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    SubjectRe: [PATCH v3 2/4] x86, mpx: hook #BR exception handler to allocate bound tables
    On 01/28/2014 01:21 PM, Andy Lutomirski wrote:
    > On Mon, Jan 27, 2014 at 7:35 PM, Ren Qiaowei <qiaowei.ren@intel.com> wrote:
    >> On 01/28/2014 04:36 AM, Andy Lutomirski wrote:
    >>>>
    >>>> + bd_entry = status & MPX_BNDSTA_ADDR_MASK;
    >>>> + if ((bd_entry >= bd_base) && (bd_entry < bd_base + bd_size))
    >>>> + allocate_bt(bd_entry);
    >>>
    >>>
    >>> What happens if this fails? Retrying forever isn't very nice.
    >>>
    >> If allocation of the bound table fail, the related entry in the bound
    >> directory is still invalid. The following access to this entry still produce
    >> #BR fault.
    >>
    >
    > By the "following access" I think you mean the same instruction that
    > just trapped -- it will trap again because the exception hasn't been
    > fixed up. Then mmap will fail again, and you'll retry again, leading
    > to an infinite loop.
    >
    I don't mean the same instruction that just trapped.

    > I think that failure to fix up the exception should either let the
    > normal bounds error through or should raise SIGBUS.
    >
    Maybe we need HPA help answer this question. Peter, what do you think
    about it? If allocation of the bound table fail, what should we do?

    >>
    >>>> + if (!user_mode(regs)) {
    >>>> + if (!fixup_exception(regs)) {
    >>>> + tsk->thread.error_code = error_code;
    >>>> + tsk->thread.trap_nr = X86_TRAP_BR;
    >>>> + die("bounds", regs, error_code);
    >>>> + }
    >>>
    >>>
    >>> Why the fixup? Unless I'm missing something, the kernel has no business
    >>> getting #BR on access to a user address.
    >>>
    >>> Or are you adding code to allow the kernel to use MPX itself? If so,
    >>> shouldn't this use an MPX-specific fixup to allow normal C code to use
    >>> this stuff?
    >>>
    >> It checks whether #BR come from user-space. You can see do_trap_no_signal().
    >
    > Wasn't #BR using do_trap before? do_trap doesn't call
    > fixup_exception. I don't see why it should do it now. (I also don't
    > think it should come from kernel space until someone adds kernel-mode
    > MPX support.)
    >
    do_trap() -> do_trap_no_signal() call similar code to check if the fault
    occurred in userspace or kernel space. You can see previous discussion
    for the first version of this patchset.

    Thanks,
    Qiaowei


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