Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from_fxsr()

[Pekka Enberg]

On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>>> While %ds still contains the userspace selector, %cs is KERNEL_CS
>>> at this point.  Always get %cs from pt_regs.
>>>
>>> It actually is possible to get the correct segments for compat tasks,
>>> but that involves using the [f]xsave instruction without a REX.W prefix.
>>>
>>> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>

On Sun, Aug 29, 2010 at 2:41 PM, Pekka Enberg <penberg@xxxxxxxxxx> wrote:
>> It might be just me but the above description doesn't explain
>> anything. What's the problem here? What is this fixing?

On Mon, Aug 30, 2010 at 3:25 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> The %cs segment being reported to a compat task is flat out wrong.  It
> is getting KERNEL_CS when it should be some userspace segment.  The
> code segment may still be wrong, because the %cs in pt_regs may not
> have been the segment where the instruction that flagged the exception
> executed from.  That could be fixed by using fxsave without a REX.W
> prefix when saving the state of compat tasks, which would save the
> segment and 32-bit offset instead of the 64-bit offset for the code
> and data pointers.  This is such a corner case that it probably isn't
> worth putting much effort into fixing unless someone demonstrates a
> real need for it.

I sort of was able to deduce most of that from the original
description. However, I still don't quite understand what the problem
causes. Just a wrong cs reported to a signal handler or something
else?
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