linux-kernel - Re: [PATCH v7 1/3] x86: Add classes to exception tables

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Date:	Wed, 6 Jan 2016 09:54:19 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Tony Luck <tony.luck@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Robert <elliott@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v7 1/3] x86: Add classes to exception tables

On Wed, Jan 6, 2016 at 4:33 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Dec 30, 2015 at 09:59:29AM -0800, Tony Luck wrote:
>> Starting with a patch from Andy Lutomirski <luto@...capital.net>
>> that used linker relocation trickery to free up a couple of bits
>> in the "fixup" field of the exception table (and generalized the
>> uaccess_err hack to use one of the classes).
>
> So I still think that the other idea Andy gave with putting the handler
> in the exception table is much cleaner and straightforward.
>
> Here's a totally untested patch which at least builds here. I think this
> approach is much more extensible and simpler for the price of a couple
> of KBs of __ex_table size.
>
> ---
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index 189679aba703..43b509c88b13 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -44,18 +44,20 @@
>
>  /* Exception table entry */
>  #ifdef __ASSEMBLY__
> -# define _ASM_EXTABLE(from,to)                                 \
> +# define _ASM_EXTABLE(from,to)                         \
>         .pushsection "__ex_table","a" ;                         \
>         .balign 8 ;                                             \
>         .long (from) - . ;                                      \
>         .long (to) - . ;                                        \
> +       .long 0 - .;                                            \

I assume that this zero is to save the couple of bytes for the
relocation entry on relocatable kernels?

If so, ...

> +inline ex_handler_t ex_fixup_handler(const struct exception_table_entry *x)
> +{
> +       return (ex_handler_t)&x->handler + x->handler;

I would check for zero here, because...

> +       new_ip  = ex_fixup_addr(e);
> +       handler = ex_fixup_handler(e);
> +
> +       if (!handler)
> +               handler = ex_handler_default;

the !handler condition here will never trigger because the offset was
already applied.

Otherwise this looks generally sane.

--Andy
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