Re: [Xen-devel] [RFC Patch v4 7/9] correct xc_domain_save()'s return value

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/09/14 03:14, Wen Congyang wrote:
> On 09/22/2014 10:06 PM, Andrew Cooper wrote:
>> On 22/09/14 14:13, Ian Campbell wrote:
>>> On Mon, 2014-09-22 at 14:03 +0100, Ian Jackson wrote:
>>>> Ian Campbell writes ("Re: [Xen-devel] [RFC Patch v4 7/9] correct 
>>>> xc_domain_save()'s return value"):
>>>>> libxc doesn't know that, if it is important then it seems like the
>>>>> failure + errno ought to be marshalled across the IPC link.
>>>> Yes, but ...
>>>>
>>>>> It may be that this can be easily handled in
>>>>> libxl__srm_callout_sendreply + helper_getreply. Ian J -- what do you
>>>>> think?
>>>> ... while that would be possbile, we have another option.
>>>>
>>>> We could say that the callbacks return errno values.  That would
>>>> simplify the API and avoid having the IPC involve accesses to global
>>>> variables (ie, things not in the functions' parameter lists).
>>>>
>>>> If we do that then it becomes the responsibility of xc_domain_save to
>>>> either change its own API to return errno, or to save the callback's
>>>> return value in errno.
>>> Hrm. libxc is already a complete mess wrt error returning/handling
>>> because some proportion of the code incorrectly does/assumes this sort
>>> of thing is happening (because people were confused about the syscall
>>> returns from the kernel vs. process context). Having a place in libxc
>>> where this is now done on purpose seems a bit like setting the rope on
>>> fire to me...
>>>
>>> Ian.
>>>
>> libxc is an absolute mess, but this is far from the only codepath (even
>> in xc_domain_save()) which ends up like this.
>>
>> The *only* safe assumption is that ==0 is success and !=0 is failure for
>> xc_domain_save(), and errno may or may not be relevant, whether rc is -1
>> or not.
> Do you mean: errno is undefined even if rc is -1?
>
> Thanks
> Wen Congyang

Correct, last time I checked.  The error handling in libxc is in dire
need of fixing from scratch.

~Andrew

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