> On Mar 20, 2026, at 8:50 AM, Dave Hansen <[email protected]> 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! :)
+1 :)
This serves as a key architectural differentiator between FRED and the legacy
IDT framework.
For additional context, here is a fix to user segment selector values:
https://lore.kernel.org/all/174069328263.10177.6796873487608898067.tip-bot2@tip-bot2/
It’s worth noting that there was an attempt to fix this bug roughly three years
ago:
https://lore.kernel.org/lkml/[email protected]/
