Re: [PATCH v2 4/6] powerpc: use common ptrace_syscall_enter hook to handle _TIF_SYSCALL_EMU

[Sudeep Holla <sudeep.holla@arm.com>]

On Mon, Mar 18, 2019 at 06:20:24PM +0100, Oleg Nesterov wrote:
> On 03/18, Sudeep Holla wrote:
> >
> > --- a/arch/powerpc/kernel/ptrace.c
> > +++ b/arch/powerpc/kernel/ptrace.c
> > @@ -3278,35 +3278,29 @@ long do_syscall_trace_enter(struct pt_regs *regs)
> >
> >  	user_exit();
> >
> > -	flags = READ_ONCE(current_thread_info()->flags) &
> > -		(_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE);
> > -
> > -	if (flags) {
> > -		int rc = tracehook_report_syscall_entry(regs);
> > +	if (unlikely(ptrace_syscall_enter(regs))) {
> > +		/*
> > +		 * A nonzero return code from tracehook_report_syscall_entry()
> > +		 * tells us to prevent the syscall execution, but we are not
> > +		 * going to execute it anyway.
> > +		 *
> > +		 * Returning -1 will skip the syscall execution. We want to
> > +		 * avoid clobbering any registers, so we don't goto the skip
> > +		 * label below.
> > +		 */
> > +		return -1;
> > +	}
> >
> > -		if (unlikely(flags & _TIF_SYSCALL_EMU)) {
> > -			/*
> > -			 * A nonzero return code from
> > -			 * tracehook_report_syscall_entry() tells us to prevent
> > -			 * the syscall execution, but we are not going to
> > -			 * execute it anyway.
> > -			 *
> > -			 * Returning -1 will skip the syscall execution. We want
> > -			 * to avoid clobbering any registers, so we don't goto
> > -			 * the skip label below.
> > -			 */
> > -			return -1;
> > -		}
> > +	flags = READ_ONCE(current_thread_info()->flags) & _TIF_SYSCALL_TRACE;
>
> Why do we need READ_ONCE() with this change?
>
> And now that we change a single bit "flags" doesn't look like a good name.
>
> Again, to me this patch just makes the code look worse. Honestly, I don't
> think that the new (badly named) ptrace_syscall_enter() hook makes any sense.
>

Worse because we end up reading current_thread_info->flags twice ?

--
Regards,
Sudeep