mount(8): Avoid truncation when fstab-formatting unionfs mount info

[jah]

Summary

When displaying unionfs mounts in fstab format (mount -p), mount(8)
currently uses strlcpy to remove the disposition prefix from the mount
name returned by getmntinfo(3). But strlcpy, like strcpy before it,
does not guaranteee correct behavior if the source and destination
buffers overlap.

Use memmove for this case instead.

PR: 283420
MFC after: 1 week

[jah]

sbin/mount/mount.c

913

Is there any concern about handling buffers that aren't NUL-terminated here?
I would guess not, since it seems that would be a serious failure on the part of the kernel to correctly populate the string (and there certainly doesn't seem to be such concern in the rest of this file), but I have to ask...

[imp]

sbin/mount/mount.c

913

Just 3 lines later we take atrlen of it again. And other options. These are actual arrays, not pointers to strings, so i think the worst case of malformed data would just produce garbled output for other fields.

But we trust the kernel as a source of data on a lot of places. The original strlcpy is clearly wrong and i would have used a pointer to this field below and offser it in the above/below: cases to avoid the copy.

[jah]

sbin/mount/mount.c

913

Yep, I noticed the same things.

I'd been assuming there was some other reason we didn't want to just offset the pointer, but it really doesn't look as though there is. It also seems dumb to destructively modify the buffer like this in case we do later end up wanting the original contents for some reason.

[jah]

Avoid destructive copy