On Fri, 31 Mar 2023 at 17:52, Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote: > > On Thu, Mar 30, 2023 at 3:27 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > On Thu, Mar 30, 2023 at 12:04:09PM -0700, 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. > > > > > > > > - 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. If features/flags are not checked in kernel, and the kernel doesn't return an error on an unknown flag/error, that makes the syscall non-extendable, meaning that adding any new feature may break existing software, which doesn't sanitize them properly. https://lwn.net/Articles/588444/ See a bunch of painful exercises from syscalls with numbers in the end: https://lwn.net/Articles/792628/ To adding an additional setsockopt() because an old one didn't have sanity checks for flags: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8917a777be3b (not the best example, as the new setsockopt() didn't check flags for sanity as well (sic!), but that's near the code I work on now) This is even documented nowadays: https://www.kernel.org/doc/html/latest/process/adding-syscalls.html#designing-the-api-planning-for-extension ...and everyone knows what happens when you blame userspace for breaking by not doing what you would have expected it to do: https://lkml.org/lkml/2012/12/23/75 [..] > > There's one reason that we may consider keeping the behavior. IMHO it is > > when there're major softwares that uses the "wrong" ABI (let's say so; > > because it's not following the man pages). If you're aware any such major > > softwares (especially open sourced) will break due to this revert patch, > > please shoot. > > Well, I did find one example, criu: > https://github.com/checkpoint-restore/criu/blob/criu-dev/criu/uffd.c#L266 Mike can speak better than me about uffd, but AFAICS, CRIU correctly detects features with kerneldat/kdat: https://github.com/checkpoint-restore/criu/blob/criu-dev/criu/kerndat.c#L1235 So, doing a sane thing in kernel shouldn't break CRIU (at least here). Thanks, Dmitry
