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    SubjectRe: Compat 32-bit syscall entry from 64-bit task!?
    On Wed, Jan 18, 2012 at 11:31 AM, Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    > The actually used code just does
    >
    >                /* Check CS register value. On x86-64 linux it is:
    >                 *      0x33    for long mode (64 bit)
    >                 *      0x23    for compatibility mode (32 bit)
    >                 * It takes only one ptrace and thus doesn't need
    >                 * to be cached.
    >                 */
    >                if (upeek(tcp, 8*CS, &val) < 0)
    >                        return -1;
    >                switch (val) {
    >                        case 0x23: currpers = 1; break;
    >                        case 0x33: currpers = 0; break;
    >
    > which is the reasonable and obvious approach.

    *sigh*

    It's reasonable, obvious, and even more wrong than it appears. On
    Xen, there's an extra 64-bit GDT entry, and it gets used by default.
    (I got bitten by this in some iteration of the vsyscall emulation
    patches -- see user_64bit_mode for the correct and
    unusable-from-user-mode way to do this.)

    --Andy
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