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    SubjectRe: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF
    On Tue, Jan 17, 2012 at 11:01 AM, Andrew Lutomirski <> wrote:
    > On Tue, Jan 17, 2012 at 8:56 AM, Will Drewry <> wrote:
    >> On Tue, Jan 17, 2012 at 10:45 AM, Oleg Nesterov <> wrote:
    >>> On 01/16, Will Drewry wrote:
    >>>> On Mon, Jan 16, 2012 at 12:37 PM, Oleg Nesterov <> wrote:
    >>>> >
    >>>> > Yes, thanks, I forgot about compat tasks again. But this is easy, just
    >>>> > we need regs_64_to_32().
    >>>> Yup - we could make the assumption that is_compat_task is always
    >>>> 32-bit and the pt_regs is always 64-bit, then copy_and_truncate with
    >>>> regs_64_to_32.  Seems kinda wonky though :/
    >>> much simpler/faster than what regset does to create the artificial
    >>> user_regs_struct32.
    >> True, I could collapse pt_regs to looks like the exported ABI pt_regs.
    >>  Then only compat processes would get the copy overhead.  That could
    >> be tidy and not break ABI.  It would mean that I have to assume that
    >> if unsigned long == 64-bit and is_compat_task(), then the task is
    >> 32-bit.  Do you think if we ever add a crazy 128-bit "supercomputer"
    >> arch that we will add a is_compat64_task() so that I could properly
    >> collapse? :)
    >> I like this idea!
    > FWIW, it's possible for a task to execute in 32-bit mode when
    > !is_compat_task or in 64-bit mode when is_compat_task.  From earlier
    > in the thread, I think you were planning to block the wrong-bitness
    > syscall entries, but it's worth double-checking that you don't open up
    > a hole when a compat task issues the 64-bit syscall instruction.

    Yup - I had to (see below).

    > (is_compat_task says whether the executable was marked as 32-bit.  The
    > actual execution mode is determined by the cs register, which the user
    > can control.  See the user_64bit_mode function in
    > arch/asm/x86/ptrace.h.  But maybe it would make more sense to have a
    > separate 32-bit and 64-bit BPF program and select which one to use
    > based on the entry point.)

    So that was my original design, but the problem was with how regviews
    decides on the user_regs_struct. It decides using TIF_IA32 while I
    can only check the cross-arch is_compat_task() which checks TS_COMPAT
    on x86. If I'm just collapsing registers for compat calls (which I am
    exploring the viability of right now), then I guess I could re-fork
    the filtering to support compat versus non-compat. The nastier bits
    there were that I don't want to allow a compat call to be allowed
    because a process only defined non-compat. I think that can be made
    manage-able though.

    I'll finish proving out the possibilities here.

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