The resultant file descriptor acts the same as one produced by open(2) with O_PATH,
meaning it can also be used as a dirfd argument to other "*at()" system calls.

This may be interpreted by somebody that fsmount returns exactly same fd as open(2) with O_PATH returns.
This is not true, because, for example, you can use move_mount with fd, returned by fsmount,
but cannot use move_mount with fd, returned by open with O_PATH.

So, please replace "acts the same as one" with "can be used as one" or something.

Also, please, replace 'to other "*at()"' with 'to "*at()"'. No at-syscalls were
mentioned in this paragraph before, so "other" is not needed here.

/* Create a new file without attaching the mount object. */
int tmpfd = openat(sfd, "tmpfile", O_CREAT | O_EXCL | O_RDWR, 0600);
unlinkat(sfd, "tmpfile", 0);

You meant mfd here, not sfd.

--
Askar Safin