Fix compat_sys_sigpending breakage

The latest change of compat_sys_sigpending in commit 8f13621abced
("sigpending(): move compat to native") has broken it in two ways.

First, it tries to write 4 bytes more than userspace expects:
sizeof(old_sigset_t) == sizeof(long) == 8 instead of
sizeof(compat_old_sigset_t) == sizeof(u32) == 4.

Second, on big endian architectures these bytes are being written in the
wrong order.

This bug was found by strace test suite.

Reported-by: Anatoly Pugachev <[e-mail]>
Inspired-by: Eugene Syromyatnikov <[e-mail]>
Fixes: 8f13621abced ("sigpending(): move compat to native")
Signed-off-by: Dmitry V. Levin <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/signal.c

···
 #ifdef CONFIG_COMPAT
 COMPAT_SYSCALL_DEFINE1(sigpending, compat_old_sigset_t __user *, set32)
 {
+#ifdef __BIG_ENDIAN
 	sigset_t set;
-	int err = do_sigpending(&set, sizeof(old_sigset_t)); 
-	if (err == 0)
-		if (copy_to_user(set32, &set, sizeof(old_sigset_t)))
-			err = -EFAULT;
+	int err = do_sigpending(&set, sizeof(set.sig[0]));
+	if (!err)
+		err = put_user(set.sig[0], set32);
 	return err;
+#else
+	return sys_rt_sigpending((sigset_t __user *)set32, sizeof(*set32));
+#endif
 }
 #endif