Re: [PATCH v9 00/24] ILP32 for ARM64

From: Arnd Bergmann
Date: Wed Oct 10 2018 - 10:18:07 EST

On 10/10/18, Eugene Syromiatnikov <esyr@xxxxxxxxxx> wrote:
> On Tue, Jul 24, 2018 at 08:39:57PM +0300, Yury Norov wrote:
>> Hi all,
>> + Pavel Machek, Palmer Dabbelt, Wookey.
>> On Wed, May 16, 2018 at 11:18:45AM +0300, Yury Norov wrote:
>> > This series enables AARCH64 with ILP32 mode.
>> >
>> > As supporting work, it introduces ARCH_32BIT_OFF_T configuration
>> > option that is enabled for existing 32-bit architectures but disabled
>> > for new arches (so 64-bit off_t userspace type is used by new
>> > userspace).
>> > Also it deprecates getrlimit and setrlimit syscalls prior to prlimit64.
>> >
>> > Based on kernel v4.16. Tested with LTP, glibc testsuite, trinity,
>> > lmbench,
>> > CPUSpec.
>> This is the update of the series based on 4.17 kernel
> Hello.
> I have some questions regarding AArch64 ILP32 implementation for which I
> failed to find an answer myself:
> * How ptrace() tracer is supposed to distinguish between ILP32 and LP64
> tracees? For MIPS N32 and x32 this is possible based on syscall
> number, but for AArch64 ILP32 I do not see such a sign. There's also
> ARM_ip is employed for signalling entering/exiting, I wonder whether
> it's possible to employ it also for signalling tracee's personality.

Don't know.

> * What's the reasoning behind capping syscall arguments to 32 bit? x32
> and MIPS N32 do not have such a restriction (and do not need special
> wrappers for syscalls that pass 64-bit values as a result, except
> when they do, as it is the case for preadv2 on x32); moreover, that
> would lead to insurmountable difficulties for AArch64 ILP32 tracers
> that try to trace LP64 tracees, as it would be impossible to pass
> 64-bit addresses to process_vm_{read,write} or ptrace PEEK/POKE.

The idea is to keep the syscall entry points as similar as possible
between arm (aarch32) emulation and aarch64-ilp32 mode when
you have a kernel that supports both.