Re: [PATCH v3] selftests/x86: Fix sysret_rip assertion failure on FRED systems

["H. Peter Anvin" <hpa@zytor.com>]

On March 31, 2026 6:59:06 PM PDT, Xin Li <xin@zytor.com> wrote:
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>
>> On Mar 30, 2026, at 11:03 PM, Xin Li <xin@zytor.com> wrote:
>> 
>> 
>>>>>> The existing 'sysret_rip' selftest asserts that 'regs->r11 ==
>>>>>> regs->flags'. This check relies on the behavior of the SYSCALL
>>>>>> instruction on legacy x86_64, which saves 'RFLAGS' into 'R11'.
>>>>>> 
>>>>>> However, on systems with FRED (Flexible Return and Event Delivery)
>>>>>> enabled, instead of using registers, all state is saved onto the stack.
>>>>>> Consequently, 'R11' retains its userspace value, causing the assertion
>>>>>> to fail.
>>>>>> 
>>>>>> Fix this by detecting if FRED is enabled and skipping the register
>>>>>> assertion in that case. The detection is done by checking if the RPL
>>>>>> bits of the GS selector are preserved after a hardware exception.
>>>>>> IDT (via IRET) clears the RPL bits of NULL selectors, while FRED (via
>>>>>> ERETU) preserves them.
>>>>>> 
>>>>> 
>>>>> I don't really like this.  I think we have two credible choices:
>>>>> 
>>>>> 1. Define the Linux ABI to be that, on FRED systems, SYSCALL preserves
>>>>> R11 and RCX on entry and exit.  And update the test to actually test
>>>>> this.
>>>>> 
>>>>> 2. Define the Linux ABI to be what it has been for quite a few years:
>>>>> SYSCALL entry copies RFLAGS to R11 and RIP to RCX and SYSCALL exit
>>>>> preserves all registers.
>>>>> 
>>>>> I'm in favor of #2.  People love making new programming languages and
>>>>> runtimes and inline asm and, these days, vibe coded crap.  And it's
>>>>> *easier* to emit a SYSCALL and forget to tell the compiler / code
>>>>> generator that RCX and R11 are clobbered than it is to remember that
>>>>> they're clobbered.  And it's easy to test on FRED (well, not really,
>>>>> but it hopefully will be some day) and it's easy to publish one's
>>>>> code, and then everyone is a bit screwed when the resulting program
>>>>> crashes sometimes on non-FRED systems.  And it will be miserable to
>>>>> debug.
>>>>> 
>>>>> (It's *really* *really* easy to screw this up in a way that sort of
>>>>> works even on non-FRED: RCX and R11 are usually clobbered across
>>>>> function calls, so one can get into a situation in which one's
>>>>> generated code usually doesn't require that SYSCALL preserve one of
>>>>> these registers until an inlining decision changes or some code gets
>>>>> reordered, and then it will start failing.  And making the failure
>>>>> depend on hardware details is just nasty.
>>>>> 
>>>>> So I think we should add the ~2 lines of code to fix the SYSCALL entry
>>>>> on FRED to match non-FRED.
>>>> 
>>>> Yes; I'm afraid I have to concur. Preserving the clobber on entry for
>>>> FRED systems is by far the safest choice. 
>>>> 
>>>> Aside from this selftest, fancy debuggers and anything that can transfer
>>>> userspace state between machines might be 'surprised'.
>>> 
>>> Thanks Andy and Peter.
>>> 
>>> Indeed, making the selftest branch on FRED vs. non-FRED behavior
>>> is not a good practice. The selftest should validate ABI consistency.
>>> 
>>> I agree with Andy's option #2, so this should be fixed in the FRED
>>> syscall entry implementation.
>>> 
>>> Li Xin, does this direction look right to you? I can assit with
>>> validation and keep the selftest aligned with the agreed ABI.
>>> 
>> 
>> Yes, consistency should take precedence over hardware-specific variations.
>> 
>> I would like to hear from Andrew Cooper and hpa before we do it.
>
>Per Andy’s suggestion, the change would be:
>
>diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
>index 88c757ac8ccd..a19898747a2c 100644
>--- a/arch/x86/entry/entry_fred.c
>+++ b/arch/x86/entry/entry_fred.c
>@@ -79,6 +79,9 @@ static __always_inline void fred_other(struct pt_regs *regs)
> {
> 	/* The compiler can fold these conditions into a single test */
> 	if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.l)) {
>+		regs->cx = regs->ip;
>+		regs->r11 = regs->flags;
>+
> 		regs->orig_ax = regs->ax;
> 		regs->ax = -ENOSYS;
> 		do_syscall_64(regs, regs->orig_ax);
>
>It adds 4 extra MOVs on this hot path, but I don’t see it's a problem here.
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We discussed this over a year ago, and at that point agreed that reserving the register was the desired behavior. Why has this changed now?