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

Andrew Lutomirski
2014-04-29T00:02:50Z

On Mon, Apr 28, 2014 at 4:08 PM, H. Peter Anvin <hpa@linux.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