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

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