work around wrong kernel type for sem_nsems member of struct semid_ds

rejecting invalid values for n is fine even in the case where a new
sem will not be created, since the kernel does its range checks on n
even in this case as well.

by default, the kernel will bound the limit well below USHRT_MAX
anyway, but it's presumably possible that an administrator could
override this limit and break things.

include/sys/sem.h

@@ -25,13 +25,21 @@ extern "C" {
 #define SETVAL		16
 #define SETALL		17
 
+#include <endian.h>
+
 struct semid_ds {
 	struct ipc_perm sem_perm;
 	long sem_otime;
 	unsigned long __unused1;
 	long sem_ctime;
 	unsigned long __unused2;
-	unsigned long sem_nsems;
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+	unsigned short sem_nsems;
+	char __sem_nsems_pad[sizeof(long)-sizeof(short)];
+#else
+	char __sem_nsems_pad[sizeof(long)-sizeof(short)];
+	unsigned short sem_nsems;
+#endif
 	unsigned long __unused3;
 	unsigned long __unused4;
 };

src/ipc/semget.c

 #include <sys/sem.h>
+#include <limits.h>
+#include <errno.h>
 #include "syscall.h"
 #include "ipc.h"
 
 int semget(key_t key, int n, int fl)
 {
+	/* The kernel uses the wrong type for the sem_nsems member
+	 * of struct semid_ds, and thus might not check that the
+	 * n fits in the correct (per POSIX) userspace type, so
+	 * we have to check here. */
+	if (n > USHRT_MAX) return __syscall_ret(-EINVAL);
 #ifdef SYS_semget
 	return syscall(SYS_semget, key, n, fl);
 #else