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Re: [Xen-devel] [PATCH v2 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: Wed, 3 Jul 2019 11:21:57 +0100
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- Cc: WeiLiu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 03 Jul 2019 10:22:12 +0000
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On 17/06/2019 09:13, Jan Beulich wrote:
> Despite -fno-omit-frame-pointer the compiler may omit the frame pointer,
> often for relatively simple leaf functions.
Actually, the problem is more widespread than this. For every function,
there is a non-zero quantity of time between the function starting and
the frame pointer being set up.
However, half of this time is spent with the old %rbp on the top of the
stack, so won't benefit from these changes.
> (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.
This far, I'm happy with.
> 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'm still opposed to this. The introduction of ? does more harm than
good IMO, because it simply can't be trusted.
Stack traces are not guaranteed-accurate, even with frame pointers
enabled. The only thing we can say for certain in any trace is where
%rip points.
~Andrew
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