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@redhat.com
Thanks,