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    SubjectRe: [BUG][PATCH -mm] avoid BUG() in __stop_machine_run()
    Rusty Russell wrote:
    > On Thursday 19 June 2008 16:59:50 Hidehiro Kawai wrote:
    >
    >> When a process loads a kernel module, __stop_machine_run() is called, and
    >> it calls sched_setscheduler() to give newly created kernel threads highest
    >> priority. However, the process can have no CAP_SYS_NICE which required
    >> for sched_setscheduler() to increase the priority. For example, SystemTap
    >> loads its module with only CAP_SYS_MODULE. In this case,
    >> sched_setscheduler() returns -EPERM, then BUG() is called.
    >>
    >
    > Hi Hidehiro,
    >
    > Nice catch. This can happen in the current code, it just doesn't
    > BUG().
    >
    >
    >> Failure of sched_setscheduler() wouldn't be a real problem, so this
    >> patch just ignores it.
    >>
    >
    > Well, it can mean that the stop_machine blocks indefinitely. Better
    > than a BUG(), but we should aim higher.
    >
    >
    >> Or, should we give the CAP_SYS_NICE capability temporarily?
    >>
    >
    > I don't think so. It can be seen from another thread, and in theory
    > that should not see something random. Worse, they can change it from
    > another thread.
    >
    > How's this?
    >
    > sched_setscheduler: add a flag to control access checks
    >
    > Hidehiro Kawai noticed that sched_setscheduler() can fail in
    > stop_machine: it calls sched_setscheduler() from insmod, which can
    > have CAP_SYS_MODULE without CAP_SYS_NICE.
    >
    > This simply introduces a flag to allow us to disable the capability
    > checks for internal callers (this is simpler than splitting the
    > sched_setscheduler() function, since it loops checking permissions).
    >
    What about?

    int sched_setscheduler(struct task_struct *p, int policy,
    struct sched_param *param)
    {
    return __sched_setscheduler(p, policy, param, true);
    }


    int sched_setscheduler_nocheck(struct task_struct *p, int policy,
    struct sched_param *param)
    {
    return __sched_setscheduler(p, policy, param, false);
    }


    (With the appropriate transformation of sched_setscheduler -> __)

    Better than scattering stray true/falses around the code.

    J


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