Re: Compat 32-bit syscall entry from 64-bit task!?

[Andrew Lutomirski]

On Wed, Jan 18, 2012 at 11:31 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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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