Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

[Sami Tolvanen]

On Thu, Jul 21, 2022 at 08:27:12PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 21, 2022 at 11:06:42AM -0700, Linus Torvalds wrote:
> > On Thu, Jul 21, 2022 at 10:56 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > this seems to work, let me go hack the kernel..
> > 
> > Am I missing something?
> > 
> > Isn't this generating
> > 
> >         movl $~IMM,%r10d
> >         negl %r10d
> >         cmpl %r10d,-4(%calldest)
> > 
> > for the sequence?
> > 
> > That seems bogus for two reasons:
> > 
> >  (a) 'neg' is not the opposite of '~'. Did you mean 'notl' or did you mean '-'?
> > 
> >      Or am I missing something entirely?
> 
> No, you're right, I'm being daft again.
> 
> >  (b) since you have that r10 use anyway, why can't you just generate the simpler
> > 
> >         movl $-IMM,%r10d
> >         addl -4(%calldest),%r10d
> > 
> >      instead? You only need ZF anyway.
> 
> Right, lemme see if I can wrangle llvm to generate that.

That looks good to me. I updated my LLVM tree to generate this code
for the checks:

https://github.com/samitolvanen/llvm-project/commits/kcfi

Sami