> 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?
