Theodore Ts'o [off-list ref] writes:

On Tue, May 09, 2017 at 02:17:46PM -0700, Eric Biggers wrote:

quoted

1.) Privacy implications.  Say the filesystem is being shared between multiple
    users, and one user unpacks foo.tar.gz into their home directory, which
    they've set to mode 700 to hide from other users.  Because of this new
    ioctl, all users will be able to see every (inode number, size in blocks)
    pair that was added to the filesystem, as well as the exact layout of the
    physical block allocations which might hint at how the files were created.
    If there is a known "fingerprint" for the unpacked foo.tar.gz in this
    regard, its presence on the filesystem will be revealed to all users.  And
    if any filesystems happen to prefer allocating blocks near the containing
    directory, the directory the files are in would likely be revealed too.

Unix/Linux has historically not been terribly concerned about trying
to protect this kind of privacy between users.  So for example, in
order to do this, you would have to call GETFSMAP continously to track
this sort of thing.  Someone who wanted to do this could probably get
this information (and much, much more) by continuously running "ps" to
see what processes are running.

(I will note. wryly, that in the bad old days, when dozens of users
were sharing a one MIPS Vax/780, it was considered a *good* thing
that social pressure could be applied when it was found that someone
was running a CPU or memory hogger on a time sharing system.  The
privacy right of someone running "xtrek" to be able to hide this from
other users on the system was never considered important at all.  :-)

Fortunately, the days of timesharing seem to well behind us.  For
those people who think that containers are as secure as VM's (hah,
hah, hah), it might be that best way to handle this is to have a mount
option that requires root access to this functionality.  For those
people who really care about this, they can disable access.

What would be the reason for not putting this behind
capable(CAP_SYS_ADMIN)?

What possible legitimate function could this functionality serve to
users who don't own your filesystem?

I have seen several people speak up how this is a concern I don't see
anyone saying here is a legitimate use for a non-system administrator.

This doesn't seem like something where abuses of time-sharing systems
can be observed.

Eric