linux-kernel - Re: [PATCH v2 2/3] MIPS: Setup an instruction emulation in VDSO protected page instead of user stack

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Date:	Fri, 10 Oct 2014 15:47:56 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	<linux-mips@...ux-mips.org>, <Zubair.Kakakhel@...tec.com>,
	<geert+renesas@...der.be>, <david.daney@...ium.com>,
	<peterz@...radead.org>, <paul.gortmaker@...driver.com>,
	<davidlohr@...com>, <macro@...ux-mips.org>, <chenhc@...ote.com>,
	<richard@....at>, <zajec5@...il.com>, <keescook@...omium.org>,
	<alex@...x-smith.me.uk>, <tglx@...utronix.de>,
	<blogic@...nwrt.org>, <jchandra@...adcom.com>,
	<paul.burton@...tec.com>, <qais.yousef@...tec.com>,
	<linux-kernel@...r.kernel.org>, <ralf@...ux-mips.org>,
	<markos.chandras@...tec.com>, <dengcheng.zhu@...tec.com>,
	<manuel.lauss@...il.com>, <akpm@...ux-foundation.org>,
	<lars.persson@...s.com>
Subject: Re: [PATCH v2 2/3] MIPS: Setup an instruction emulation in VDSO protected
 page instead of user stack

On 10/10/2014 03:03 AM, James Hogan wrote:
> I just mean an (illegal/undefined) sequence of FPU branch instructions 
> in one anothers delay slots shouldn't be able to crash the kernel. 
> Actually 2 of them would be enough to verify the kernel didn't get too 
> confused. Maybe the second will be detected & ignored, or maybe it 
> doesn't matter if the first emuframe gets overwritten by the second 
> one from the kernels point of view.

Yes, I am looking into that sequences. I try to keep both emulators 
isolated from the rest of kernel and from each other as much as possible 
but intercalls via illegal combinations are still possible.


 > From Peter Zijlstra:

 > Right, look at uprobes, it does exactly all this with a single page.
 > Slot allocation will block waiting for a free slot when all are in use.

I don't see a reason to change my 300 lines design into much more 
lengthy code. That code has more links to the rest of kernel and high 
possibility to execute atomic operation/locks/mutex/etc - I can't do it 
for emulation of MIPS locking instructions.

- Leonid.
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