fix type of semctl variadic argument

per POSIX, the variadic argument has type union semun, which may
contain a pointer or int; the type read depends on the command being
issued. this allows the userspace part of the implementation to be
type-correct without requiring special-casing for different commands.
the kernel always expects to receive the argument interpreted as
unsigned long (or equivalently, a pointer), and does its own handling
of extracting the int portion from the representation, as needed.

this change fixes two possible issues: most immediately, reading the
argument as a (signed) long and passing it to the syscall would
perform incorrect sign-extension of pointers on the upcoming x32
target. the other possible issue is that some archs may use different
(user-space) argument-passing convention for unions, preventing va_arg
from correctly obtaining the argument when the type long (or even
unsigned long or void *) is passed to it.

src/ipc/semctl.c

@@ -3,16 +3,22 @@
 #include "syscall.h"
 #include "ipc.h"
 
+struct semun {
+	int val;
+	struct semid_ds *buf;
+	unsigned short *array;
+};
+
 int semctl(int id, int num, int cmd, ...)
 {
-	long arg;
+	struct semun arg;
 	va_list ap;
 	va_start(ap, cmd);
-	arg = va_arg(ap, long);
+	arg = va_arg(ap, struct semun);
 	va_end(ap);
 #ifdef SYS_semctl
-	return syscall(SYS_semctl, id, num, cmd | IPC_64, arg);
+	return syscall(SYS_semctl, id, num, cmd | IPC_64, arg.buf);
 #else
-	return syscall(SYS_ipc, IPCOP_semctl, id, num, cmd | IPC_64, &arg);
+	return syscall(SYS_ipc, IPCOP_semctl, id, num, cmd | IPC_64, &arg.buf);
 #endif
 }