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    SubjectRe: [PATCH v8 5/9] seccomp: split mode set routines
    On 06/25, Andy Lutomirski wrote:
    >
    > On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg@redhat.com> wrote:
    > > On 06/25, Andy Lutomirski wrote:
    > >>
    > >> Write the filter, then smp_mb (or maybe a weaker barrier is okay),
    > >> then set the bit.
    > >
    > > Yes, exactly, this is what I meant. Plas rmb() in __secure_computing().
    > >
    > > But I still can't understand the rest of your discussion about the
    > > ordering we need ;)
    >
    > Let me try again from scratch.
    >
    > Currently there are three relevant variables: TIF_SECCOMP,
    > seccomp.mode, and seccomp.filter. __secure_computing needs
    > seccomp.mode and seccomp.filter to be in sync, and it wants (but
    > doesn't really need) TIF_SECCOMP to be in sync as well.
    >
    > My suggestion is to rearrange it a bit. Move mode into seccomp.filter
    > (so that filter == NULL implies no seccomp) and don't check
    > TIF_SECCOMP in secure_computing. Then turning on seccomp is entirely
    > atomic except for the fact that the seccomp hooks won't be called if
    > filter != NULL but !TIF_SECCOMP. This removes all ordering
    > requirements.

    Ah, got it, thanks. Perhaps I missed somehing, but to me this looks like
    unnecessary complication at first glance.

    We alredy have TIF_SECCOMP, we need it anyway, and we should only care
    about the case when this bit is actually set, so that we can race with
    the 1st call of __secure_computing().

    Otherwise we are fine: we can miss the new filter anyway, ->mode can't
    be changed it is already nonzero.

    > Alternatively, __secure_computing could still BUG_ON(!seccomp.filter).
    > In that case, filter needs to be set before TIF_SECCOMP is set, but
    > that's straightforward.

    Yep. And this is how seccomp_assign_mode() already works? It is called
    after we change ->filter chain, it changes ->mode before set(TIF_SECCOMP)
    just it lacks a barrier.

    Oleg.



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