----- On Jul 10, 2018, at 2:16 AM, Michael Ellerman mpe@xxxxxxxxxxxxxx wrote:
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes:
>> ----- On Jul 8, 2018, at 5:03 PM, Mathieu Desnoyers
>> mathieu.desnoyers@xxxxxxxxxxxx wrote:
>>
>>> In preparation to use __u64 for the rseq_cs pointer field, 32-bit
>>> architectures need to read this 64-bit value located in user-space
>>> addresses.
>>>
>>> __get_user is used to read this value, given that its access check has
>>> already been performed with access_ok() on rseq registration.
>>>
>>> arm does not implement 8-byte __get_user. Rather than trying to
>>> improve __get_user on ARM, use get_user/put_user across rseq instead.
>>>
>>> If those end up showing up in benchmarks, the proper approach would be to
>>> use user_access_begin() / unsafe_get/put_user() / user_access_end()
>>> anyway.
>>
>> So, another twist to this story: ppc32 does not implement u64 get_user().
>
> Or __get_user() for that matter.
>
> But we should just fix it.
>
> We have the asm to do it, it's just the fact that __gu_val is unsigned
> long causes the size > sizeof(x) check here to fail:
>
> #define __get_user_size(x, ptr, size, retval) \
> do { \
> retval = 0; \
> __chk_user_ptr(ptr); \
> if (size > sizeof(x)) \
> (x) = __get_user_bad(); \
>
>
>
> We seem to be able to fix that with the __inttype() trick that x86 uses.
>
> That's probably not 4.18 material though. But if you want to go with
> copy_from_user() for now you could then switch to get_user() for 4.19.
I agree. Let's use copy_from_user() for 4.18. Once get_user() ends up supporting
u64 on ppc32 for 4.19, rseq will happily move back to it.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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