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Date: Sun, 29 Aug 2010 20:25:16 -0400 From: Brian Gerst <brgerst@...il.com> To: Pekka Enberg <penberg@...nel.org> Cc: hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from_fxsr() On Sun, Aug 29, 2010 at 2:41 PM, Pekka Enberg <penberg@...nel.org> wrote: > On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@...il.com> 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@...il.com> > > It might be just me but the above description doesn't explain > anything. What's the problem here? What is this fixing? 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. -- Brian Gerst -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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