Re: [PATCH 01/29] Revert "userfaultfd: don't fail on unrecognized features"

[Peter Xu]

On Thu, Mar 30, 2023 at 08:31:30PM +0200, David Hildenbrand wrote:
> On 30.03.23 17:56, Peter Xu wrote:
> > This is a proposal to revert commit 914eedcb9ba0ff53c33808.
> > 
> > I found this when writting a simple UFFDIO_API test to be the first unit
> > test in this set.  Two things breaks with the commit:
> > 
> >    - UFFDIO_API check was lost and missing.  According to man page, the
> >    kernel should reject ioctl(UFFDIO_API) if uffdio_api.api != 0xaa.  This
> >    check is needed if the api version will be extended in the future, or
> >    user app won't be able to identify which is a new kernel.
> 
> Agreed.
> 
> > 
> >    - Feature flags checks were removed, which means UFFDIO_API with a
> >    feature that does not exist will also succeed.  According to the man
> >    page, we should (and it makes sense) to reject ioctl(UFFDIO_API) if
> >    unknown features passed in.
> > 
> 
> Agreed.
> 
> I understand the motivation of the original commit, but it should not have
> changed existing checks/functionality. Introducing a different way to enable
> such functionality on explicit request would be better. But maybe simple
> feature probing (is X support? is Y supported? is Z supported) might be
> easier without requiring ABI changes.

Yes, I mentioned a similar "proposal" of UFFDIO_FEATURES here too, simply
returning the feature bitmask before UFFDIO_API:

https://lore.kernel.org/all/ZCSUTSbAcwBINiNk@x1n/

But I think current way is still fine; so maybe we'd just not bother.

> 
> I assume we better add
> 
> Fixes: 914eedcb9ba0 ("userfaultfd: don't fail on unrecognized features")

Yes I'll add it.

> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks,

-- 
Peter Xu