On Fri, 31 Mar 2023 at 17:52, Axel Rasmussen axelrasmussen@google.com wrote:
On Thu, Mar 30, 2023 at 3:27 PM Peter Xu peterx@redhat.com 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@redhat.com 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/?i... (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#designin...
...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#L123...
So, doing a sane thing in kernel shouldn't break CRIU (at least here).
Thanks, Dmitry