restore: set up correct sa_restorer for SIGCHLD handler (v3)

Alexander, could you help me with ARM? I found in glibc sources, that
sa_restoer should look like this:

ports/sysdeps/unix/sysv/linux/arm/sigrestorer.S
....
ENTRY(__cr_restore_rt)
        CFI
        mov     r7, $SYS_ify(rt_sigreturn)
        swi     0x0
        .fnend
END(__cr_restore_rt)

v2: Move architecture-dependent code in the proper place
v3: Don't add a separate handler for collecting zombies

Cc: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

arch/arm/include/asm/types.h

@@ -99,6 +99,8 @@ static inline void ksigfillset(k_rtsigset_t *set)
 		set->sig[i] = (unsigned long)-1;
 }
 
+#define SA_RESTORER	0x04000000
+
 typedef struct {
 	rt_sighandler_t	rt_sa_handler;
 	unsigned long	rt_sa_flags;

arch/x86/include/asm/types.h

@@ -108,6 +108,8 @@ static inline void ksigfillset(k_rtsigset_t *set)
 		set->sig[i] = (unsigned long)-1;
 }
 
+#define SA_RESTORER	0x04000000
+
 typedef struct {
 	rt_sighandler_t	rt_sa_handler;
 	unsigned long	rt_sa_flags;

arch/x86/syscall-common-x86-64.S

@@ -14,3 +14,8 @@ ENTRY(__syscall_common)
 	syscall
 	ret
 END(__syscall_common)
+
+ENTRY(__cr_restore_rt)
+	movq $__NR_rt_sigreturn, %rax
+	syscall
+END(__cr_restore_rt)

pie/restorer.c

@@ -11,6 +11,7 @@
 #include <unistd.h>
 #include <sched.h>
 #include <sys/resource.h>
+#include <signal.h>
 
 #include "compiler.h"
 #include "asm/types.h"
@@ -39,6 +40,9 @@
 static struct task_entries *task_entries;
 static futex_t thread_inprogress;
 
+extern void cr_restore_rt (void) asm ("__cr_restore_rt")
+			__attribute__ ((visibility ("hidden")));
+
 static void sigchld_handler(int signal, siginfo_t *siginfo, void *data)
 {
 	char *r;
@@ -433,8 +437,11 @@ long __export_restore_task(struct task_restore_core_args *args)
 	rt_sigaction_t act;
 
 	task_entries = args->task_entries;
-	sys_sigaction(SIGCHLD, NULL, &act, sizeof(k_rtsigset_t));
+
+	ksigfillset(&act.rt_sa_mask);
 	act.rt_sa_handler = sigchld_handler;
+	act.rt_sa_flags = SA_SIGINFO | SA_RESTORER | SA_RESTART;
+	act.rt_sa_restorer = cr_restore_rt;
 	sys_sigaction(SIGCHLD, &act, NULL, sizeof(k_rtsigset_t));
 
 	log_set_fd(args->logfd);