Re: [RFC PATCH v2 6/6] prctl.2: Add tagged address ABI control prctls (arm64)

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Dave

Is there a plan for future work on this patch?

Thanks,

Michael

On Mon, 15 Jun 2020 at 16:51, Dave Martin <Dave.Martin@arm.com> wrote:
>
> On Wed, Jun 10, 2020 at 06:42:05PM +0100, Catalin Marinas wrote:
> > On Wed, Jun 10, 2020 at 05:42:09PM +0100, Dave P Martin wrote:
> > > On Wed, Jun 10, 2020 at 04:26:34PM +0100, Catalin Marinas wrote:
> > > > On Wed, Jun 10, 2020 at 11:06:42AM +0100, Dave P Martin wrote:
> > > > > On Tue, Jun 09, 2020 at 06:22:32PM +0100, Catalin Marinas wrote:
> > > > > > On Wed, May 27, 2020 at 10:17:38PM +0100, Dave P Martin wrote:
> > > > > > > +.IP
> > > > > > > +The level of support is selected by
> > > > > > > +.IR "(unsigned int) arg2" ,
> > > > > >
> > > > > > We use (unsigned long) for arg2.
> > > > >
> > > > > Hmmm, not quite sure how I came up with unsigned int here.  I'll just
> > > > > drop this: the type in the prctl() prototype is unsigned long anyway.
> > > > >
> > > > > The type is actually moot in this case, since the valid values all fit
> > > > > in an unsigned int.
> > > >
> > > > Passing an int doesn't require that the top 32-bit of the long are
> > > > zeroed (in case anyone writes the low-level SVC by hand).
> > >
> > > Fair point, I was forgetting that wrinkle.  Anyway, the convention in
> > > this page seems to be that if the type is unsigned long, we don't
> > > mention it, because the prctl() prototype says that already.
> > >
> > > Question: the glibc prototype for prctl is variadic, so surely any
> > > calls that don't explicitly cast the args to unsigned long are already
> > > theoretically broken?  The #defines (and 0) are all implicitly int.
> > > This probably affects lots of prctls.
> > >
> > > We may get away with it because the compiler is almost certainly going
> > > to favour a mov over a ldr for getting small integers into regs, and mov
> > > <Wd> fortunately zeroes the top bits for us anyway.
> >
> > So does LDR Wd.
> >
> > Anyway, I think glibc (or my reading of it) has something like like:
> >
> >   register long _x1 asm ("x1") = _x1tmp;
> >
> > before invoking the SVC. I assume this would do the right conversion to
> > long. I can't tell about other libraries but I'd say it's their
> > responsibility to convert the args to long before calling the kernel's
> > prctl().
>
> Ignore me.  I was worrying that glibc would propagate junk in the high
> bits of int arguments, due to treating them as longs.  Actually, it
> will, but it doesn't matter where we explicitly cast the argument to int
> inside the kernel (thanks as usual to -fno-strict-overflow).
>
> Cheers
> ---Dave



-- 
Michael Kerrisk
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