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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