linux-kernel - Re: [PATCH] x86/extable: Add a comment about early exception handlers

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Date:	Mon, 4 Apr 2016 18:16:34 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	X86 ML <x86@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KVM list <kvm@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86/extable: Add a comment about early exception handlers

On Mon, Apr 04, 2016 at 08:46:22AM -0700, Andy Lutomirski wrote:
> Borislav asked for a comment explaining why all exception handlers are
> allowed early.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/mm/extable.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 98b5f45d9d79..36fe03bc81ee 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -132,6 +132,20 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
>  	if (regs->cs != __KERNEL_CS)
>  		goto fail;
>  
> +	/*
> +	 * The full exception fixup machinery is available as soon as
> +	 * the early IDT is loaded.  This means that it is the
> +	 * responsibility of extable users to either function correctly
> +	 * when handlers are invoked early or to simply avoid causing
> +	 * exceptions before they're ready to handle them.
> +	 *
> +	 * This is better than filtering which handlers can be used,
> +	 * because refusing to call a handler here is guaranteed to
> +	 * result in a hard-to-debug panic.
> +	 *
> +	 * Keep in mind that not all vectors actually get here.  Early
> +	 * fage faults, for example, are special.
> +	 */
>  	if (fixup_exception(regs, trapnr))
>  		return;
>  
> -- 

Thanks!

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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