Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

[Andy Lutomirski]

On Thu, Jan 25, 2018 at 10:48 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Jan 22, 2018 at 10:55 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> Honestly, I'd rather get rid of the fast-path entirely. Compared to
>> all the PTI mess, it's not even noticeable.
>
> So I looked at how that would be.
>
> Patch attached. Not really "tested", but I'm running the kernel with
> this patch now, and 'strace' etc works, and honestly, it seems very
> obvious.
>
> Also, code generation for 'do_syscall_64()' does not look horrible. In
> fact, it doesn't look all that much worse than the fast-path ever did.
>
> So the biggest impact of this is the extra register saves
> (SAVE_EXTRA_REGS) from setting up the full ptregs. And honestly, I
> hate how that stupid macro still uses "movq reg,off(%rsp)" instead of
> "pushq %reg".
>
> Considering the diffstat:
>
>  2 files changed, 2 insertions(+), 121 deletions(-)
>
> and how those 100+ lines are nasty assembly code, I do think we should
> just do it.

Feel free to Acked-by: Andy Lutomirski <luto@xxxxxxxxxx> that patch.

Or I can grab it and send it to -tip.

Re: the trampoline not showing up: if I find some time, I'll try to
wire it up correctly in kallsyms.

--Andy