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Re: [Xen-devel] [PATCH 2/2] x86/traps: widen condition for logging top-of-stack


  • To: Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 7 Jun 2019 19:01:11 +0100
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  • Cc: WeiLiu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 07 Jun 2019 18:01:28 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 31/05/2019 10:22, Jan Beulich wrote:
> Despite -fno-omit-frame-pointer the compiler may omit the frame pointer,
> often for relatively simple leaf functions. (To give a specific example,
> the case I've run into this with is _pci_hide_device() and gcc 8.
> Interestingly the even more simple neighboring iommu_has_feature() does
> get a frame pointer set up, around just a single instruction. But this
> may be a result of the size-of-asm() effects discussed elsewhere.)
>
> Log the top-of-stack value if it looks valid _or_ if RIP looks invalid.
>
> Also annotate non-frame-pointer-based stack trace entries with a
> question mark, to signal clearly that any one of them may not actually
> be part of the call stack.

I very specifically didn't do that before, because it give the false
impression that when a question mark isn't present, the logging line is
accurate.

This is not true for %rbp corruption, asm blocks which don't respect the
frame pointer ABI (arguably also corruption), any fault raised from
within an EFI call.

Porting Xen to use objtool would be a definite improvement, but cannot
guard against unexpected %rbp corruption and the EFI case.

~Andrew

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