linux-kernel - Re: [PATCH v2 16/39] x86/bpf: Add ENDBR instructions to prologue and trampoline

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Message-ID: <20220225224619.foms7v6h2qd5qoe2@treble>
Date:   Fri, 25 Feb 2022 14:46:19 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 16/39] x86/bpf: Add ENDBR instructions to prologue and
 trampoline

On Fri, Feb 25, 2022 at 01:24:58PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 24, 2022 at 03:37:31PM -0800, Josh Poimboeuf wrote:
> 
> > > @@ -2028,10 +2052,11 @@ int arch_prepare_bpf_trampoline(struct b
> > >  		/* skip patched call instruction and point orig_call to actual
> > >  		 * body of the kernel function.
> > >  		 */
> > > -		orig_call += X86_PATCH_SIZE;
> > > +		orig_call += X86_PATCH_SIZE + 4*HAS_KERNEL_IBT;
> > 
> > All the "4*HAS_KERNEL_IBT" everywhere is cute, but you might as well
> > just have IBT_ENDBR_SIZE (here and in other patches).
> 
> So there's two forms of this, only one has the 4 included:
> 
>   (x * (1 + HAS_KERNEL_IBT))
>   (x + 4*HAS_KERNEL_IBT)
> 
> If I include the 4, then the first form would become something like:
> 
>   (x * (1 + !!IBT_ENDBR_SIZE))
> 
> that ok?

I don't have a strong preference, but there's no harming in having both
IBT_ENDBR_SIZE and HAS_KERNEL_IBT if it would help readability.

-- 
Josh