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

[Thomas Gleixner]

On Thu, 25 Jan 2018, Andy Lutomirski wrote:
> 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.

That would be nice, so we can route it through x86/pti which provides it
for backporting cleanly.

Thanks,

	tglx