cr-exec: initialize kdat.{task_size, has_compat_sigreturn} on criu exec

For `criu exec` we are searching for a place for syscall injection.
While searching for a VMA with PROT_EXEC and with needed size,
we check that VMA is lower than task_size.
The callpath for it is:
cr_exec => parasite_prep_ctl => get_vma_by_ip

Firstly, I thought to omit kdat.task_size checking if it's not inited:
> if (vma_area->e->start >= kdat.task_size && kdat.task_size)
but I think it's a hack then a proper solution.
Besides, this code still can choose VMA over task_size on ARM
and try to inject syscall there (IIRC, ARM has kernel-mapped
VMA in that area).

So, lets init kdat.task_size for `criu exec`.
Also lets init kdat.has_compat_sigreturn so we could exec into
compatible applications.

Cc: Christopher Covington <cov@codeaurora.org>
Cc: Andrew Vagin <avagin@virtuozzo.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cr-exec.c

@@ -7,6 +7,7 @@
 #include "vma.h"
 #include "log.h"
 #include "util.h"
+#include "kerndat.h"
 
 struct syscall_exec_desc {
 	char *name;
@@ -125,6 +126,11 @@ int cr_exec(int pid, char **opt)
 		goto out;
 	}
 
+	if (kerndat_init_cr_exec()) {
+		pr_err("Failed to init kerndat\n");
+		goto out;
+	}
+
 	si = find_syscall(sys_name);
 	if (!si) {
 		pr_err("Unknown syscall [%s]\n", sys_name);

criu/include/kerndat.h

@@ -12,6 +12,7 @@ struct stat;
 
 extern int kerndat_init(void);
 extern int kerndat_init_rst(void);
+extern int kerndat_init_cr_exec(void);
 extern int kerndat_get_dirty_track(void);
 extern int kerndat_fdinfo_has_lock(void);
 extern int kerndat_loginuid(bool only_dump);

criu/kerndat.c

@@ -506,3 +506,12 @@ int kerndat_init_rst(void)
 
 	return ret;
 }
+
+int kerndat_init_cr_exec(void)
+{
+	int ret;
+
+	ret = get_task_size();
+
+	return ret;
+}