Re: [RFC PATCH for 4.18 3/5] rseq: uapi: declare rseq_cs field as union, update includes

[Andy Lutomirski]

On Fri, Jul 6, 2018 at 12:31 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Jul 6, 2018 at 12:23 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>>
>> For -rc, I would favor the following simpler approach. Or I could even
>> just use get_user() instead. Thoughts ?
>
> Please just use "get_user()".
>
> In fact, we should be thinking seriosly about just removing
> __get_user() entirely. It's wrong. It optimizes the wrong thing
> entirely. It _used_ to be that the range check was noticeable, and it
> really isn't any more. These days the expensive parts are the SMAP
> costs, and both get_user() and __get_user() have those, except
> get_user() is safer and doesn't waste I$ on inlining the code to
> disable and re-enable SMAP.

If Al and Christoph ever manage to get rid of set_fs(), I bet we can
rewrite access_ok() and get_user() so that gcc can fold redundant
checks together and generate optimal code for get_user() of
consecutive struct fields all by itself.  Or maybe I'm giving gcc more
credit than it deserves.