Re: [PATCH for 4.18 2/6] rseq: use get_user/put_user rather than __get_user/__put_user

----- On Jul 10, 2018, at 2:16 AM, Michael Ellerman mpe@ellerman.id.au wrote:

Mathieu Desnoyers [off-list ref] writes:

quoted

----- On Jul 8, 2018, at 5:03 PM, Mathieu Desnoyers
mathieu.desnoyers@efficios.com wrote:

quoted

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