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    SubjectRe: [PATCH] x86/mpx: Correctly report do_mpx_bt_fault() failures to user-space
    Hi Dave,

    On Mon, Apr 17, 2017 at 08:38:03AM -0700, Dave Hansen wrote:
    > Just to be clear, the thing you're calling "correct" is this do_trap(),
    > right?
    >
    > do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, NULL);

    Yes, because it signals the right trap_nr and error_code to user-space.

    > do_mpx_bt_fault() can fail for a bunch of reasons:
    > * unexpected or invalid value in BNDCSR
    > * out of memory (physical or virtual)
    > * unresolvable fault walking/filling bounds tables
    > * !valid and non-empty bad entry in the bounds tables
    >
    > This will end up sending a signal that *looks* like a X86_TRAP_BR for
    > all of those, including those that are not really bounds-related, like
    > unresolvable faults. We also don't populate enough information in the
    > siginfo that gets delivered for userspace to resolve the fault.
    >
    > I'm not sure this patch is the right thing.

    The problem is, without this patch the trap_nr reported to user-space is
    0, which maps to divide-by-zero. I think this is wrong, and since all
    failure cases from do_mpx_bt_fault() can only happen in the #BR
    exception handler, I think that reporting X86_TRAP_BR for all failure
    cases is the right thing to do.

    I don't know whether user-space (with this patch) already gets enough
    information from do_trap() to handle all of the above cases, but it is a
    step in the right direction.


    Joerg

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