Re: [PATCH v1 1/1] x86/fred/signal: Prevent single-step upon ERETU completion

[Xin Li <xin@zytor.com>]

On 5/22/2025 12:20 AM, H. Peter Anvin wrote:
> On 5/21/25 23:05, Xin Li (Intel) wrote:
>>   
>> +/*
>> + * To prevent infinite SIGTRAP handler loop if TF is used without an external
>> + * debugger, clear the software event flag in the augmented SS, ensuring no
>> + * single-step trap is pending upon ERETU completion.
>> + *
>> + * Note, this function should be called in sigreturn() before the original state
>> + * is restored to make sure the TF is read from the entry frame.
>> + */
>> +static __always_inline void prevent_single_step_upon_eretu(struct pt_regs *regs)
>> +{
>> +	/*
>> +	 * If the trap flag (TF) is set, i.e., the sigreturn() SYSCALL instruction
>> +	 * is being single-stepped, do not clear the software event flag in the
>> +	 * augmented SS, thus a debugger won't skip over the following instruction.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_X86_FRED) && cpu_feature_enabled(X86_FEATURE_FRED) &&
>> +	    !(regs->flags & X86_EFLAGS_TF))
>> +		regs->fred_ss.swevent = 0;
>> +}
>> +
> 
> Minor nit (and I should have caught this when I saw your patch earlier):
> 
> cpu_feature_enabled(X86_FEATURE_FRED) is unnecessary here, because when
> FRED is not enabled, regs->fred_ss.swevent will always be 0, and this
> bit has no function, so there is no point in making that extra test.

Yeah, less conditions, less complexity.  I will remove it.

Thanks!
     Xin