On Mon, Sep 21, 2020 at 9:15 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote: > > On 21/09/2020 19:10, Pavel Begunkov wrote: > > On 20/09/2020 01:22, Andy Lutomirski wrote: > >> > >>> On Sep 19, 2020, at 2:16 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > >>> > >>> On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: > >>>>> On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig <hch@xxxxxx> wrote: > >>>>> On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote: > >>>>>> Said that, why not provide a variant that would take an explicit > >>>>>> "is it compat" argument and use it there? And have the normal > >>>>>> one pass in_compat_syscall() to that... > >>>>> > >>>>> That would help to not introduce a regression with this series yes. > >>>>> But it wouldn't fix existing bugs when io_uring is used to access > >>>>> read or write methods that use in_compat_syscall(). One example that > >>>>> I recently ran into is drivers/scsi/sg.c. > >>> > >>> Ah, so reading /dev/input/event* would suffer from the same issue, > >>> and that one would in fact be broken by your patch in the hypothetical > >>> case that someone tried to use io_uring to read /dev/input/event on x32... > >>> > >>> For reference, I checked the socket timestamp handling that has a > >>> number of corner cases with time32/time64 formats in compat mode, > >>> but none of those appear to be affected by the problem. > >>> > >>>> Aside from the potentially nasty use of per-task variables, one thing > >>>> I don't like about PF_FORCE_COMPAT is that it's one-way. If we're > >>>> going to have a generic mechanism for this, shouldn't we allow a full > >>>> override of the syscall arch instead of just allowing forcing compat > >>>> so that a compat syscall can do a non-compat operation? > >>> > >>> The only reason it's needed here is that the caller is in a kernel > >>> thread rather than a system call. Are there any possible scenarios > >>> where one would actually need the opposite? > >>> > >> > >> I can certainly imagine needing to force x32 mode from a kernel thread. > >> > >> As for the other direction: what exactly are the desired bitness/arch semantics of io_uring? Is the operation bitness chosen by the io_uring creation or by the io_uring_enter() bitness? > > > > It's rather the second one. Even though AFAIR it wasn't discussed > > specifically, that how it works now (_partially_). > > Double checked -- I'm wrong, that's the former one. Most of it is based > on a flag that was set an creation. > Could we get away with making io_uring_enter() return -EINVAL (or maybe -ENOTTY?) if you try to do it with bitness that doesn't match the io_uring? And disable SQPOLL in compat mode? --Andy
