From: Kees Cook

Sent: 16 June 2020 04:25

For both pidfd and seccomp, the __user pointer is not used. Update
__fd_install_received() to make writing to ufd optional. (ufd
itself cannot checked for NULL because this changes the SCM_RIGHTS
interface behavior.) In these cases, the new fd needs to be returned
on success.  Update the existing callers to handle it. Add new wrapper
fd_install_received() for pidfd and seccomp that does not use the ufd
argument.

...> 

 static inline int fd_install_received_user(struct file *file, int __user *ufd,
 					   unsigned int o_flags)
 {
-	return __fd_install_received(file, ufd, o_flags);
+	return __fd_install_received(file, true, ufd, o_flags);
+}

Can you get rid of the 'return user' parameter by adding
	if (!ufd) return -EFAULT;
to the above wrapper, then checking for NULL in the function?

Or does that do the wrong horrid things in the fail path?

	David

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