[cxenstored] Potential bug in special watch event firing

[cxenstored] Potential bug in special watch event firing

[Raphael Ning]

While reading C xenstored code, I noticed that it may send special events like @releaseDomain to (privileged) clients that only watch the root node (/). That's probably not the intended behaviour.

For example, when firing @releaseDomain, fire_watches() is called with exact == false:

https://github.com/xen-project/xen/blob/2419a159fb943c24a6f2439604b9fdb1478fcd08/tools/xenstore/xenstored_domain.c#L234

As a result, it will check whether @releaseDomain is a child of the path being watched:

https://github.com/xen-project/xen/blob/2419a159fb943c24a6f2439604b9fdb1478fcd08/tools/xenstore/xenstored_watch.c#L195

And when the parent path is /, is_child() unconditionally returns true:

https://github.com/xen-project/xen/blob/2419a159fb943c24a6f2439604b9fdb1478fcd08/tools/xenstore/xenstored_watch.c#L66

The xenstore documentation is not clear about whether @<wspecial> should be considered children of /. But I find it counterintuitive that clients may be notified about a special event without explicitly subscribing to it. Is it a bug?


Thanks,

Raphael

Re: [cxenstored] Potential bug in special watch event firing

[Juergen Gross]

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Hi Raphael,

On 26.04.22 19:02, Raphael Ning wrote:
> While reading C xenstored code, I noticed that it may send special events like @releaseDomain to (privileged) clients that only watch the root node (/). That's probably not the intended behaviour.
> 
> For example, when firing @releaseDomain, fire_watches() is called with exact == false:
> 
> https://github.com/xen-project/xen/blob/2419a159fb943c24a6f2439604b9fdb1478fcd08/tools/xenstore/xenstored_domain.c#L234
> 
> As a result, it will check whether @releaseDomain is a child of the path being watched:
> 
> https://github.com/xen-project/xen/blob/2419a159fb943c24a6f2439604b9fdb1478fcd08/tools/xenstore/xenstored_watch.c#L195
> 
> And when the parent path is /, is_child() unconditionally returns true:
> 
> https://github.com/xen-project/xen/blob/2419a159fb943c24a6f2439604b9fdb1478fcd08/tools/xenstore/xenstored_watch.c#L66
> 
> The xenstore documentation is not clear about whether @<wspecial> should be considered children of /. But I find it counterintuitive that clients may be notified about a special event without explicitly subscribing to it. Is it a bug?

I agree this is a bug.


Juergen

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Re: [cxenstored] Potential bug in special watch event firing

[Raphael Ning]

Hi Juergen,

On 27/04/2022 08:30, Juergen Gross wrote:
> [...]
>> The xenstore documentation is not clear about whether @<wspecial> should be considered children of /. But I find it counterintuitive that clients may be notified about a special event without explicitly subscribing to it. Is it a bug?
>
> I agree this is a bug.


Thanks for confirming that. I can send a patch to fix it, but it probably won't happen until later next week. If anyone else can get it done sooner, please feel free to do so.

Raphael