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

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Date:	Fri, 8 Jan 2016 17:29:27 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	"Williams, Dan J" <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 Fri, Jan 8, 2016 at 11:29 AM, Luck, Tony <tony.luck@...el.com> wrote:
>>> +EXPORT_SYMBOL(ex_handler_default);
>>
>> Why not EXPORT_SYMBOL_GPL() ?
>>
>> We do not care about external modules.
>
> I thought the guideline was that new features are GPL, but changes
> to existing features shouldn't break by adding new GPL requirements.
>
> The point is moot though because  the shared hallucinations wore
> off this morning and I realized that having the "handler" be a pointer
> to a function can't work. We're storing the 32-bit signed offset from
> the extable to the target address. This is fine if the table and the
> address are close together. But for modules we have an exception
> table wherever vmalloc() loaded the module, and a function back
> in the base kernel.
>
> So back to your ".long 0" for the default case.  And if we want to allow
> modules to use any of the new handlers, then we can't use
> relative function pointers for them either.
>
> So I'm looking at making the new field just a simple integer and using
> it to index an array of function pointers (like in v7).
>
> Unless someone has a better idea?

Aren't modules loaded in the top 2GB of address space like the main
kernel?  Otherwise rip-relative addressing wouldn't work and modules
would have to be compiled as PIC.

--
Brian Gerst