The handling of ctl_table.flags looks racy on SMP or preemptible.
    That's not at all a serious problem in this usage, but such handling of
    ctl_table.flags may be a problem in the future. Which means that if
    some future user of this field *is* sensitive to races then people are
    going to have to come back to this code and add the needed locking.

    So we should at least think about what that locking is to be, and
    document it in some fashion. Do we already hold an appropriate lock at
    this time? If so, what is it?

    If some such future user of ctl_table.flags has to add a new lock to
    the ctl_table for this purpose then we just eliminated your use-16-bit
    space saving trick and we may as well use a ulong and operate on it
    with bitops.


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