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

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Message-ID: <CALCETrXsC9eiQ8yF555-8G88pYEms4bDsS060e24FoadAOK+kw@mail.gmail.com>
Date:	Wed, 6 Jan 2016 10:07:19 -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 9:59 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Jan 06, 2016 at 09:54:19AM -0800, Andy Lutomirski wrote:
>> I assume that this zero is to save the couple of bytes for the
>> relocation entry on relocatable kernels?
>
> I didn't want to touch all _ASM_EXTABLE() macro invocations by adding a
> third param @handler which is redundant as we know which it is.

I see.  You could shove the .long ex_handler_default - . into the
macro, but that would indeed bloat the kernel image a bit more
(although not the in-memory size of the kernel).

>
>> > +       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.
>
> Actually, if I do "0 - .", that would overflow the int because current
> location is virtual address and that's 64-bit. Or would gas simply
> truncate it? Lemme check...
>
> Anyway, what we should do instead is simply
>
>         .long 0
>
> to denote that the @handler is implicit.
>
> Right?

Agreed.  I just think that your current fixup_ex_handler
implementation needs adjustment if you do it that way.

--Andy
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