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

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

[wenyang.linux]

From: Wen Yang <wenyang.linux@foxmail.com>

By explicitly initializing ksig->info.si_code and uniformly using ksig->info,
get_signal() function could be slightly optimized, as folowes:

	clear_siginfo(&ksig->info);
	ksig->info.si_signo = signr = SIGKILL;          --> missed si_code
	sigdelset(&current->pending.signal, SIGKILL);
	trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,  --> unnecessary SEND_SIG_NOINFO
 			&sighand->action[SIGKILL - 1]);
			recalc_sigpending();
	goto fatal;
...

        fatal:
...
                  if (sig_kernel_coredump(signr)) {
...
                          do_coredump(&ksig->info);    --> contains si_code
                }

No functional change intended.

Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/signal.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 9b40109f0c56..8cab55bbec2f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -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,
 				&sighand->action[SIGKILL - 1]);
 			recalc_sigpending();
 			goto fatal;
-- 
2.25.1

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

[Oleg Nesterov]

On 02/23, wenyang.linux@foxmail.com wrote:
>
> From: Wen Yang <wenyang.linux@foxmail.com>
>
> 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.

> @@ -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 ksig->info 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.

Oleg.

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@foxmail.com wrote:
>>
>> From: Wen Yang <wenyang.linux@foxmail.com>
>>
>> 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.
> 

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

[Oleg Nesterov]

On 02/23, Wen Yang wrote:
>
> On 2024/2/23 03:05, Oleg Nesterov wrote:
> >On 02/23, wenyang.linux@foxmail.com wrote:
> >>
> >>From: Wen Yang <wenyang.linux@foxmail.com>
> >>
> >>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.

I don't really understand what does this mean. But I can say that
SEND_SIG_NOINFO is exactly what we should use, this signal has no
info.

In fact, SIGKILL can never have the info, see the sig == SIGKILL
check in __send_signal_locked() but this is offtopic.

> 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.

IIRC, do_coredump() paths use only siginfo->si_signo, but this doesn't
matter.

do_coredump() can't be called, sig_kernel_coredump(SIGKILL) is false.

> >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.

See above.

Oleg.