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 15:56:56 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
CC:	James Hogan <james.hogan@...tec.com>, 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 03:47 PM, Leonid Yegoshin wrote:
> 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.
>

It isn't just the number of lines of code that is important.

Doesn't your solution consume an extra page for each thread requiring 
emulation?  That could be a significant amount of memory in a system 
with many threads.

Are you are using this to emulate atomic operations in addition to FPU 
branch delay slot instructions?  Where is the code that does that?

David Daney

> - Leonid.
>
>
>

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