linux-kernel - Re: [RFC] x86: xsave/xrstor support, ucontext

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Date:	Thu, 22 May 2008 19:27:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Mikael Pettersson <mikpe@...uu.se>,
	Andi Kleen <andi@...stfloor.org>, mingo@...e.hu,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, roland@...hat.com, drepper@...hat.com,
	Hongjiu.lu@...el.com, linux-kernel@...r.kernel.org,
	arjan@...ux.intel.com, rmk+lkml@....linux.org.uk, dan@...ian.org,
	asit.k.mallick@...el.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

Suresh Siddha wrote:
>>>
>>> The kernel needs to accept one(*) of the formats it can produce, which
>>> is not necessarily what it last produced. It's not inconceivable that
>>> user-space will construct sigframes on the fly (to emulate setcontext),
>>> or that it will mangle sigframes (e.g. to map non-rt to rt before 
>>> sigreturn).
>>>
>>> (*) The format is determined by which version of sys_sigreturn the
>>> user invokes.
>>>
>> No.  You CANNOT restore from a frame that doesn't have the full state - 
>> you don't have enough information to do so!
> 
> What I was doing in the RFC is: restore the state what ever that was present and
> init the state that was not present in the stack frame.

Either way, I find it somewhat surprising that the user would invoke a 
different sys_sigreturn especially if using a restorer function (which 
gcc always does.)

I really think I need to understand your application better, 
*especially* in the light of the fact you wouldn't at the moment know 
how even get the size of the frame.

	-hpa
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