Re: [PATCH] signal: Don't init struct kernel_siginfo fields to zero again

[Rae Kim]

It looks like compiler optimization is smart enough to know that
assigning zero is unnecessary after clear_siginfo() which is memset()
under the hood. At least in my x86_64 machine, w/ or w/o this patch,
there is no difference in final compiled machine code. (I've compared
"objdump -d" results for "__send_signal()", "do_tkill()", and
"collect_signal()")

Wouldn't it be nicer to have more information for both human and
compiler since it doesn't generate extra machine code?

On Sun, Dec 20, 2020 at 03:21:35PM +0100, Oleg Nesterov wrote:
> On 12/20, Leesoo Ahn wrote:
> >
> > clear_siginfo() is responsible for clearing struct kernel_siginfo object.
> > It's obvious that manually initializing those fields is needless as
> > a commit[1] explains why the function introduced and its guarantee that
> > all bits in the struct are cleared after it.
> >
> > [1]: commit 8c5dbf2ae00b ("signal: Introduce clear_siginfo")
> >
> > Signed-off-by: Leesoo Ahn <lsahn@xxxxxxxxxx>
> 
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> 
> 
> > ---
> >  kernel/signal.c | 21 ---------------------
> >  1 file changed, 21 deletions(-)
> > 
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index 5736c55aaa1a..8f49fa3ade33 100644
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -603,10 +603,7 @@ static void collect_signal(int sig, struct sigpending *list, kernel_siginfo_t *i
> >  		 */
> >  		clear_siginfo(info);
> >  		info->si_signo = sig;
> > -		info->si_errno = 0;
> >  		info->si_code = SI_USER;
> > -		info->si_pid = 0;
> > -		info->si_uid = 0;
> >  	}
> >  }
> >  
> > @@ -1120,7 +1117,6 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
> >  		case (unsigned long) SEND_SIG_NOINFO:
> >  			clear_siginfo(&q->info);
> >  			q->info.si_signo = sig;
> > -			q->info.si_errno = 0;
> >  			q->info.si_code = SI_USER;
> >  			q->info.si_pid = task_tgid_nr_ns(current,
> >  							task_active_pid_ns(t));
> > @@ -1133,10 +1129,7 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
> >  		case (unsigned long) SEND_SIG_PRIV:
> >  			clear_siginfo(&q->info);
> >  			q->info.si_signo = sig;
> > -			q->info.si_errno = 0;
> >  			q->info.si_code = SI_KERNEL;
> > -			q->info.si_pid = 0;
> > -			q->info.si_uid = 0;
> >  			break;
> >  		default:
> >  			copy_siginfo(&q->info, info);
> > @@ -1623,10 +1616,7 @@ void force_sig(int sig)
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = sig;
> > -	info.si_errno = 0;
> >  	info.si_code = SI_KERNEL;
> > -	info.si_pid = 0;
> > -	info.si_uid = 0;
> >  	force_sig_info(&info);
> >  }
> >  EXPORT_SYMBOL(force_sig);
> > @@ -1659,7 +1649,6 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = sig;
> > -	info.si_errno = 0;
> >  	info.si_code  = code;
> >  	info.si_addr  = addr;
> >  #ifdef __ARCH_SI_TRAPNO
> > @@ -1691,7 +1680,6 @@ int send_sig_fault(int sig, int code, void __user *addr
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = sig;
> > -	info.si_errno = 0;
> >  	info.si_code  = code;
> >  	info.si_addr  = addr;
> >  #ifdef __ARCH_SI_TRAPNO
> > @@ -1712,7 +1700,6 @@ int force_sig_mceerr(int code, void __user *addr, short lsb)
> >  	WARN_ON((code != BUS_MCEERR_AO) && (code != BUS_MCEERR_AR));
> >  	clear_siginfo(&info);
> >  	info.si_signo = SIGBUS;
> > -	info.si_errno = 0;
> >  	info.si_code = code;
> >  	info.si_addr = addr;
> >  	info.si_addr_lsb = lsb;
> > @@ -1726,7 +1713,6 @@ int send_sig_mceerr(int code, void __user *addr, short lsb, struct task_struct *
> >  	WARN_ON((code != BUS_MCEERR_AO) && (code != BUS_MCEERR_AR));
> >  	clear_siginfo(&info);
> >  	info.si_signo = SIGBUS;
> > -	info.si_errno = 0;
> >  	info.si_code = code;
> >  	info.si_addr = addr;
> >  	info.si_addr_lsb = lsb;
> > @@ -1740,7 +1726,6 @@ int force_sig_bnderr(void __user *addr, void __user *lower, void __user *upper)
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = SIGSEGV;
> > -	info.si_errno = 0;
> >  	info.si_code  = SEGV_BNDERR;
> >  	info.si_addr  = addr;
> >  	info.si_lower = lower;
> > @@ -1755,7 +1740,6 @@ int force_sig_pkuerr(void __user *addr, u32 pkey)
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = SIGSEGV;
> > -	info.si_errno = 0;
> >  	info.si_code  = SEGV_PKUERR;
> >  	info.si_addr  = addr;
> >  	info.si_pkey  = pkey;
> > @@ -1934,7 +1918,6 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = sig;
> > -	info.si_errno = 0;
> >  	/*
> >  	 * We are under tasklist_lock here so our parent is tied to
> >  	 * us and cannot change.
> > @@ -2033,7 +2016,6 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = SIGCHLD;
> > -	info.si_errno = 0;
> >  	/*
> >  	 * see comment in do_notify_parent() about the following 4 lines
> >  	 */
> > @@ -2506,7 +2488,6 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info)
> >  	if (signr != info->si_signo) {
> >  		clear_siginfo(info);
> >  		info->si_signo = signr;
> > -		info->si_errno = 0;
> >  		info->si_code = SI_USER;
> >  		rcu_read_lock();
> >  		info->si_pid = task_pid_vnr(current->parent);
> > @@ -3660,7 +3641,6 @@ static inline void prepare_kill_siginfo(int sig, struct kernel_siginfo *info)
> >  {
> >  	clear_siginfo(info);
> >  	info->si_signo = sig;
> > -	info->si_errno = 0;
> >  	info->si_code = SI_USER;
> >  	info->si_pid = task_tgid_vnr(current);
> >  	info->si_uid = from_kuid_munged(current_user_ns(), current_uid());
> > @@ -3833,7 +3813,6 @@ static int do_tkill(pid_t tgid, pid_t pid, int sig)
> >  
> >  	clear_siginfo(&info);
> >  	info.si_signo = sig;
> > -	info.si_errno = 0;
> >  	info.si_code = SI_TKILL;
> >  	info.si_pid = task_tgid_vnr(current);
> >  	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
> > -- 
> > 2.26.2
> > 
>