Re: [LTP] [PATCH] userfaultfd05: handle kernels rejecting WP feature in UFFDIO_API

[Petr Vorel <pvorel@suse.cz>]

> > >       if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
> > > -             if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP))
> > > -                     tst_brk(TCONF, "UFFD write-protect unsupported");
> > > +             int err = errno;
> > > +             if (err == EINVAL) {
> > > +                     uffdio_api.api = UFFD_API;
> > > +                     uffdio_api.features = 0;
> > > +
> > > +                     if (ioctl(uffd, UFFDIO_API, &uffdio_api) == 0)
> > > +                             tst_brk(TCONF, "UFFD write-protect unsupported");
> > > +             }

> > Wouldn't be better in this case to check kconfig for
> > CONFIG_HAVE_ARCH_USERFAULTFD_WP (untested, but it should work

> That's true, it would be simpler, let's go with this method.

> > Back to our discussion about how often using kconfig [1]. While I prefer to
> > avoid using it for tristate (kernel might be configured but module missing), but
> > here is just a feature.

> > [1] https://lore.kernel.org/ltp/CAASaF6wOSvi+07Pq5O6+f1Hkrq6WWMgpCaooJxWrO9uOvRM3pw@mail.gmail.com/

> I don’t think there is a single “standard” answer for feature detection;
> it really depends on the specific situation.

> For the UFFD-WP feature the situation is: there isn’t really a boot
> parameter or runtime knob that disables UFFD-WP independently once the
> interface is present. Given that, a simpler approach is to rely on Kconfig
> checks.

> This is especially relevant here because different kernels report “WP
> unsupported” differently (e.g. return -1/EINVAL vs return 0 with a
> different feature mask), which makes runtime-based detection more
> complicated.

Yes, for the discussion when to use I'd propose to *not* use kconfig if:
* boot parameter to enable/disable exist
* check for tristate (functionality which can be compiled as module)
* kernel new functionality which is unlikely to be backported (use .min_kver instead)

Unless some objection, I'll post a patch for ground rules.

Kind regards,
Petr

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