Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 2, 2018, at 7:37 PM, Andy Lutomirski luto@amacapital.net wrote:

>> On Jul 2, 2018, at 4:22 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> wrote:
>> 
>> ----- On Jul 2, 2018, at 7:16 PM, Mathieu Desnoyers
>> mathieu.desnoyers@efficios.com wrote:
>> 
>>> ----- On Jul 2, 2018, at 7:06 PM, Linus Torvalds torvalds@linux-foundation.org
>>> wrote:
>>> 
>>>> On Mon, Jul 2, 2018 at 4:00 PM Mathieu Desnoyers
>>>> <mathieu.desnoyers@efficios.com> wrote:
>>>>> 
>>>>> Unfortunately, that rseq->rseq_cs field needs to be updated by user-space
>>>>> with single-copy atomicity. Therefore, we want 32-bit user-space to initialize
>>>>> the padding with 0, and only update the low bits with single-copy atomicity.
>>>> 
>>>> Well... It's actually still single-copy atomicity as a 64-bit value.
>>>> 
>>>> Why? Because it doesn't matter how you write the upper bits. You'll be
>>>> writing the same value to them (zero) anyway.
>>>> 
>>>> So who cares if the write ends up being two instructions, because the
>>>> write to the upper bits doesn't actually *do* anything.
>>>> 
>>>> Hmm?
>>> 
>>> Are there any kind of guarantees that a __u64 update on a 32-bit architecture
>>> won't be torn into something daft like byte-per-byte stores when performed
>>> from C code ?
>>> 
>>> I don't worry whether the upper bits get updated or how, but I really care
>>> about not having store tearing of the low bits update.
>> 
>> For the records, most updates of those low bits are done in assembly
>> from critical sections, for which we control exactly how the update is
>> performed.
>> 
>> However, there is one helper function in user-space that updates that value
>> from C through a volatile store, e.g.:
>> 
>> static inline void rseq_prepare_unload(void)
>> {
>>        __rseq_abi.rseq_cs = 0;
>> }
> 
> How about making the field be:
> 
> union {
> __u64 rseq_cs;
> struct {
>   __u32 rseq_cs_low;
>   __u32 rseq_cs_high;
> };
> };
> 
> 32-bit user code that cares about performance can just write to rseq_cs_low
> because it already knows that rseq_cs_high == 0.
> 
> The header could even supply a static inline helper write_rseq_cs() that
> atomically writes a pointer and just does the right thing for 64-bit, for
> 32-bit BE, and for 32-bit LE.
> 
> I think the union really is needed because we can’t rely on user code being
> built with -fno-strict-aliasing.  Or the helper could use inline asm.
> 
> Anyway, the point is that we get optimal code generation (a single instruction
> write of the correct number of bits) without any compat magic in the kernel.

That works for me! Any objection from anyone else for this approach ?

Thanks,

Mathieu

> 
>> 
>> Thanks,
>> 
>> Mathieu
>> 
>>> 
>>> Thanks,
>>> 
>>> Mathieu
>>> 
>>> 
>>> --
>>> Mathieu Desnoyers
>>> EfficiOS Inc.
>>> http://www.efficios.com
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com