On Fri, Mar 20, 2026 at 08:50:54AM -0700, Dave Hansen wrote:
> On 3/20/26 08:47, Andrew Cooper wrote:
> >> First, CPUID doesn't tell you if FRED is in use. Is it even on by
> >> default yet? There might not be a better way to do this than checking
> >> CPUID, but checking CPUID is imprecise at best.
> > A reliable way to distinguish IDT and FRED mode is to:
> >
> > 1) Load $3 into %fs (x86_64) or %gs (i386) (i.e. whichever isn't thread
> > local stoage)
> > 2) execute a breakpoint, ignore the signal
> > 3) Look to see whether %fs/%gs holds 3 or 0
> >
> > IRET has a fun behaviour where it zeroes NULL selectors even if they had
> > a non-zero RPL.
> >
> > ERETU doesn't do this; Andy Luto and I asked for this minor information
> > leak to be removed, and Intel agreed as it served no purpose anyone
> > could identify.
> >
> > As a consequence, you can use it to determine whether the kernel used
> > IRET or ERET to return back to userspace.
>
> I was thinking of just grepping /proc/cpuinfo for "fred", but that
> sounds much more fun! :)
Thank you both for the review and suggestions. The behavioral difference
between IRET and ERETU is a more robust way to detect FRED activation
than checking CPUID.
How about the following implementation to add a helper function to
determine if FRED is enabled at runtime:
static void empty_handler(int sig, siginfo_t *info, void *ctx_void)
{
}
static bool is_fred_enabled(void)
{
unsigned short gs_val;
sethandler(SIGTRAP, empty_handler, 0);
/*
* Distinguish IDT and FRED mode by loading GS with a non-zero RPL and
* triggering an exception:
* IDT (IRET) clears RPL bits of NULL selectors.
* FRED (ERETU) preserves them.
*
* If GS is loaded with 3 (Index=0, RPL=3), and we trigger an exception:
* Legacy should restore GS as 0.
* FRED should preserve GS as 3.
*/
asm volatile(
"mov $3, %%ax\n\t"
"mov %%ax, %%gs\n\t"
"int3\n\t"
"mov %%gs, %%ax\n\t"
"mov %%ax, %0\n\t"
: "=r" (gs_val)
:
: "ax", "memory"
);
clearhandler(SIGTRAP);
return gs_val == 3;
}
