Re: block-rbd: One function call less in rbd_dev_probe_parent() after error detection

[Ilya Dryomov]

On Tue, Nov 24, 2015 at 9:34 PM, SF Markus Elfring
<elfring@xxxxxxxxxxxxxxxxxxxxx> wrote:
>> Well, there isn't any _literal_ linking (e.g. adding to a link list,
>> etc) in this case.  We just bump some refs and do probe to fill in the
>> newly allocated parent.
>
> Thanks for your clarification.
>
>
>> The actual linking (rbd_dev->parent = parent) is done right before
>> returning so we never have to undo it in rbd_dev_probe_parent() and
>> that's the only reason your patch probably doesn't break anything.
>
> Is this function implementation just also affected by an issue
> which is mentioned in the Linux document "CodingStyle" as "one err bugs"?

No, why?  "one err bug" as per CodingStyle is a NULL deref on line 2 if
foo is NULL.  If it was just "err: kfree(foo); return ret;", a NULL foo
would be perfectly OK.

1       err:
2               kfree(foo->bar);
3               kfree(foo);
4               return ret;

If you can spot such a NULL deref in rbd_dev_probe_parent(), I'd gladly
take a patch.

>
>
>> Think about what happens if, after your patch is applied, someone moves
>> that assignment up or adds an extra step that can fail after it...
>
> Is such a software maintenance concern really enough to delay (or reject)
> my second update suggestion in this small patch series?

Yes - it's rejected because it messes up the order of cleanup for no
good reason.  I realize why you think the patch is correct and it's not
without merit, but it just doesn't fit the weird rbd_dev_probe_parent()
contract.

Thanks,

                Ilya
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