Re: [PATCH 07/14] x86/ibt: Clean up is_endbr()

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Sep 29, 2024 at 10:32:38AM -0700, Alexei Starovoitov wrote:
> On Fri, Sep 27, 2024 at 12:50 PM Peter Zijlstra <peterz@infradead.org> wrote:

> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1027,19 +1027,9 @@ static const struct bpf_func_proto bpf_g
> >  #ifdef CONFIG_X86_KERNEL_IBT
> >  static unsigned long get_entry_ip(unsigned long fentry_ip)
> >  {
> > -       u32 instr;
> > +       if (is_endbr((void *)(fentry_ip - ENDBR_INSN_SIZE)))
> > +               return fentry_ip - ENDBR_INSN_SIZE;
> >
> > -       /* We want to be extra safe in case entry ip is on the page edge,
> > -        * but otherwise we need to avoid get_kernel_nofault()'s overhead.
> > -        */
> > -       if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
> > -               if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
> > -                       return fentry_ip;
> > -       } else {
> > -               instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
> > -       }
> > -       if (is_endbr(instr))
> > -               fentry_ip -= ENDBR_INSN_SIZE;
> >         return fentry_ip;
> 
> Pls don't.
> 
> This re-introduces the overhead that we want to avoid.
> 
> Just call __is_endbr() here and keep the optimization.

Well, I could do that ofcourse, but as I wrote elsewhere, the right
thing to do is to optimize get_kernel_nofault(), its current
implementation is needlessly expensive. All we really need is a load
with an exception entry, the STAC/CLAC and speculation nonsense should
not be needed.

Fixing get_kernel_nofault() benefits all users.