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

On Fri, Jul 06, 2018 at 12:56:58PM -0700, Linus Torvalds wrote:

On Fri, Jul 6, 2018 at 12:38 PM Mathieu Desnoyers
[off-list ref] wrote:

quoted

Should I change all 4 bytes __get_user()/__put_user() in kernel/rseq.c
for get_user()/put_user() to ensure consistency ?

Probably.

*If* this actually turns out to be somethinig that shows up on
profiles, it's almost certainly going to be the STAC/CLAC instructions
("perf report" tends to report them as three one-byte nop's because
that's how they look before instruction replacement).

And then it's not __get/put_user() that will improve things, but doing a

        user_access_begin();

        .. do unsafe_get/put_user() ..

        user_access_end();

that will improve performance.

But it is *very* seldom useful. We have it in a handful of places in
the kernel, and the most noticeable one is
lib/{strnlen,strncpy_from}_user.c

Also, __get_user() is probably going to become the same as get_user()
when I finish the Spectre v1 ARM mitigations, because there'll be no
point in __get_user() being any different.  For those mitigations,
we're going to have to check the pointer against the address limit
inside __get_user() and NULL it out, just like get_user() does, which
makes the whole distinction between the two completely pointless.

Is this not also the case on other architectures affected by Spectre
variant 1, hmm?

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