Re: [PATCH v2 29/29] y2038: add 64-bit time_t syscalls to all 32-bit architectures

Geert Uytterhoeven
2019-01-21T08:19:11Z

Hi Russell,

On Sat, Jan 19, 2019 at 3:29 PM Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
> On Fri, Jan 18, 2019 at 11:53:25AM -0800, Andy Lutomirski wrote:
> > On Fri, Jan 18, 2019 at 11:33 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Fri, Jan 18, 2019 at 7:50 PM Andy Lutomirski <luto@kernel.org> wrote:
> > > > On Fri, Jan 18, 2019 at 8:25 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > > > > - Once we get to 512, we clash with the x32 numbers (unless
> > > > > we remove x32 support first), and probably have to skip
> > > > > a few more. I also considered using the 512..547 space
> > > > > for 32-bit-only calls (which never clash with x32), but
> > > > > that also seems to add a bit of complexity.
> > > >
> > > > I have a patch that I'll send soon to make x32 use its own table. As
> > > > far as I'm concerned, 547 is *it*. 548 is just a normal number and is
> > > > not special. But let's please not reuse 512..547 for other purposes
> > > > on x86 variants -- that way lies even more confusion, IMO.
> > >
> > > Fair enough, the space for those numbers is cheap enough here.
> > > I take it you mean we also should not reuse that number space if
> > > we were to decide to remove x32 soon, but you are not worried
> > > about clashing with arch/alpha when everything else uses consistent
> > > numbers?
> > >
> >
> > I think we have two issues if we reuse those numbers for new syscalls.
> > First, I'd really like to see new syscalls be numbered consistently
> > everywhere, or at least on all x86 variants, and we can't on x32
> > because they mean something else. Perhaps more importantly, due to
> > what is arguably a rather severe bug, issuing a native x86_64 syscall
> > (x32 bit clear) with nr in the range 512..547 does *not* return
> > -ENOSYS on a kernel with x32 enabled. Instead it does something that
> > is somewhat arbitrary. With my patch applied, it will return -ENOSYS,
> > but old kernels will still exist, and this will break syscall probing.
> >
> > Can we perhaps just start the consistent numbers above 547 or maybe
> > block out 512..547 in the new regime?
>
> I don't think you gain much with that kind of scheme - it won't take
> very long before an architecture misses having a syscall added, and
> then someone else adds their own. Been there with ARM - I was keeping
> the syscall table in the same order as x86 for new syscalls, but now

Same for m68k, and probably other architectures.

> that others have been adding syscalls to the table since I converted
> ARM to the tabular form, that's now gone out the window.
>
> So, I think it's completely pointless to do what you're suggesting.
> We'll just end up with a big hole in the middle of the syscall table
> and then revert back to random numbering of syscalls thereafter again.

I believe the plan is to add future syscalls for all architectures in a
single commit, to keep everything in sync.

Regardless, I'm wondering what to do with the holes marked "room for
arch specific calls".
When is a syscall really arch-specific, and can it be added there, and
when does it turn out (later) that it isn't, breaking the
synchronization again?

The pkey syscalls may be a bad example, as AFAIU they can be implemented
on some architectures, but not on some others. Still, I had skipped them
when adding new syscalls to m68k.

Perhaps we should get rid of the notion of "arch-specific syscalls", and
reserve a slot everywhere anyway?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds