Android and compatibility with deprecated armv7 instructions

[Ard Biesheuvel]

On 4 July 2014 10:24, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Fri, Jul 04, 2014 at 08:08:05AM +0100, Ard Biesheuvel wrote:
>> On 4 July 2014 00:47, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
>> > On Thu, Jul 03, 2014 at 11:16:16PM +0100, Måns Rullgård wrote:
>> >> Will Deacon <will.deacon at arm.com> writes:
>> >>
>> >> > On Thu, Jul 03, 2014 at 06:05:58PM +0100, Russell King - ARM Linux wrote:
>> >> >> On Thu, Jul 03, 2014 at 05:22:30PM +0100, Grant Likely wrote:
>> >> >> > So, no. I completely reject any notion that breaking existing apps is
>> >> >> > okay. If we're going to say that v8 still supports 32-bit apps, then
>> >> >> > it has to be all of v7, not just the 'good' bits. Nor do I think
>> >> >> > saying "it's just a bunch of games" justifies anything. We're kernel
>> >> >> > engineers. Applications are applications and we don't break userspace.
>> >> >> > Period.
>> >> >>
>> >> >> +1 on all points above.  I'd go further - if we're going to say that v8
>> >> >> still supports 32-bit apps, that covers at least v6 *as well*.
>> >> >
>> >> > We've never pretended to support anything other than ARMv8 in the compat
>> >> > layer. uname even reports this in the machine name.
>> >> >
>> >> > If people are suddenly so concerned about *full* compatibility with an ARMv7
>> >> > kernel, that needs a lot more than just SWP emulation:
>> >> >
>> >> >   - Alignment fixups for ldm/stm
>> >>
>> >> No ARM variant ever supported unaligned ldm/stm.
>> >
>> > Quite right but that's not the point being discussed.  Please note that
>> > the sentence says "with an ARMv7 *kernel*" - we are not talking about
>> > the architecture there.
>> >
>> > So, what's more to the point is that on 32-bit ARM userspace under Linux,
>> > we _have_ supported it since early 2000 up to present.  It's not currently
>> > supported on 64-bit ARM running Linux, even when running a 32-bit binary
>> > in userspace.
>> >
>> > Ergo, it's a user visible ABI change, one which we don't know whether it
>> > matters.  In all probability, it doesn't because (hopefully) no one ever
>> > does unaligned LDMs/STMs - I think it would require hand-crafted assembly,
>> > at which point you're talking about optimising something, and you'd be
>> > silly to do it as it would invoke the alignment fault handling which
>> > would be slow.
>> >
>>
>> Well, if something like this
>>
>> struct bar {
>>     long l[4];
>> };
>>
>> void foo(struct bar *dst, struct bar const *src)
>> {
>>     *dst = *src;
>> }
>>
>> produces this:
>>
>> foo:
>> @ args = 0, pretend = 0, frame = 0
>> @ frame_needed = 0, uses_anonymous_args = 0
>> @ link register save eliminated.
>>     mov ip, r0
>>     ldmia r1, {r0, r1, r2, r3}
>>     stmia ip, {r0, r1, r2, r3}
>>     bx lr
>>
>> won't it take just a single cast from some unaligned char* to struct
>> bar* to trigger this?
>
> Is this even allowed by the C ABI? The compiler generates the LDMs
> because function foo() gets a struct pointer which is guaranteed to be
> aligned.
>

Well, I was merely responding to Russell's assertion that it would
require hand-crafted assembly to trigger alignment faults in ldm/stm
instructions.

The point is not whether the C ABI allows it, the point is whether
a) the current 32-bit ARM kernel allows it, and
b) how likely it is to appear in existing code

-- 
Ard.