Re: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases

[Oleg Nesterov <oleg@redhat.com>]

Hi Andy,

I am really sorry for delay.

This is on top of the recent change from Kees, right? Could me remind me
where can I found the tree this series based on? So that I could actually
apply these changes...

On 07/21, Andy Lutomirski wrote:
>
> +long syscall_trace_enter_phase2(struct pt_regs *regs, u32 arch,
> +				unsigned long phase1_result)
>  {
>  	long ret = 0;
> +	u32 work = ACCESS_ONCE(current_thread_info()->flags) &
> +		_TIF_WORK_SYSCALL_ENTRY;
> +
> +	BUG_ON(regs != task_pt_regs(current));
>  
>  	user_exit();
>  
> @@ -1458,17 +1562,20 @@ long syscall_trace_enter(struct pt_regs *regs)
>  	 * do_debug() and we need to set it again to restore the user
>  	 * state.  If we entered on the slow path, TF was already set.
>  	 */
> -	if (test_thread_flag(TIF_SINGLESTEP))
> +	if (work & _TIF_SINGLESTEP)
>  		regs->flags |= X86_EFLAGS_TF;

This looks suspicious, but perhaps I misread this change.

If I understand correctly, syscall_trace_enter() can avoid _phase2() above.
But we should always call user_exit() unconditionally?

And we should always set X86_EFLAGS_TF if TIF_SINGLESTEP? IIRC, TF can be
actually cleared on a 32bit kernel if we step over sysenter insn?

Oleg.