linux-kernel - Re: [PATCH 07/14] x86/ibt: Clean up is

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Message-ID: <20240930083026.GG5594@noisy.programming.kicks-ass.net>
Date: Mon, 30 Sep 2024 10:30:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	alyssa.milburn@...el.com, scott.d.constable@...el.com,
	Joao Moreira <joao@...rdrivepizza.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Jose E. Marchesi" <jose.marchesi@...cle.com>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>, ojeda@...nel.org,
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 07/14] x86/ibt: Clean up is_endbr()

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@...radead.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.