ZFS compiler warnings

[Benjamin Kaduk]

On Fri, 10 Apr 2015, Garrett Wollman wrote:

> I notice on my 10.1 kernel builds warnings like the following:
>
> /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c:2855:12: warning: comparison of constant -1 with expression of type 'zfs_prop_t' is always true [-Wtautological-constant-out-of-range-compare]
>                 if (prop != ZPROP_INVAL && !zfs_prop_inheritable(prop))
>                     ~~~~ ^  ~~~~~~~~~~~
> /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c:3810:11: warning: comparison of constant -1 with expression of type 'zfs_prop_t' is always false [-Wtautological-constant-out-of-range-compare]
>         if (prop == ZPROP_INVAL) {
>             ~~~~ ^  ~~~~~~~~~~~
>
> The warnings indicate that these checks are being optimized away by
> clang.  The code in head is the same.  Does anyone have an idea of how
> this should be fixed, in a way that would be acceptable upstream?  (Do
> we have someone who is the "official" liaison with the OpenZFS
> project?)
>
> My guess is that the definition of the zfs_prop_t enum itself needs to
> include these two constants explicitly -- but that also has the effect
> of changing the type of ZPROP_INVAL, which then means that
> zpool_prop_t needs to have its own version of "invalid property"
> defined, and that then requires cascading changes in a bunch of other
> places.

n1256.pdf lists as a "common warning" (annex I) the case where a value is
assigned to a variable of enumerated type and the value is not from the
same set of enumerated constants, so any assignment of ZPROP_INVAL to a
variable of type zpool_prop_t should trigger a warning with the current
code.  (If there are no such assignments, then why are we bothering to
check against it?)

> The following is a minimal change that fixes the warnings in the
> kernel alone (beware whitespace lossage) -- I haven't checked the
> userland side:

It seems like the right thing to do, to me, but I am not really a ZFS guy.

-Ben