linux-kernel - Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

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Date:	Mon, 28 Apr 2014 17:02:13 -0700
From:	Andrew Lutomirski <amluto@...il.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, comex <comexk@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Borislav Petkov <bp@...en8.de>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Alexandre Julliard <julliard@...ehq.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

On Mon, Apr 28, 2014 at 4:08 PM, H. Peter Anvin <hpa@...ux.intel.com> wrote:
> On 04/28/2014 04:05 PM, H. Peter Anvin wrote:
>>
>> So I tried writing this bit up, but it fails in some rather spectacular
>> ways.  Furthermore, I have been unable to debug it under Qemu, because
>> breakpoints don't work right (common Qemu problem, sadly.)
>>
>> The kernel code is at:
>>
>> https://git.kernel.org/cgit/linux/kernel/git/hpa/espfix64.git/
>>
>> There are two tests:
>>
>> git://git.zytor.com/users/hpa/test16/test16.git, build it, and run
>> ./run16 test/hello.elf
>> http://www.zytor.com/~hpa/ldttest.c
>>
>> The former will exercise the irq_return_ldt path, but not the fault
>> path; the latter will exercise the fault path, but doesn't actually use
>> a 16-bit segment.
>>
>> Under the 3.14 stock kernel, the former should die with SIGBUS and the
>> latter should pass.
>>
>
> Current status of the above code: if I remove the randomization in
> espfix_64.c then the first test passes; the second generally crashes the
> machine.  With the randomization there, both generally crash the machine.
>
> All my testing so far has been under KVM or Qemu, so there is always the
> possibility that I'm chasing a KVM/Qemu bug, but I suspect it is
> something simpler than that.

I'm compiling your branch.  In the mean time, two possibly stupid questions:

What's the assembly code in the double-fault entry for?

Have you tried hbreak in qemu?  I've had better luck with hbreak than
regular break in the past.

--Andy
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