linux-kernel - Re: [patch 02/15] x86/cpu: Uninline CR4 accessors

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Date:   Mon, 20 Apr 2020 02:01:02 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [patch 02/15] x86/cpu: Uninline CR4 accessors

> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -387,7 +387,30 @@ void native_write_cr4(unsigned long val)
>  			  bits_missing);
>  	}
>  }
> -EXPORT_SYMBOL(native_write_cr4);
> +#if IS_MODULE(CONFIG_LKDTM)
> +EXPORT_SYMBOL_GPL(native_write_cr4);
> +#endif

While this is better than what we had before we really need to have
a discussion on lkdtm - it needs a lot of crap that otherwise wouldn't
be exported, and I'm really worried about people enabling it and thus
adding exports even if they are conditional.  Can we force the code
to be built in require a boot option for it to be activated?