[PATCH 86/87] fs: switch timespec64 fields in inode to discrete integers

Theodore Ts'o tytso at mit.edu
Fri Sep 29 07:26:56 AEST 2023

On Thu, Sep 28, 2023 at 01:40:55PM -0400, Jeff Layton wrote:
> Correct. We'd lose some fidelity in currently stored timestamps, but as
> Linus and Ted pointed out, anything below ~100ns granularity is
> effectively just noise, as that's the floor overhead for calling into
> the kernel. It's hard to argue that any application needs that sort of
> timestamp resolution, at least with contemporary hardware. 
> Doing that would mean that tests that store specific values in the
> atime/mtime and expect to be able to fetch exactly that value back would
> break though, so we'd have to be OK with that if we want to try it. The
> good news is that it's relatively easy to experiment with new ways to
> store timestamps with these wrappers in place.

The reason why we store 1ns granularity in ext4's on-disk format (and
accept that we only support times only a couple of centuries into the
future, as opposed shooting for an on-disk format good for several
millennia :-), was in case there was userspace that might try to store
a very fine-grained timestamp and want to be able to get it back
bit-for-bit identical.

For example, what if someone was trying to implement some kind of
steganographic scheme where they going store a secret message (or more
likely, a 256-bit AES key) in the nanosecond fields of the file's
{c,m,a,cr}time timestamps, "hiding in plain sight".  Not that I think
that we have to support something like that, since the field is for
*timestamps* not cryptographic bits, so if we break someone who is
doing that, do we care?

I don't think anyone will complain about breaking the userspace API
--- especially since if, say, the CIA was using this for their spies'
drop boxes, they probably wouldn't want to admit it.  :-)

       	    	     	      	      	    - Ted

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