Re: [PATCH v5 2/5] vfs: Add checks for filesystem timestamp limits

[Arnd Bergmann <arnd@arndb.de>]

On Sun, Apr 9, 2017 at 4:58 AM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
>>> Allow read only mounts for filesystems that do not
>>> have maximum timestamps beyond the y2038 expiry
>>> timestamp.
>>
>> This option seems arbitrary and pointless.
>>
>> Nobody sane should ever enable it except for testing, but for testing
>> it would be much better to simply specify what the limit should be:
>> 2038 is not magical for all filesystems, because the base may be
>> different.
>
> Yes, the way the patch is right now, it is meant only for testing
> y2038 readiness.
> The feature is meant for system wide tests and not individual filesystem tests.

There is one global option that I want to see, and that is for completely
disabling all components that are known to be broken in y2038.

We could do this with just a compile-time option that primarily
turns off all drivers using the 32-bit time_t, but the same compile-time
option can also force the file system to be read-only.

I don't see this just as something we want to do for testing, but
also as a safeguard for people shipping embedded systems with
long service life: If something can go wrong after write-mounting
an ext3 file system after 2038, it's better to force a behavior now
that can be reasonably expected not to change.

Between doing a compile-time option or a boot-time option, doing
it purely compile-time is probably better as it gives us the possible
additional checking when we hide the time_t definition.

We can do the boot-time option as well, to set a particular limit
other than the one enforced at compile time. Passing a year
number like "fstimestampcheck=2099" would address Linus'
concern about the cutoff being arbitrary.

I would also make the default limit higher than 2038, as at
least the Apple HFS/HFS+ file systems break only a bit later
in 2040. However, I don't think any other file system breaks
until 2099 (some Microsoft file systems), which would be
the next reasonably default cutoff IMO.

>> And honestly, for testing, it would be much better to just make it a
>> mount option rather than some crazy system-wide one.
>
> The patch allows the y2038 number to be changed at compile time. I can
> extend the sysctl and boot option to allow changing of this limit also
> if that is preferred.
>
> We also proposed the mount option route in the RFC. But, we received
> no preferences/ comments. We proceeded with the sysctl option because
> this allows us to extend this feature into disallowing writes on non
> updatable time filesystems.
>
> I could change this to providing a mount option instead if you think
> that is better.

I don't see much value in a mount option that prevents the use,
but maybe a mount option to override the global setting to make
an exception for someone who does want to mount a particular
(known-broken) file system despite having the stricter global setting.

         Arnd