linux-kernel - Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall

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Date:	Wed, 15 Jun 2011 11:59:27 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall_64.c

On Wed, Jun 15, 2011 at 3:33 AM, Andrew Lutomirski <luto@....edu> wrote:
> On Tue, Jun 14, 2011 at 5:31 PM, Ingo Molnar <mingo@...e.hu> wrote:
>>
>> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>
>>> On Tue, Jun 14, 2011 at 2:16 PM, Ingo Molnar <mingo@...e.hu> wrote:
>>> >
>>> > I think correctness trumps code size and turning BUG() and BUG_ON()
>>> > into a NOP is just crazy ...
>>>
>>> Umm. It's even CRAZIER to turn it into a "compiler generates random code".
>>
>> Sigh, i assumed it got turned into an infinite loop - that is what
>> i've done in a prior patch.
>>
>> You are right, unreachable() is bogus and you'd also be right to
>> suggest that i should not comment on patches after 11pm ;-)
>
> What we want is a magic GCC trick that says "don't warn about code
> paths that go through here but generate the same code as you would
> without this annotation."  I don't think such a thing exists.
>
No, I don't think we need such kind of thing. I think, we should less
rely on GCC. Here, we need to reconsider the use of BUG. When
vsyscall_nr is default, it hits BUG. Here is the code comment:

               " * If we get here, then vsyscall_nr indicates that int 0xcc
                 * happened at an address in the vsyscall page that doesn't
                 * contain int 0xcc.  That can't happen. "

If that can't happen, I think we can treat it as a FAULT. So, rather
than calling BUG we can ground it into EFAULT. Does it break ABI
compatibility?

Thanks,
Rakib
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