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:

quoted

On Wed, Jun 10, 2020 at 11:06:42AM +0100, Dave P Martin wrote:

quoted

On Tue, Jun 09, 2020 at 06:22:32PM +0100, Catalin Marinas wrote:

quoted

On Wed, May 27, 2020 at 10:17:38PM +0100, Dave P Martin wrote:

quoted

+.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().

-- 
Catalin

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel