Re: [PATCH v2] x86/traps: Handle trap flag when instruction is emulated

[Andrew Cooper <andrew.cooper3@citrix.com>]

> Simulate the trap flag (TF) behavior when the kernel emulates UIMP
> instructions and iopl instructions.

You'll want to do this for CPUID too when CPUID Faulting is active.

> diff
> <https://lore.kernel.org/lkml/20250901162527.18247-2-samuele@cerea.dev/#iZ31arch:x86:kernel:traps.c>
> --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index
> 36354b470590..bea28473866b 100644 --- a/arch/x86/kernel/traps.c +++
> b/arch/x86/kernel/traps.c
> @@ -705,6 +718,7 @@ static bool fixup_iopl_exception(struct pt_regs
> *regs)  	}
>  
>  	regs->ip += 1;
> + emulate_trap_flag(regs);  	return true;
>  }
>  

There's a fun bug in fixup_iopl_exception() which you're turning from
latent to real.

Not all STI/CLI instructions are 1 byte long.  If they have a redundant
prefix, the old logic would simply brute-force through the instruction 1
prefix byte at a time.

But now, you'll generate SIGTRAP in the middle of the instruction.

~Andrew