linux-kernel - Re: [RFC][PATCH] KGDB: remove kgdb-own fault handling (was: Re: [git pull] x86 arch updates for v2.6.25)

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Date:	Fri, 8 Feb 2008 13:58:44 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Kiszka <jan.kiszka@....de>
cc:	Andi Kleen <andi@...stfloor.org>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [RFC][PATCH] KGDB: remove kgdb-own fault handling (was: Re: [git
 pull] x86 arch updates for v2.6.25)

On Fri, 8 Feb 2008, Jan Kiszka wrote:
> 
> Well, let's try it this way: Find below a patch against kgdb.git that
> removes the special fault handling (this wouldn't be the first feature I
> recently removed from kgdb :->). Light testing revealed no obvious
> problems yet.

That is indeed horrible code. No way will I merge anything that has things 
like that even in it's *history* (ie somebody needs to re-generate the 
tree without code like that - some things should not be allowed to exist).

That said, while just using "probe_kernel_addr()" is certainly much 
better, it's still really inefficient. If you actually want to do a "safe 
memory copy", then the right way to do that is basically to do

	pagefault_disable();
	leftover = __copy_from_user_inatomic(dst, src, count);
	pagefault_enable();

	if (leftover)
		handle_the_fact_that_the_copy_didnt_complete();

which should even be reasonably efficient and should work in all contexts 
(hardware interrupts disabled, spinlocks held, you name it).

So all those "kgdb_{get|set}_mem()" things seem bogus (they also have 
insane calling semantics - return NULL or errptr? Why not just return an 
integer error code?

		Linus
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