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Re: [RESEND PATCH 03/12] golang/xenlight: fix string conversion in generated toC functions

  • To: Nick Rosbrook <rosbrookn@xxxxxxxxx>
  • From: George Dunlap <George.Dunlap@xxxxxxxxxx>
  • Date: Thu, 1 Jul 2021 14:09:47 +0000
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  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Nick Rosbrook <rosbrookn@xxxxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
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  • Thread-topic: [RESEND PATCH 03/12] golang/xenlight: fix string conversion in generated toC functions

> On Jun 21, 2021, at 5:11 PM, Nick Rosbrook <rosbrookn@xxxxxxxxx> wrote:
> On Fri, Jun 18, 2021 at 11:00:26AM +0000, George Dunlap wrote:
>>> On May 24, 2021, at 9:36 PM, Nick Rosbrook <rosbrookn@xxxxxxxxx> wrote:
>>> In gengotypes.py, the toC functions only set C string fields when
>>> the Go strings are non-empty. However, to prevent segfaults in some
>>> cases, these fields should always at least be set to nil so that the C
>>> memory is zeroed out.
>>> Update gengotypes.py so that the generated code always sets these fields
>>> to nil first, and then proceeds to check if the Go string is non-empty.
>>> And, commit the new generated code.
>>> Signed-off-by: Nick Rosbrook <rosbrookn@xxxxxxxxxxxx>
>> So wait — if you do
>> var foo C.typename
>> Then golang won’t automatically zero out `foo`?
>> That seems like a bug really; but assuming this fixes real behavior you’ve 
>> encountered:
> I would have to dig in again to figure out exactly what Go/cgo is doing
> here, and whether or not this is a bug. But, the behavior I observed was
> that without these nil assignments, I would sometimes get segfaults in
> libxl_string_copy. This patch ensures that libxl__str_dup is not called
> in the empty string case, thus avoiding the segfault.

I skimmed through the CGo page again when I was looking at this, and didn’t see 
anything specified about what happens if something is passed to a C function 
before being used by golang.  If you get a chance, I think it would be good to 
try to file a ticket with the golang project, pointing out the observed 
behavior, and asking them to either:

1. Document that the golang compiler may not initialize a structure before 
passing it in to a C function

2. Document that it *will* initialize values to zero, and fix the bug.




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