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

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

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.

The only reason for IS_ENABLED(CONFIG_X86_FRED) (which is implied in
cpu_feature_enabled() anyway via !CONFIG_X86_DISABLED_FEATURE_FRED) is
to eliminate the code entirely if FRED is not compiled in.

Although the way it goes, it sounds like CONFIG_X86_FRED might go away
soon anyway, since KVM wants to use some of the FRED code
unconditionally (and Xen might follow, too.)

	-hpa