Re: [PATCH 01/29] Revert "userfaultfd: don't fail on unrecognized features"
From: David Hildenbrand
Date: Mon Apr 03 2023 - 03:49:26 EST
On 30.03.23 21:04, Axel Rasmussen wrote:
On Thu, Mar 30, 2023 at 8:57 AM Peter Xu <peterx@xxxxxxxxxx> 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.
100% agreed, this was a mistake.
- 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.
I still strongly disagree with reverting this part, my feeling is
still that doing so makes things more complicated for no reason.
Re: David's point, it's clearly wrong to change semantics so a thing
that used to work now fails. But this instead makes it more permissive
- existing userspace programs continue to work as-is, but *also* one
can achieve the same thing more simply (combine probing +
configuration into one step). I don't see any problem with that,
generally.
But, if David and others don't find my argument convincing, it isn't
the end of the world. It just means I have to go update my userspace
code to be a bit more complicated. :)
I'd probably find it more convincing if we'd started out with that
approach ;) . User space would have to deal with the behavior of old
kernels either way already? IOW, old kernels would reject the new flags,
new kernels would not reject them but mask them out. So changing that
behavior after the effects is somewhat suboptimal IMHO ... and rather
makes things more complicated.
--
Thanks,
David / dhildenb