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
