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It was forbidden to create mount units for a symlink. But the reason is that the mount unit needs to know the real path that will appear in /proc/self/mountinfo. The kernel dereferences *all* the symlinks in the path at mount time (I checked this with `mount -c` running under `strace`). This will have no effect on most systems. As recommended by docs, most systems use /etc/fstab, as opposed to native mount unit files. fstab-generator dereferences symlinks for backwards compatibility. A relatively minor issue regarding Time Of Check / Time Of Use also exists here. I can't see how to get rid of it entirely. If we pass an absolute path to mount, the racing process can replace it with a symlink. If we chdir() to the mount point and pass ".", the racing process can move the directory. The latter might potentially be nicer, except that it breaks WorkingDirectory=. I'm not saying the race is relevant to security - I just want to consider how bad the effect is. Currently, it can make the mount unit active (and hence the job return success), despite there never being a matching entry in /proc/self/mountinfo. This wart will be removed in the next commit; i.e. it will make the mount unit fail instead.
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