linux-kernel - Re: Q: put_user_try & co (Was: [PATCH 1/5] Split wait_noreap

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Message-ID: <4A146E78.6000600@zytor.com>
Date:	Wed, 20 May 2009 13:56:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland McGrath <roland@...hat.com>
CC:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Q: put_user_try & co (Was: [PATCH 1/5] Split wait_noreap_copyout())

Roland McGrath wrote:
> 
> You mean &&__efault_label here (it's a funny syntax, but that's how it is).
> &&label is a GCC extension that I'm not sure the kernel has used before.
> 
> I think it can be touchy to have an asm jump into compiled code that way.
> e.g., perhaps the compiler produced:
> 
> 	mov reg, 40(sp)
> 	mov $123, reg
> 	#APP
> 	... inside of your asm ...
> 	#NO_APP
> 	mov 40(sp), reg
> 
> or some such thing.  If you jump away from inside the asm, you won't ever
> do "mov 40(sp), reg".  But the compiler might think that reg has its
> original value at the __efault_label: code location.
> 
> Perhaps more important than any particular compiler-confusion scenario we
> can come up with is simply that this would be an obscure corner of code
> generation in the compiler that the kernel has not evoked before.  There
> might be bugs or oddities in various compilers of various vintages, that
> we don't know about because they never came up before.
> 

Yes, it seems like a bad idea to me.

	-hpa
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