Re: [PATCH 2/2] kernel/signal.c: explicitly initialize si_code and use ksig->info uniformly

[Wen Yang]

On 2024/2/23 03:05, Oleg Nesterov wrote:
> On 02/23, wenyang.linux@xxxxxxxxxxx wrote:
> > From: Wen Yang <wenyang.linux@xxxxxxxxxxx>
> >
> > By explicitly initializing ksig->info.si_code and uniformly using ksig->info,
> > get_signal() function could be slightly optimized, as folowes:
>
> I don't understand. Why do you think it will be optimized? in what sense?
>
> > 	clear_siginfo(&ksig->info);
> > 	ksig->info.si_signo = signr = SIGKILL;          --> missed si_code
>
> because we do not need to set .si_code in this case?
>
> > 	sigdelset(&current->pending.signal, SIGKILL);
> > 	trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,  --> unnecessary SEND_SIG_NOINFO
>
> Why do you think the usage of SEND_SIG_NOINFO is "unnecessary" or bad?
> To me this looks good.

Since it is called "SEND_SIG_NOINFO", but here it is neither SEND_SIG
nor NOINFO.
It is get_signal() here, and ksig->info has also been partially
initialized before calling trace_signal_deliver(). Below "goto fatal", 
do_coredump() also use the initialized ksig->info.


> > @@ -2732,8 +2732,9 @@ bool get_signal(struct ksignal *ksig)
> >   		     signal->group_exec_task) {
> >   			clear_siginfo(&ksig->info);
> >   			ksig->info.si_signo = signr = SIGKILL;
> > +			ksig->info.si_code = SI_USER;
> >   			sigdelset(&current->pending.signal, SIGKILL);
> > -			trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
> > +			trace_signal_deliver(SIGKILL, &ksig->info,
>
> Well. to me this look like the minor but unnecessary pessimization.
>
> AFAICS, we do not need to initialize .si_code. The usage if ksig->info
> instead of  means that TP_STORE_SIGINFO() will actually read
> the memory.
>
> Sorry, I don't understand the point at all :/
>
> and it seems that we can simply kill clear_siginfo(), but this is
> another story.



This is not right.

ksig->info will be passed to user space through do_coredump(), and the 
clear_siginfo() cannot be killed.


bool get_signal(struct ksignal *ksig)
{
..
                if ((signal->flags & SIGNAL_GROUP_EXIT) ||
                     signal->group_exec_task) {
                        clear_siginfo(&ksig->info);
                        ksig->info.si_signo = signr = SIGKILL;
..
                        goto fatal;
                }

        fatal:
..
                        do_coredump(&ksig->info);
                }



void do_coredump(const kernel_siginfo_t *siginfo)
{
..
        struct coredump_params cprm = {
                .siginfo = siginfo,
..
        };

..
       sub_info = call_usermodehelper_setup(..., &cprm);
..
       call_usermodehelper_exec(sub_info,...);




> Oleg.