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    SubjectRe: Compat 32-bit syscall entry from 64-bit task!?
    On Thu, Jan 19, 2012 at 11:21 AM, Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    > On Thu, Jan 19, 2012 at 8:01 AM, Jamie Lokier <jamie@shareable.org> wrote:
    >> Andrew Lutomirski wrote:
    >>> 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.)
    >>
    >> Here it is:
    >>
    >>        static inline bool user_64bit_mode(struct pt_regs *regs)
    >
    > This is pointless, even if it worked, which it clearly doesn't on Xen
    > (or other random situations).
    >
    > Why would you care?
    >
    > The issue is *not* whether somebody is running in 32-bit mode or 64-bit mode.
    >
    > The problem is the system call itself, and that can be 32-bit or
    > 64-bit independently of the execution mode. So knowing the user-mode
    > mode is simply not relevant.

    Unless you're writing a debugger and you want to disassemble the code
    that's being executed (i.e. normal code, not a system call). I wonder
    how gdb guesses whether the cpu is in long mode.

    --Andy
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